Thermocline

The thermocline is the region of the ocean where temperature decreases most rapidly with increasing depth. It separates the warm, well-mixed upper layer from the colder, deep water below. A thermocline is present throughout the year in the tropics and middle latitudes. It is more difficult to discern in high latitudes, where temperature is more uniform with depth. The presence of a very shallow thermocline in the eastern equatorial Pacific Ocean has important implications for global climate.

The thermocline exists because the ocean absorbs most of the sun’s heat in a shallow layer near the surface. The heat absorbed from the sun increases the temperature of the surface relative to that of the deep ocean, maintaining the thermocline. This is in contrast to the atmosphere, where a much larger portion of incident solar radiation passes through to the Earth’s surface. Two important properties of the thermocline are its depth and its strength, or how rapidly temperature decreases with increasing depth. The thermocline’s depth is influenced by the winds at the surface of the ocean. In the Atlantic and Pacific oceans, surface winds push warm surface water away from the equator toward the poles, bringing the thermocline close to the surface at the equator.

Water that diverges at the equator accumulates in the subtropics, increasing the depth of the thermocline there. The thermocline is generally 82 to 656 ft. (25 to 200 m) deep in the equatorial regions and up to 3,281 ft. (1,000 m) deep in the subtropics.

The thermocline is strongest in the tropics and weakest in high latitudes. This reflects the fact that the surface temperature of the ocean generally decreases from the tropics to the poles, whereas the temperature of the deep ocean is nearly the same at all latitudes. As a result, the temperature contrast between the upper ocean and the deep ocean is greatest in the tropics. The temperature can drop by as much as 18 degrees F (10 degrees C) in less than 164 ft. (50 m) in the tropical thermocline.

In the extratropical oceans, the strength and depth of the thermocline vary from season to season. There is a main thermocline throughout the year, between 656 and 3,281 ft. (200 and 1,000 m). During summer, the sun heats the ocean’s surface more strongly than in winter. Most of the additional heat is absorbed in a very shallow surface layer, generating a sharper “seasonal” thermocline above the main thermocline. The seasonal thermocline is similar to the tropical thermocline in terms of its strength and depth. It erodes in the winter as the surface cools relative to the temperature in the main thermocline.

Tropical Oceans

The existence of a strong and shallow thermocline in the tropical oceans has important implications for climate. In the equatorial Pacific Ocean, westward surface winds lead to an accumulation of warm surface water in the west, depressing the thermocline there and raising it to near the surface in the east. The shallow thermocline in the east enables cold, nutrient-rich water to be mixed upward into the surface layer. Every few years, the thermocline in the eastern equatorial Pacific deepens in association with an El Niño event. The mixing of cold, nutrient-rich thermocline water into the surface layer is reduced, the surface temperature of the eastern equatorial Pacific Ocean increases, and biological productivity decreases. The warmer surface temperatures associated with El Niño affect atmospheric circulation in the tropics and alter weather patterns throughout the world.

The depth of the eastern equatorial Pacific thermocline has varied significantly in association with changes in global climate. For example, during the early Pliocene period (between 4.5 and 3 million years ago, the most recent period with global temperatures significantly higher than today), the eastern Pacific thermocline was much deeper than it is today, much like it is during a modern El Niño event.

Tertiary Period

The Tertiary period (ca. 66.4 to 1.8 million years ago [Ma]) was an interval of enormous geologic, climatic, oceanographic, and biologic change. It spans the transition from a globally warm world of relatively high sea levels to a world of lower sea levels, polar glaciation, and sharply differentiated climate zones. Over the past decade, however, it has become increasingly clear that Tertiary climatic history was not a simple unidirectional cooling driven by a single cause, but a much more complicated pattern of change controlled by a complex and dynamic linkage between changes in atmospheric carbon dioxide (CO2) levels and ocean circulation, both probably ultimately driven by tectonic evolution of ocean–continent geometry. Although satisfactory explanations for many aspects of Tertiary climate history are available, many areas remain incompletely understood.

The early Tertiary (Paleocene and most of the Eocene epochs, ca. 66–50 Ma) was characterized by a continuation of Cretaceous warm equable climates extending from pole to pole. Global temperatures may have been as much as 18–22 degrees F (10–12 degrees C) higher than present, and pole-to-equator temperature gradients were about 9 degrees F (5 degrees C) during the Paleocene, as compared with about 45 degrees F (25 degrees C) today.

The Paleocene–Eocene boundary (about 54 Ma) was marked by a geologically brief episode of global warming known as the Paleocene–Eocene thermal maximum (PETM), characterized by an increase in sea surface temperatures of 9–11 degrees F (5–6 degrees C), in conjunction with ocean acidification, a decline in productivity, and a large and abrupt decrease in the proportion of isotopically heavy terrestrial sedimentary carbon in the oceans. The PETM is thought to have lasted only about 170,000 to 220,000 years, with most of the temperature and isotopic change occurring in the first 10,000 to 20,000 years. Its causes remain unclear, but it was probably associated with dissolution of methane hydrates on the ocean floor, which would then have caused greenhouse warming. Possible triggers for this hydrate release include an increase in volcanism, leading to an increase in atmospheric CO2 and consequent sudden initiation of greenhouse warming; a change in ocean circulation; or massive regional submarine slope collapse.

Global temperatures warmed still further during the early Eocene, reaching their highest levels of the past 65 million years during an interval sometimes called the early Eocene climatic optimum (52–50 Ma). Global cooling began during the early middle Eocene (ca. 50 Ma) and accelerated rapidly across the Eocene–Oligocene boundary (ca. 34 Ma), at which time Antarctic continental glaciation began. This shift is frequently referred to as a change from a greenhouse to an icehouse climate regime, and it is one of the most fundamental reorganizations of global climate known in the geological record.

Initiation of Antarctic glaciation has long been attributed to the tectonic opening of Southern Ocean gateways, especially the Drake Passage between South America and the Antarctic Peninsula, which allowed establishment of the Antarctic Circumpolar Current and the consequent isolation of the southern continent from warmer low-latitude waters. This has been questioned recently, however, as a result of the redating of the formation of these gateways, as well as modeling results that point to a greater role for reduced atmospheric CO2.

Equatorial Upwelling

Equatorial Upwelling (UE) is upward water’s motion in the upper layer of the equatorial ocean. It occurs when a persistent easterly wind is blowing over the equatorial zone. Maximum upward velocity in the UE occurs just at the equator. The EU is a result of a permanent divergence of a westward surface South Equatorial Current in the narrow equator vicinity forced by the southeast trade wind. Divergence of the westward current at the equator is caused by the change of the sign of the Coriolis force between the Northern and Southern Hemispheres. As a consequence of divergence, the upper thermocline becomes shallower at the equator. Strong permanent equatorial divergence also causes an intense entrainment of more cold water of thermocline into the upper mixed layer because associated vertical velocity in the equatorial thermocline is typically ~10-5 m per sec. This leads to cooling of the upper mixed layer. As a result, the sea surface temperature is about 1.8 degrees F (1 degree C) lower in the vicinity of the equator than in the interior equatorial ocean outside of it.

Location of Pure Equatorial Upwelling

Pure UE occurs in the narrow vicinity of the equator, just within the divergent zone. Because of the slope of equatorial thermocline in a zonal direction (the thermocline is deeper in the western equatorial Atlantic and Pacific oceans than in the eastern) and the generation of coastal upwelling in the eastern equatorial oceans, UE manifestation, as relatively cold surface water, is more pronounced just in the upper layer of the eastern equatorial oceans. Therefore, such cooler sea-surface water looks like a long, thin tongue along the equator, spreading from the eastern equatorial oceans. There is also high biological activity in the vicinity of this relatively cold tongue.

The thickness of the UE is restricted by the upper boundary of equatorial undercurrent because the eastward current is accompanied by equatorial convergence and, hence, downward water motion. That is why this thickness varies from about 330–660 ft. (100–200 m) (in the western equatorial Atlantic or Pacific oceans, respectively) to 33–66 ft. (10–20 m) (in the eastern equatorial Atlantic and Pacific oceans, respectively). The UE is quite a persistent phenomenon in the Atlantic and Pacific oceans because the westward surface South Equatorial Current occurs there in the equator’s vicinity almost throughout the entire year. However, the UE intensity varies from season to season and from year to year. Seasonally, it is at a maximum in the equatorial Atlantic and Pacific when the South Equatorial Current intensifies, following a seasonal cycle of the southeast trade wind (with some delay, which does not typically exceed a month); that is, in boreal late summer to early fall. Interannual variations of UE are mostly due to the El Niño/La Niño phenomena, especially in the Pacific Ocean. Just before the development of an El Niño event (the anomalous warming of the upper layer in the equatorial Pacific), the southeast trade wind dramatically weakens and UE is over.

In contrast, during a La Niño event (a cold episode in the equatorial Pacific Ocean), UE is strongly developed as a result of anomalous intensification of the southeast trade wind, and hence the South Equatorial Current. Interannual variability of UE in the equatorial Atlantic follows to Pacific variability with some delay, which is typically not more than a few months. However, the magnitude of interannual UE variations in the Atlantic Ocean is not as large as in the Pacific Ocean. A seasonal cycle prevails in the equatorial Atlantic, where the magnitude of seasonal UE variations is two to three times larger than interannual variations.

In the Indian Ocean, UE (as a persistent phenomenon) occurs only in boreal winter, when the northeast monsoon has been developing. The UE is most pronounced in the western part of this basin. Seasonal UE variability is at maximum just in the Indian Ocean. Interannual UE variability in the Indian Ocean is controlled by Indo-Ocean Dipole, which is the inherent Indo-Ocean mode interrelated with the Pacific interannual variability (the El Niño/La Niño phenomena), as can be seen in the 2007 results from researchers Swadhin Behera, Toshio Yamagata, Alexander Polonsky, and colleagues. Low-frequency (decade-to-decade) variability of the southeast trade wind and/or northeast monsoon would generate quasi-equilibrium Upwelling Equatorial variations. A more (less) intense southeast trade wind and northeast monsoon would lead to more (less) intense Upwelling Equatorial.

Coastal Upwelling

Coastal upwelling occurs when water along a coastline flows offshore and deeper water—usually relatively cool, rich in nutrients, and high in partial pressure of carbon dioxide—flows upward to fill its place. Upwelling areas are notable for their effect on carbon cycling, as upwelling not only brings dissolved inorganic carbon to the surface, where it is released into the atmosphere, but also stimulates phytoplankton blooms that further remove some of that carbon through photosynthesis; a small percentage of this bloom also sinks in the form of organic matter (organic carbon) to deep water and becomes buried in sediment, creating a long-term carbon sink. There is considerable interest among carbon-cycle scientists regarding the reciprocal interactions between upwelling systems and climate change. Although such upwelling can in principle occur along any coastline, marine or freshwater, some marine coastlines (e.g., Peru, the western United States, northwest Africa, and southwest Africa) are renowned for their annual upwelling events that are the source of major blooms of diatoms and dinoflagellates, which become the base for extensive marine food webs and coastal fishing industries.

Influence on Carbon Cycling

In the past several decades, major research programs have developed around the influence of coastal upwelling ecosystems on ocean carbon cycling and atmospheric carbon dioxide, how natural climate change (such as glacial–interglacial cycles) has affected coastal upwelling and associated biological productivity over a range of time scales, and how human-induced climate change is affecting coastal upwelling rates and timing and the associated fisheries.

Carbon dioxide exchange between coastal surface water and the atmosphere varies considerably in time and space. Because the pattern is complicated and dynamic relative to the number of direct measurements, considerable uncertainty lingers regarding the net carbon flux through the system over the course of a year. In general, outgassing occurs near the coastline, where upwelled water outcrops at the surface. This water is often rich in carbon dioxide arising from the respiration of organisms ingesting organic matter that sank from the surface to deeper water (which may be the sea bottom along the continental shelf). As upwelled water moves from shore, phytoplankton bloom in response to dissolved nitrogen, phosphorus, and other nutrients and begin to use up some of the dissolved inorganic carbon, reducing the partial pressure of carbon dioxide.

Because this process occurs over a period of several weeks, the rate of uptake of dissolved inorganic carbon also changes through time, so that net outgassing will occur early in an upwelling event, gradually changing to net ingassing. Much of the phytoplankton is recycled in the surface layer, prolonging the bloom, but some of the nutrients and carbon escape the system through the fecal material of heterotrophs feeding on the phytoplankton. The nutrients of remineralized organic matter that sink may come to the surface in future years through upwelling, or the organic matter may sink below the depth of upwelled water into the deep sea, or get buried in sediment.

The latter two processes can take carbon out of the atmosphere for thousands or millions of years, respectively. Although these processes occur in other aquatic areas, enough of the global ocean carbon flux in a given year occurs through coastal upwelling zones to affect atmospheric carbon dioxide. The strength and direction of surface winds that drive coastal upwelling vary over a broad

spectrum of time scales. Changes in global heat retention through time affect the potential for temperature gradients that influence wind speed, and the distribution of land masses and topographic

features such as mountains affect coastal shape, coastal currents, sea level and coastal profile, and atmospheric circulation patterns. 

Temperature and precipitation patterns and sea level, among other variables, affect nutrient distribution in the oceans. All of these affect upwelling strength, biological productivity, carbon burial, and net effect on the global carbon cycle. Much research has been dedicated to understanding upwelling changes during glacial–interglacial cycles, tracking responses to changed wind speeds and to lowered sea level, and therefore steeper coastal profiles. Other research has examined how to predict occurrences of upwelling in, for example, the Mesozoic, under the assumption that upwelling is responsible for the accumulation of some petroleum deposits.

Both models and empirical observations of several coastal upwelling areas, such as off the coast of California and northwest Africa, suggest that atmospheric warming is leading to greater rates of upwelling. This increase is driven by a greater land–ocean temperature gradient and therefore greater wind speeds. This can lead both to greater outgassing of carbon dioxide (if not balanced by increased productivity) and loss of certain fish that cannot maintain their population position because of higher offshore current velocities.

Vostok Ice Core

Russia’s Vostok station is located in east Antarctica. The Vostok station holds the record for the lowest temperature ever recorded at minus 129 degrees F (minus 89 degrees C). Soviet researchers began deep drilling at the Vostok Station in 1980. The ice cores brought to the surface in segments provide information (chemistry, structure, and inclusions) about climate conditions, similar to tree ring samples. 

The information from air bubbles allows for measurement of the atmospheric concentration of greenhouse gases (e.g., carbon dioxide, methane, nitrogen, helium, sodium, and organic carbon). Besides presenting an extraordinary human effort, spanning two decades in one of the most inhospitable places on Earth, the drilling at Vostok has produced one of the richest scientific treasure troves of all time. Previously, analysis revealed tracking between carbon dioxide and temperature, and that the magnitude of carbon dioxide swings could account for the magnitude of temperature swings. 

Scientific Treasure Trove

The first hole drilling stopped in 1985 because of problems. A second hole drilled with French- Russian cooperation produced an ice core 2,083 m long, or 1.33 mi. With a climate record of 160,000 years, drilling on this hole ended in 1990. A third hole was drilled, with collaboration among Russia, France, and the United States. 

The drilling reached a depth of 2.25 mi. (3.6 km) and in January 1998 produced the deepest ice core recovered at the time (now exceeded by the European Project for Ice Coring in Antarctica)— 11,886 ft. (3,623 m) deep, containing a climate record of 420,000 years, for a total of four climate cycles. Drilling stopped at this depth because the researchers were recovering accretion ice refrozen to the bottom of the glacier, indicating the presence of an underlying lake, and did not want to put themselves in danger from the release of pressurized lake water or risking contamination of the lake. Researchers have found microbes in the glacial ice from the Vostok core and four times more in the glacial-accretion ice transition, suggesting that the underground lake contains microbes and organic carbon. 

Polar snowfall can be preserved in annual layers within an ice sheet to provide a climate record. These layers can be studied to develop an accurate picture of the climate history, extending over long time periods (the deepest Vostok core extends over a 400,000-year time frame). Impurities (volcanic debris, sea salt, organic material, and interstellar particles) are also deposited with snow, making those layers distinctive.

Air bubbles trap gases in the ice and allow for testing to determine the air’s composition at distinctive periods in the climate record. Water pockets may also become trapped the deeper the ice core is, and closer to the underlying rock or water. Researchers can determine the composition of water in comparison with heavy water isotopes to indicate environmental temperature; cold periods are those with moisture removed from the atmosphere.

Studies on the second Vostok core showed a correlation between carbon dioxide and temperature over the past 160,000 years, and provided evidence linking climate change with the greenhouse effect. The trapped air bubbles provided gas isotopes, and when compared with the temperature variations, matched up to show that greenhouse gases were the primary driver of climate change over time. The climate variation of ice ages also matched solar records.

Initial Vostok studies, when combined with later research, provide an inclusive representation of the multiple factors involved in climate change by using a multidisciplinary approach to climate change research, using astronomical tables, chemistry, and physics. The Vostok ice cores indicate periods of ice ages, contain gases for comparison with temperature changes, and highlight the last ice age of 8 degrees F (4.4 degrees C) cooler than the present, taking place about 18,000 years ago.

Vostok’s cores have provided significant evidence of greenhouse gas variations driving climate change and have provided information for the modeling of future climate changes in relation to greenhouse gas concentrations.The third Vostok core, recovered in 1998, provides additional confirmation and extends the historical record through the four most recent glacial cycles, showing that increased concentrations of greenhouse gases have forced the temperature higher and can be compared with the geological record of the same time frames.

The Vostok ice core has become the standard for creating timescales from cores recovered from other parts of the world. Researchers are able to plot the isotopes of their samples with similar isotope ratios of the known sample to provide an accurate time period reference.

Hurricane Beta

Caribbean Sea–Central America, October 26–30, 2005 

A record-setting tropical cyclone of many degrees, Hurricane Beta was the first North Atlantic tropical system to be given a “B” identifier from the Greek alphabet. It was the 23rd named tropical system—and the 14th mature-stage hurricane—to develop during the historic 2005 North Atlantic hurricane season. It also, on October 30, 2005, became one of a handful of tropical systems to make a direct landfall on the eastern coast of Nicaragua. A late-season tropical cyclone, Beta was born on October 27, 2005, from a tropical depression that had lingered in the southwestern Caribbean Sea for several days before intensifying.

Moving to the north, and then abruptly to the northeast, the system now a tropical storm with a central pressure of 29.20 inches (989 mb) deepened into a Category 1 hurricane on October 29, while still located less than 100 miles (161 km) off Central America’s famed “Mosquito Coast.” Between the mid-morning hours of August 29 and the early morning hours of August 30, Beta’s central barometric pressure slipped from 29.14 inches (987 mb) to 28.34 inches (960 mb), boasting its sustained wind speeds from 75 MPH (121 km/h) to 115 MPH (185 km/h) in just over a 24-hour period. On October 30, as Beta’s gusts were clocked at nearly 140 MPH (225 km/h), it became the seventh major hurricane to develop during the 2005 hurricane season. Fortunately for those interests along the Costa Rican-Nicaraguan coastlines, Beta began to weaken as it turned due west, and then southwestward, and was of powerful Category 2 intensity as it came ashore in central Nicaragua, near the small coastal town of Sandy Bay, on October 30, 2005. 

A central pressure at landfall of 28.49 inches (965 mb) produced sustained wind speeds of 109 MPH (175 km/h), which uprooted trees, sank small watercraft, and caused extensive structural damage to small buildings and harbor facilities. A slow-moving hurricane, Beta’s heavy rains caused several flash flood conditions in Nicaragua and Honduras. Dozens of people were injured on the offshore island of Providencia, while several injuries in Honduras and Nicaragua were also reported. Quickly downgraded to a tropical storm as it moved inland over Nicaragua, by October 30 Beta had dissipated. While Hurricane Beta was a powerful hurricane at landfall, local emergency management authorities attributed the lack of deaths to a combination of preparedness measures, most important among them the many evacuations that preceded the storm’s landfall, and the fact that the system came ashore in a sparsely inhabited section of the coastline.

Because the identifier Beta is only used when the standard A-W naming list for a particular North Atlantic season is exhausted, it remains in use on the Greek alphabet tropical cyclone naming list.

Bermuda High

The name given to the high-pressure anticyclone that dominates wind patterns over the North Atlantic Ocean. Influenced by the progression of the seasons, the clockwise-spinning Bermuda High serves to determine the tracks or trajectories of Atlantic hurricanes as they move westward across the ocean’s subtropical reaches. During the winter months when air and water temperatures in the Northern Hemisphere are low, the Bermuda High is relatively small and is positioned in the ocean’s southeast quadrant. But during the summer months of July through September when water temperatures in the region are much warmer, the Bermuda High strengthens considerably, growing to encompass the entire center of the North Atlantic. In this position it influences a host of meteorological factors, including the steering currents of hurricanes. Because the Bermuda High is an area of settled high-pressure,low-pressure tropical cyclones cannot encroach upon it. 

Instead, they must either progress along its southernmost flank and undergo recurvature as they round the anticyclone’s western edge, or else move up from the Caribbean Sea and continue with the northeasterly curl until it guides them into the North Atlantic’s cooling spaces. If during a particular season the Bermuda High is of notably large size and is situated farther to the west than is normal, that year’s crop of hurricanes will be more likely to make landfall in the Gulf of Mexico or the United States’s eastern shores. Conversely, if the Bermuda High is less intense than usual, a number of Atlantic hurricanes can find their way to the west coast of Europe. Such an occurrence was seen during the 1966 hurricane season, when Hurricane Faith brought 100-MPH (161-km/h) winds to Norway, and during the 1987 season, when tropical storm Arlene delivered flooding rains to Portugal.

Tropical Storm Bertha

Storm North Atlantic Ocean, August 30–September 4, 1984 On August 30, 1984, 

Tropical Storm Bertha formed over the southeastern North Atlantic Ocean. Like most tropical cyclones that originate south of 10 degrees North, the tropical depression that would eventually be upgraded and named Tropical Storm Bertha experienced some initial difficulty in organizing its complex circulation system. Meteorologists who have studied this phenomenon attribute it to the diminishing intensity of the Coriolis effect, which diminishes the closer an object is to the equator. Bertha, which steadily moved to the northwest and away from the equator, did not intensify until it reached 15 degrees North, at which point it was upgraded to a tropical storm. At its peak, Bertha generated a central barometric pressure of 29.73 inches (1,007 mb) and sustained winds of 40 MPH (64 km/h), making it a very weak system. Wind shear and cooler seasurface temperatures again downgraded Tropical Storm Bertha to tropical depression intensity, and by September 4, 1984, Bertha had dissipated over the mid-North Atlantic Ocean.

Tropical Storm Bertha

Storm North Atlantic Ocean, August 30–September 4, 1984 On August 30, 1984, 

Tropical Storm Bertha formed over the southeastern North Atlantic Ocean. Like most tropical cyclones that originate south of 10 degrees North, the tropical depression that would eventually be upgraded and named Tropical Storm Bertha experienced some initial difficulty in organizing its complex circulation system. Meteorologists who have studied this phenomenon attribute it to the diminishing intensity of the Coriolis effect, which diminishes the closer an object is to the equator. Bertha, which steadily moved to the northwest and away from the equator, did not intensify until it reached 15 degrees North, at which point it was upgraded to a tropical storm. At its peak, Bertha generated a central barometric pressure of 29.73 inches (1,007 mb) and sustained winds of 40 MPH (64 km/h), making it a very weak system. Wind shear and cooler seasurface temperatures again downgraded Tropical Storm Bertha to tropical depression intensity, and by September 4, 1984, Bertha had dissipated over the mid-North Atlantic Ocean.

Tropical Storm Beryl

North Atlantic Ocean–Northeastern United States, July 18–20, 2006 

The fourth North Atlantic tropical cyclone identified as Beryl originated off the coast of North Carolina on the morning of July 18, 2006. Initially dubbed Tropical Depression No. 2 (TD 2), the system slowly moved to the north-northeast, prompting the posting of tropical storm watches for the North Carolina coastline. While these watches were later dropped, the system continued to intensify, becoming Tropical Storm Beryl during the early morning hours of July 19, and steadily strengthening thereafter. 

As of 11:00 p.m. EDT on July 19, Beryl was a powerful tropical storm, with a central barometric pressure of 29.58 inches (1,002 mb) and sustained wind speeds of 60 MPH (95 km/h). As Beryl drew away from Carolina’s Outer Banks and headed to the northnortheast at nine MPH (15 km/h), tropical storm watches were posted for southeastern Massachusetts, including Nantucket Island and Martha’s Vineyard. The name Beryl is scheduled to reappear during the 2012 North Atlantic hurricane season.

Precipitation

All forms of water that fall from clouds to the ground are precipitation. Rain, snow, sleet, and hail are the four main types of precipitation. Clouds contain water droplets that are so small that the upward movement of air in the cloud can keep the droplets from falling. In order for these droplets to become heavy enough to fall, their size must increase by 50 to 100 times.

Coalescence One way that cloud droplets can increase in size is by coalescence. In a warm cloud, coalescence is the primary process responsible for the formation of precipitation. Coalescence (koh uh LEH sunts)occurs when cloud droplets collide and join together to form a larger droplet. These collisions occur as larger droplets fall and collide with smaller droplets. As the process continues, the droplets eventually become too heavy to remain suspended in the cloud and fall to Earth as precipitation. Rain is precipitation that reaches Earth’s surface as a liquid. Raindrops typically have diameters between 0.5 mm and 5 mm.

Snow, sleet, and hail The type of precipitation that reaches Earth depends on the vertical variation of temperature in the atmosphere. In cold clouds where the air temperature is far below freezing, ice crystals can form that finally fall to the ground as snow. Sometimes, even if ice crystals form in a cloud, they can reach the ground as rain if they fall through air warmer than 0°C and melt.

In some cases, air currents in a cloud can cause cloud droplets to move up and down through freezing and nonfreezing air, forming ice pellets that fall to the ground as sleet. Sleet can also occur when raindrops freeze as they fall through freezing air near the surface. If the up-and-down motion in a cloud is especially strong and occurs over large stretches of the atmosphere, large ice pellets known as hail can form. Most hailstones are smaller in diameter than a dime, but some stones have been found to weigh more than 0.5 kg. Larger stones are often produced during severe thunderstorms.

The water cycle More than 97 percent of Earth’s water is in the oceans. At any one time, only a small percentage of water is present in the atmosphere. Still, this water is vitally important because, as it continually moves between the atmosphere and Earth’s surface, it nourishes living things. The constant movement of water between the atmosphere and Earth’s surface is known as the water cycle.

Radiation from the Sun causes liquid water to evaporate. Water evaporates from lakes, streams, and oceans and rises into Earth’s atmosphere. As water vapor rises, it cools and condenses to form clouds. Water droplets combine to form larger drops that fall to Earth as precipitation. This water soaks into the ground and enters lakes, streams, and oceans, or it falls directly into bodies of water and eventually evaporates, continuing the water cycle.

Types of Clouds

You have probably noticed that clouds have different shapes. Some clouds look like puffy cotton balls, while others have a thin, feathery appearance. These differences in cloud shape are due to differences in the processes that cause clouds to form. Cloud formation can also take place at different altitudes — sometimes even right at Earth’s surface, in which case the cloud is known as fog.

Clouds are generally classified according to a system developed in 1803, and only minor changes have been made since it was first introduced. Figure 11.20 shows the different types of clouds. This system classifies clouds by the altitudes at which they form and by their shapes. There are four classes of clouds based on the altitudes at which they form: low, middle, and high. In addition, there are clouds with vertical development. Low clouds typically form below 2000 m. Middle clouds form mainly between 2000 m and 6000 m. High clouds form above 6000 m. Unlike the other three classes of clouds, those with vertical development can form at all altitudes.

• Low clouds Clouds can form when warm, moist air rises, expands, and cools. If conditions are stable, the air mass stops rising at the altitude where its temperature is the same as that of the surrounding air. If a cloud has formed, it will flatten out and winds will spread it horizontally into stratocumulus or layered cumulus clouds.Cumulus (KYEW myuh lus) clouds are puffy, lumpy-looking clouds that usually occur below 2000 m. Another type of cloud that forms at heights below 2000 m is a stratus (STRAY tus), a layered sheetlike cloud that covers much or all of the sky in a given area. Stratus clouds often form when fog lifts away from Earth’s surface.

• Middle clouds Altocumulus and altostratus clouds form at altitudes between 2000 m and 6000 m. They are made up of ice crystals and water droplets due to the colder temperatures generally present at these altitudes. Middle clouds are usually layered. Altocumulus clouds are white or gray in color and form large, round masses or wavy rows. Altostratus clouds have a gray appearance, and they form thin sheets of clouds. Middle clouds sometimes produce mild precipitation.

• High clouds High clouds, made up of ice crystals, form at heights of 6000 m where temperatures are below freezing. Some, such as cirrus (SIHR us) clouds, often have a wispy, indistinct appearance. Another type of cirrus cloud, called a cirrostratus, forms as a continuous layer that can cover the sky. Cirrostratus clouds vary in thickness from almost transparent to dense enough to block out the Sun or the Moon.

• Vertical development clouds If the air that makes up a cumulus cloud is unstable, the cloud will be warmer than the surface or surrounding air and will continue to grow upward. As it rises, water vapor condenses, and the air continues to increase in temperature due to the release of latent heat. The cloud can grow through middle altitudes as a towering cumulonimbus and, if conditions are right, it can reach nearly 18,000 m. Its top is then composed of ice crystals. Strong winds can spread the top of the cloud into an anvil shape. What began as a small mass of unstable moist air is now an atmospheric giant, capable of producing the torrential rains, strong winds, and hail characteristic of some thunderstorms.

Cloud Formation

A cloud can form when a rising air mass cools. Recall that Earth’s surface heats and cools by different amounts in different places. This uneven heating and cooling of the surface causes air masses near the surface to warm and cool. As an air mass is heated, it becomes less dense than the cooler air around it. This causes the warmer air mass to be pushed upward by the denser, cooler air.

However, as the warm air mass rises, it expands and cools adiabatically. The cooling of an air mass as it rises can cause water vapor in the air mass to condense. Recall that the lifted condensation level is the height at which condensation of water vapor occurs in an air mass. When a rising air mass reaches the lifted condensation level, water vapor condenses around condensation nuclei. A condensation nucleus is a small particle in the atmosphere around which water droplets can form.

These particles are usually less than about 0.001 mm in diameter and can be made of ice, salt, dust, and other materials. The droplets that form can be liquid water or ice, depending on the surrounding temperature. When the number of these droplets is large enough, a cloud is visible.

Atmospheric stability As an air mass rises, it cools. However, the air mass will continue to rise as long as it is warmer than the surrounding air. Under some conditions, an air mass that has started to rise sinks back to its original position. When this happens, the air is considered stable because it resists rising. The stability of air masses determines the type of clouds that form and the associated weather patterns.

Stable air The stability of an air mass depends on how the temperature of the air mass changes relative to the atmosphere. The air temperature near Earth’s surface decreases with altitude. As a result, the atmosphere becomes cooler as the air mass rises. At the same time, the rising air mass is also becoming cooler. Suppose that the temperature of the atmosphere decreases more slowly with increasing altitude than does the temperature of the rising air mass. Then the rising air mass will cool more quickly than the atmosphere. The air mass will finally reach an altitude at which it is colder than the atmosphere. It will then sink back to the altitude at which its density is the same as the atmosphere. Because the air mass stops rising and sinks downward, it is stable. Fair weather clouds form under stable conditions. 

Unstable air Suppose that the temperature of the surrounding air cools faster than the temperature of the rising air mass. Then the air mass will always be less dense than the surrounding air. As a result, the air mass will continue to rise. The atmosphere is then considered to be unstable. Unstable conditions can produce the type of clouds associated with thunderstorms.

Atmospheric lifting Clouds can form when moist air rises, expands, and cools. Air rises when it is heated and becomes warmer than the surrounding air. This process is known as con vective lifting. Clouds can also form when air is forced upward or lifted by mechanical processes. Two of these processes are orographic lifting and convergence.

Orographic lifting Clouds can form when air is forced to rise over elevated land or other topographic barriers. This can happen, for example, when an air mass approaches a mountain range. Orographic lifting occurs when an air mass is forced to rise over a topographic barrier. The rising air mass expands and cools, with water droplets condensing when the temperature falls below the dew point. Many of the rainiest places on Earth are located on the windward sides of mountain slopes, such as the coastal side of the Sierra Nevadas. The formation of clouds and the resulting heavy precipitation along the west coast of Canada are also primarily due to orographic lifting.

Convergence Air can be lifted by convergence, which occurs when air flows into the same area from different directions. Then some of the air is forced upward. This process is even more pronounced when air masses at different temperatures collide. When a warm air mass and a cooler air mass collide, the warmer, less-dense air is forced upward over the denser, cooler air. As the warm air rises, it cools adiabatically. If the rising air cools to the dew-point temperature, then water vapor can condense on condensation nuclei and form a cloud. This cloud formation mechanism is common at middle latitudes where severe storm systems form along the cold polar front. Convergence also occurs near the equator where the trade winds meet at the intertropical convergence zone. 

Humidity

The distribution and movement of water vapor in the atmosphere play an important role in determining the weather of any region. Humidity is the amount of water vapor in the atmosphere at a given location on Earth’s surface. Two ways of expressing the water vapor content of the atmosphere are relative humidity and dew point. 

Relative humidity Consider a flask containing water. Some water molecules evaporate, leaving the liquid and becoming part of the water vapor in the flask. At the same time, other water molecules condense, returning from the vapor to become part of the liquid. Just as the amount of water vapor in the flask might vary, so does the amount of water vapor in the atmosphere. Water on Earth’s surface evaporates and enters the atmosphere and condenses to form clouds and precipitation.

In the example of the flask, if the rate of evaporation is greater than the rate of condensation, the amount of water vapor in the flask increases. Saturation occurs when the amount of water vapor in a volume of air has reached the maximum amount. 

The amount of water vapor in a volume of air relative to the amount of water vapor needed for that volume of air to reach saturation is called relative humidity. Relative humidity is expressed as a percentage. When a certain volume of air is saturated, its relative humidity is 100 percent. If you hear a weather forecaster say that the relative humidity is 50 percent, it means that the air contains 50 percent of the water vapor needed for the air to be saturated.

Dew point Another common way of describing the moisture content of air is the dew point. The dew point is the temperature to which air must be cooled at constant pressure to reach saturation. The dew point is often called the condensation temperature because it is the temperature at which water vapor in air condenses into water called dew. If the dew point is nearly the same as the air temperature, then the relative humidity is high.

Latent heat As water vapor in the air condenses, thermal energy is released. Where does this energy

come from? To change liquid water to water vapor, thermal energy is added to the water by heating it. The water vapor then contains more thermal energy than the liquid water. This is the energy that is released when condensation occurs. The extra thermal energy contained in water vapor compared to liquid water is called latent heat.

Energy Transfer in the Atmosphere

All materials are made of particles, such as atoms and molecules. These particles are always moving, even if the object is not moving. The particles move in all directions with various speeds — a type of motion called random motion. A moving object has a form of energy called kinetic energy. As a result, the particles moving in random motion have kinetic energy. The total energy of the particles in an object due to their random motion is called thermal energy.

Heat is the transfer of thermal energy from a region of higher temperature to a region of lower temperature. In the atmosphere, thermal energy can be transferred by radiation, conduction, and convection.

Radiation Light from the Sun heats some portions of Earth’s surface at all times, just as the heat lamp uses the process of radiation to warm food. Radiation is the transfer of thermal energy by electromagnetic waves. The heat lamp emits visible light and infrared waves that travel from the lamp and are absorbed by the food. The thermal energy carried by these waves causes the temperature of the food to increase. In the same way, thermal energy is transferred from the Sun to Earth by radiation. The solar energy that reaches Earth is absorbed and reflected by Earth’s atmosphere and Earth’s surface. 

Absorption and reflection Most of the solar energy that reaches Earth is in the form of visible light waves and infrared waves. Almost all of the visible light waves pass through the atmosphere and strike Earth’s surface. Most of these waves are absorbed by Earth’s surface. As the surface absorbs these visible light waves, it also emits infrared waves. The atmosphere absorbs some infrared waves from the Sun and emits infrared waves with different wavelengths.

About 30 percent of solar radiation is reflected into space by Earth’s surface, the atmosphere, or clouds. Another 20 percent is absorbed by the atmosphere and clouds. About 50 percent of solar radiation is absorbed directly or indirectly by Earth’s surface and keeps Earth’s surface warm.

Rate of absorption The rate of absorption for any particular area varies depending on the physical characteristics of the area and the amount of solar radiation it receives. Different areas absorb energy and heat at different rates. For example, water heats and cools more slowly than land. Also, as a general rule, darker objects absorb energy faster than light-colored objects. For instance, a black asphalt driveway heats faster on a sunny day than a light-colored concrete driveway.

Conduction Another process of energy transfer can occur when two objects at different temperatures are in contact. Conduction is the transfer of thermal energy between objects when their atoms or molecules collide. Conduction can occur more easily in solids and liquids, where particles are close together, than in gases, where particles are farther apart. Because air is a mixture of gases, it is a poor conductor of thermal energy. In the atmosphere, conduction occurs between Earth’s surface and the lowest part of the atmosphere.

Convection Throughout much of the atmosphere, thermal energy is transferred by a process called convection. The process of convection occurs mainly in liquids and gases. Convection is the transfer of thermal energy by the movement of heated material from one place to another. As water at the bottom of the pan is heated, it expands and becomes less dense than the water around it. Because it is less dense, it is forced upward. As it rises, it transfers thermal energy to the cooler water around it, and cools. It then becomes denser than the water around it and sinks to the bottom of the pan, where it is reheated. 

A similar process occurs in the atmosphere. Parcels of air near Earth’s surface are heated, become less dense than the surrounding air, and rise. As the warm air rises, it cools and its density increases. When it cools below the temperature of the surrounding air, the air parcel becomes denser than the air around it and sinks. As it sinks, it warms again, and the process repeats. Convection currents, as these movements of air are called, are the main mechanism for energy transfer in the atmosphere.

Atmospheric Layers

The atmosphere is classified into five different layers,These layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each layer differs in composition and temperature profile.

• Troposphere The layer closest to Earth’s surface, the troposphere, contains most of the mass of the atmosphere. Weather occurs in the troposphere. In the troposphere, air temperature decreases as altitude increases. The altitude at which the temperature stops decreasing is called the tropopause. The height of the tropopause varies from about 16 km above Earth’s surface in the tropics to about 9 km above it at the poles. Temperatures at the tropopause can be as low as –60°C.

• Stratosphere Above the tropopause is the stratosphere, a layer in which the air temperature mainly increases with altitude and contains the ozone layer. In the lower stratosphere below the ozone layer, the temperature stays constant with altitude. However, starting at the bottom of the ozone layer, the temperature in the stratosphere increases as altitude increases. This heating is caused by ozone molecules, which absorb ultraviolet radiation from the Sun. At the stratopause, air temperature stops increasing with altitude. The stratopause is about 48 km above Earth’s surface. About 99.9 percent of the mass of Earth’s atmosphere is below the stratopause.

• Mesosphere Above the stratopause is the mesosphere, which is about 50 km to 100 km above Earth’s surface. In the mesosphere, air temperature decreases with altitude. This temperature decrease occurs because very little solar radiation is absorbed in this layer. The top of the mesosphere, where temperatures stop decreasing with altitude, is called the mesopause.

• Thermosphere The thermosphere is the layer between about 100 km and 500 km above Earth’s surface. In this layer, the extremely low density of air causes the temperature to rise. Temperatures in this layer can be more than 1000°C. The ionosphere, which is made of electrically charged particles, is part of the thermosphere.

• Exosphere The exosphere is the outermost layer of Earth’s atmosphere. The exosphere extends from about 500 km to more than 10,000 km above Earth’s surface. There is no clear boundary at the top of the exosphere. Instead, the exosphere can be thought of as the transitional region between Earth’s atmosphere and outer space. The number of atoms and molecules in the exosphere becomes very small as altitude increases.

In the exosphere, atoms and molecules are so far apart that they rarely collide with each other. In this layer, some atoms and molecules are moving fast enough that they are able to escape into outer space.

Atmospheric Composition

The ancient Greeks thought that air was one of the four fundamental elements from which all other substances were made. In fact, air is a combination of gases, such as nitrogen and oxygen, and particles, such as dust, water droplets, and ice crystals. These gases and particles form Earth’s atmosphere, which surrounds Earth and extends from Earth’s surface to outer space.

Permanent atmospheric gases About 99 percent of the atmosphere is composed of nitrogen (N2) and oxygen (O2). The remaining 1 percent consists of argon (Ar), carbon dioxide (CO2), water vapor (H2O), and other trace gases, The amounts of nitrogen and oxygen in the atmosphere are fairly constant over recent time. However, over Earth’s history, the composition of the atmosphere has changed greatly. For example, Earth’s early atmosphere probably contained mostly helium (He), hydrogen (H2), methane (CH4), and ammonia (NH3). Today, oxygen and nitrogen are continually being recycled between the atmosphere, living organisms, the oceans, and Earth’s crust.

Variable atmospheric gases The concentrations of some atmospheric gases are not as constant over time as the concentrations of nitrogen and oxygen. Gases such as water vapor and ozone (O3) can vary significantly from place to place. The concentrations of some of these gases, such as water vapor and carbon dioxide, play an important role in regulating the amount of energy the atmosphere absorbs and emits back to Earth’s surface.

Water vapor Water vapor is the invisible, gaseous form of water. The amount of water vapor in the atmosphere can vary greatly over time and from one place to another. At a given place and time, the concentration of water vapor can be as much as 4 percent or as little as nearly zero. The concentration varies with the seasons, with the altitude of a particular mass of air, and with the properties of the surface beneath the air. Air over deserts, for instance, contains much less water vapor than the air over oceans.

Carbon dioxide Carbon dioxide, another variable gas, currently makes up about 0.039 percent of the atmosphere. During the past 150 years, measurements have shown that the concentration of atmospheric carbon dioxide has increased from about 0.028 percent to its present value. Carbon dioxide is also cycled between the atmosphere, the oceans, living organisms, and Earth’s rocks.

The recent increase in atmospheric carbon dioxide is due primarily to the burning of fossil fuels, such as oil, coal, and natural gas. These fuels are burned to heat buildings, produce electricity, and power vehicles. Burning fossil fuels can also produce other gases, such as sulfur dioxide and nitrous oxides, that can cause various respiratory illnesses, as well as other environmental problems.

Ozone Molecules of ozone are formed by the addition of an oxygen atom to an oxygen molecule. Most atmospheric ozone is found in the ozone layer, 20 km to 50 km above Earth’s surface,The maximum concentration of ozone in this layer—9.8 × 1012 molecules/cm3—is only about 0.0012 percent of the atmosphere. 

The ozone concentration in the ozone layer varies seasonally at higher latitudes, reaching a minimum in the spring. The greatest seasonal changes occur over Antarctica. During the past several decades, measured ozone levels over Antarctica in the spring have dropped significantly. This decrease is due to the presence of chemicals called chlorofluorocarbons (CFCs) that react with ozone and break it down in the atmosphere.

Atmospheric particles Earth’s atmosphere also contains variable amounts of solids in the form of tiny particles, such as dust, salt, and ice. Fine particles of dust and soil are carried into the atmosphere by wind. Winds also pick up salt particles from ocean spray. Airborne microorganisms, such as fungi and bacteria, can also be found attached to microscopic dust particles in the atmosphere.

Chemical Weathering

Chemical weathering is the process by which rocks and minerals undergo changes in their composition. Agents of chemical weathering include water, oxygen, carbon dioxide, and acid precipitation. The interaction of these agents with rock can cause some substances to dissolve, and some new minerals to form. The new minerals have properties different than those that were in the original rock. For example, iron often combines with oxygen to form iron oxide, such as in hematite.

The composition of a rock determines the effects that chemical weathering will have on it. Some minerals, such as calcite, which is composed of calcium carbonate, can decompose completely in acidic water. Limestone and marble are made almost entirely from calcite, and are therefore greatly affected by chemical weathering. Buildings and monuments made of these rocks usually show signs of wear as a result of chemical weathering. 

Temperature is another significant factor in chemical weathering because it influences the rate at which chemical interactions occur. Chemical reaction rates increase as temperature increases. With all other factors being equal, the rate of chemical weathering reactions doubles with each 10°C increase in temperature. Effect of water Water is an important agent in chemical weathering because it can dissolve many kinds of minerals and rocks. Water also plays an active role in many reactions by serving as a medium in which the reactions can occur. Water can also react directly with minerals in a chemical reaction. In one common reaction with water, large molecules of the mineral break down into smaller molecules. This reaction decomposes and transforms many silicate minerals.

For example, potassium feldspar decomposes into kaolinite, a fine-grained clay mineral common in soils. Effect of oxygen An important element in chemical weathering is oxygen. The chemical reaction of oxygen with other substances is called oxidation. Approximately 21 percent of Earth’s atmosphere is oxygen gas. Iron in rocks and minerals combines with this atmospheric oxygen to form minerals with the oxidized form of iron. A common mineral that contains the oxidized form of iron is hematite. 

Effect of carbon dioxide Another atmospheric gas that contributes to the chemical weathering process is carbon dioxide. Carbon dioxide is a gas that occurs naturally in the atmosphere as a product of living organisms. When carbon dioxide combines with water in the atmosphere, it forms a very weak acid called carbonic acid that falls to Earth’s surface as precipitation. Precipitation includes rain, snow, sleet, and fog. Natural precipitation has a pH of 5.6. The slight acidity of precipitation causes it to dissolve certain rocks, such as limestone. 

Decaying organic matter and respiration produce high levels of carbon dioxide. When slightly acidic water from precipitation seeps into the ground and combines with carbon dioxide in the soil, carbonic acid becomes an agent in the chemical weathering process. Carbonic acid slowly reacts with minerals such as calcite in limestone and marble to dissolve rocks. After many years, limestone caverns can form where the carbonic acid flowed through cracks in limestone rocks and reacted with calcite. Effect of acid precipitation Another agent of chemical weathering is acid precipitation, which is caused by sulfur dioxide and nitrogen oxides that are released into the atmosphere, in large part by human activities. Sulfur dioxide is primarily the product of industrial burning of fossil fuels. Motor-vehicle exhausts also contribute to the emissions of nitrogen oxides. These two gases combine with oxygen and water in the atmosphere, forming sulfuric and nitric acids, which are strong acids.

The acidity of a solution is described using the pH scale, as you learned in Chapter 3. Acid precipitation is precipitation that has a pH value below 5.6—the pH of normal rainfall. Because strong acids can be harmful to many organisms and destructive to humanmade structures, acid precipitation often creates problems. Many plant and animal populations cannot survive even slight changes in acidity. 

Mechanical Weathering

Weathering is the process in which materials on or near Earth’s surface break down and change. Mechanical weathering is a type of weathering in which rocks and minerals break down into smaller pieces. This process is also called physical weathering. Mechanical weathering does not involve any change in a rock’s composition, only changes in the size and shape of the rock. A variety of factors are involved in mechanical weathering, including changes in temperature and pressure.

Effect of temperature Temperature plays a role in mechanical weathering. When water freezes, it expands and increases in volume by 9 percent. You have observed this increase in volume if you have ever frozen water in an ice cube tray. In many places on Earth’s surface, water collects in the cracks of rocks and rock layers. If the temperature drops to the freezing point, water freezes, expands, exerts pressure on the rocks, and can cause the cracks to widen slightly. When the temperature increases, the ice melts in the cracks of rocks and rock layers. The freeze-thaw cycles of water in the cracks of rocks is called frost wedging. Frost wedging is responsible for the formation of potholes in many roads in the northern United States where winter temperatures vary frequently between freezing and thawing.

Effect of pressure Another factor involved in mechanical weathering is pressure. Roots of trees and other plants can exert pressure on rocks when they wedge themselves into the cracks in rocks. As the roots grow and expand, they exert increasing amounts of pressure which often causes the rocks to split.

On a much larger scale, pressure also functions within Earth. Bedrock at great depths is under tremendous pressure from the overlying rock layers. A large mass of rock, such as a batholith, may originally form under great pressure from the weight of several kilometers of rock above it. When the overlying rock layers are removed by processes such as erosion or even mining, the pressure on the bedrock is reduced. The bedrock surface that was buried expands, and long, curved cracks can form. 

These cracks, also known as joints, occur parallel to the surface of the rocks. Reduction of pressure also allows existing cracks in the bedrock to widen. For example, when several layers of overlying rocks are removed from a deep mine, the sudden decrease of pressure can cause large pieces of rocks to explode off the walls of the mine tunnels.

Over time, the outer layers of rock can be stripped away in succession, similar to the way an onion’s layers can be peeled. The process by which outer rock layers are stripped away is called exfoliation. Exfoliation often results in dome-shaped formations, such as Moxham Mountain in New York and Half Dome in Yosemite National Park in California.

Volcanism

Members of the scientific community largely concur that the Earth is undergoing a change in climate and that global warming is occurring at an increasing rate. This acceleration in the late 20th century is mainly a result of carbon dioxide (CO2) emissions generated by human activity. Carbon dioxide acts like a glass barrier over the Earth, preventing heat from leaking into the environment and thus creating a greenhouse effect. In its latest report, the United Nations Intergovernmental Panel on Climate Change (IPCC) shows that greenhouse gases are an integral factor in global warming, more so than natural causes such as solar activity and volcanoes.

Influence on Weather

Evidence suggests that volcanism, which refers to phenomena such as the outward flow of pyroclastic materials and the upsurge of gas and steam connected to the movement of molten rock, can influence short-term weather and may have an effect on long-term climate change. Similar to human activity, volcanism leads to both global warming and cooling. The effect of volcanism on climate depends on the interaction between the sun’s heat and the volcanic debris. Scientists believe that ongoing volcanic eruptions have maintained the Earth’s temperate climate for millions of years and are responsible for the gases in today’s atmosphere. Volcanoes that erupt explosively can send particles many miles away from the volcano and high into the stratosphere where the Earth’s ozone is concentrated. In addition, the debris can be dispersed for months or even years.

Volcanic dust blown into the atmosphere reduces the sunlight that reaches the Earth’s surface and cause temporary cooling; the degree of cooling is dependent on the volume of dust and the duration is dependent on the size of the dust particles. Additionally, the strength of gases can vary greatly among volcanoes. Water vapor is typically the most abundant volcanic gas, followed by CO2 and sulfur dioxide. Other principal volcanic gasses include hydrogen sulfide, hydrogen chloride, and fluorine.

Volcanoes that discharge great quantities of sulfur compounds affect the climate more significantly than those that release only dust. In fact, the greatest volcanic effect on the Earth’s shortterm weather patterns is caused by sulfur dioxide gas. When sulfur dioxide and other volcanic gases mix with oxygen and water vapor in the presence of sunlight, the result is vog, or volcanic smog. Vog poses a health hazard by exacerbating respiratory conditions. Sulfur dioxide in the atmosphere is often transformed into sulfur trioxide which, when combined with water, forms sulfuric acid that reflect the sun’s heat and triggers cooling of the Earth’s surface. Acid rain contains greater than normal amounts of sulfuric and nitric acids, and when deposited on the Earth’s surface, becomes a critical environmental problem that can affect lakes, streams, forests, and the inhabitants of these ecosystems.

Volcanoes also discharge water and CO2 in large quantities in the form of atmospheric gases; in the atmosphere, these gases can absorb and retain heat radiation emanating from the ground. Estimates suggest that water makes up to 99 percent of gas in volcanic expulsions. This short-term warming of the air causes water to become rain within a matter of hours or days, and it causes the CO2 to dissolve in the ocean or to be absorbed by plants. The majority of the heat energy connected to global warming exists in the ocean. If the oceanic depth at which heat is stored is decreased, then global temperature increases are expected to be greater than predicted.

Volcanic eruptions combined with humanmade chlorofluorocarbons (CFCs) also can contribute to ozone depletion. CFCs were developed in the early 1930s; because they were nontoxic, nonflammable, and met a number of safety criteria, CFCs were used in industrial, commercial, and household applications such as refrigeration units and aerosol propellants. In February 1992, however, following evidence that CFCs contributed to depletion of the ozone layer, the U.S. government announced plans to phase out the production of CFCs by December 1995. Members of the Montreal Protocol in 1992 followed suit and agreed to an accelerated phaseout by the end of 1995.

The ozone layer, which rests in the stratosphere and begins at 7.5 mi. (12 km) above the Earth, is a shield that protects living beings from ultraviolet-B (UV-B), the sun’s most harmful UV radiation. In high doses, UV-B can lead to cellular damage in plants and animals. Scientists believe that global warming will lead to a weakened ozone layer. As the Earth’s surface temperature rises, the stratosphere will become colder, slowing the natural repair process of the ozone layer. Decreased ozone in the stratosphere results in lower temperatures. Unlike ozone depletion created by human-made CFCs, which will take decades to repair, scientific theories indicate that as volcanic activity diminishes, the damaged caused by the volcanoes ias gradually repaired as volcanic activity diminishes.

Finally, hydrogen fluoride gas can concentrate in rain or on ash particles, contaminating grass, streams, and lakes with excess fluorine. Excess fluorine in grass and water supplies can poison the animals that eat and drink at contaminated sites and eventually causes fluorosis, which destroys bones. In fact, excessive fluorine can lead to a major cause of injury and death in livestock during ash eruptions.

Today, millions of people live near active or potentially active volcanoes. The area around the Pacific Ocean in the Ring of Fire, known as the Cascade Volcanic Arc, accounts for about 75 percent of the world’s volcanoes. In 1991, Mount Pinatubo in the Philippines emitted about 22 million tons of sulfur dioxide, which combined with water to form sulfuric acid, decreasing global temperatures for approximately a year. Additionally, in May 1980, Mount St. Helens, located in the state of Washington in the Cascade Volcanic Arc, released approximately 520 tons (472 million metric tons) of ash into the atmosphere. This volume of ash can have a short-term cooling effect hundreds or even thousands of miles away. Very cold temperatures leading to crop failures and famine in North America and Europe, followed the eruption of Mt. Tambora, Indonesia, in 1815.

One of Iceland’s largest volcanoes, the Eyjafjallajökull, showed signs of activity in March 2010 after almost 200 years of dormancy. The eruption on April 16 disrupted air traffic in northern Europe and beyond as volcanic ash spread across northern and central Europe. Although it was marked as the worst travel disruption during peacetime, ash from the Eyjafjallajökull volcano reached 55,000 ft. (16,764 m), which was less than the almost 78,000 ft. (23,774 m) reached by ash from Mt. Pinatubo. Because volcanic ash generally leaves the environment within a few years or even months, Eyjafjallajkull was expected to have a short-term impact on climate. Scientists believe that the temporary cooling of the planet by a volcanic eruption does not offset CO2 from the burning of fossil fuels.

Worldwatch Institute

The Worldwatch Institute has stated that it is “dedicated to fostering the evolution of an environmentally sustainable and socially just society, where human needs are met in ways that do not threaten the health of the natural environment or the prospects of future generations.” It describes itself as “an independent, globally focused environmental and social policy research organization” with a “unique blend of interdisciplinary research and accessible writing.” Worldwatch is essentially a think tank, with its closest environmental movement analogues being Resources for the Future, the World Resources Institute, and the Earth Policy Institute. The latter is headed by Lester Brown, who founded Worldwatch in 1974, and served as its president through 2000. Worldwatch’s current executive director is Robert Engelman, a former environmental reporter and a founder of the Society of Environmental Journalists.

Worldwatch prides itself on its accessible writing style and its fact-based analysis of critical global issues. It focuses on the underlying causes of these issues and seeks, through education and dissemination of information, to inspire people to act in positive ways. A search of its Website shows large numbers of publications related to climate change, which it has addressed in its publications since at least 1984. Worldwatch Reports (formerly Worldwatch Papers), one of its signature publications, has sought to educate the public regarding “pressing economic, environmental, and social issues” since 1975. Since 1984, Worldwatch has published State of the World, a widely read and influential annual report; the 2009 volume, “Into a Warming World,” focused on climate change. 

Although Worldwatch does not lobby Congress directly, this comprehensive report is read by legislators as well as world leaders, students, and ordinary individuals and has been translated into 25 languages. In 1992, Vital Signs: The Trends That Are Shaping Our Future was first published. It was designed to be an accessible, annual series with brief entries on more than 50 issues that affect the world each year. From 1988 to 2010, the group published a bimonthly magazine, World Watch. Many of those articles, along with summaries of current Worldwatch research and blogs, and a subscription service to Vital Signs Online are available on the Institute’s Website.

Worldwatch is not a one-issue organization, having written about a very wide range of environmental issues including energy, water pollution and water availability, soil erosion and other agricultural concerns, population, biodiversity, materials recycling and conservation, forests, and toxic materials. However, it seeks to foster recognition that these issues are inextricably tied to issues of social justice and peace. It began paying consistent attention to the relationship between social and environmental issues, particularly in international settings, much earlier than most environmental organizations. It began calling attention to the need for a sustainable society in at least 1982, five years before sustainability began to gain widespread attention with the publication of the Brundtland Commission report, Our Common Future.

The institute’s three major program areas are food and agriculture, environment and society, and climate and energy. Projects as of 2011 included providing research and policy advice in the following areas: low-carbon, economic development strategies; environmental and climate impacts of new natural gas reserves and extraction methods; status reports on renewable energy and energy efficiency; security implications of transitioning to low-carbon technologies; climate and energy challenges in India and China; connecting green industry and sustainability advocates with policymakers via its New Economy Council; and ReVolt, a blog on international climate and energy policy. Current research publications include findings from the World Nuclear Industry Status Report, focusing on the effects of the nuclear industry on climate change, and a report on the interrelation and correlation between climate change, population, and women’s lives.

A desire to inspire change in societal attitudes and actions from a grassroots perspective is a hallmark of this organization. It seeks to effect change, not by force from the top, but by educating the public and thereby inspiring them to demand change. It carries on this vision with 26 staff members.

World Weather Watch

The World Weather Watch is the central program of the World Meteorological Organization (WMO), the United Nations’ agency for cooperation among national weather bureaus, founded in 1950. The Fourth World Meteorological Congress approved the idea of the program in 1963, and the WMO, which has 188 member countries and territories, subsequently established the World Weather Watch to make an integrated worldwide weather-forecasting system available.

The World Weather Watch includes the Tropical Cyclone Program, the Antarctic Activities Program, an Emergency Response Activities Program for environmental emergencies, and the Instruments and Methods of Observation Program to ensure the quality of the observations that are vital for weather forecasting and climate monitoring. Through the World Weather Watch, a system is in place for countries around the world to obtain daily weather forecasts. The core components of the World Weather Watch—the Global Observatory System (GOS), the Global Telecommunications System (GTS), and the Global Data-Processing and Forecasting System (GDPFS)—enable the World Weather Watch to provide basic meteorological data to the WMO and other related international organizations.

The GOS allows for observing, documenting, and communicating data about the weather and climate for the creation of forecast and warning services. Monitoring the climate and environment is a priority of the WMO, and the GOS is critical to the effective and efficient operations of the WMO. Long-term objectives of the GOS include the standardization of observation practices and the optimization of global observation systems.

The GTS consists of land and satellite telecommunication links that connect meteorological telecommunication centers. The GTS provides efficient and reliable communication service among the three World Meteorological Centers in Melbourne, Moscow, and Washington, and the 15 Regional Telecommunication Hubs that make up the Main Telecommunication Network. The six Regional Meteorological Telecommunication Networks, covering Africa; Asia; South America, North America, Central America, and the Caribbean; the south–west Pacific; and Europe, ensure the collection and distribution of data to members of the WMO. The National Meteorological Telecommunication Networks make it possible for the National Meteorological Centers to collect data and to receive and disseminate weather information on a national level.

The primary aim of the GDPFS is to prepare and provide meteorological analyses to members in the most cost-effective manner possible. The GDPFS is organized to implement functions at international, regional, and national levels through the World Meteorological Centers, Regional Specialized Meteorological Centers, and National Meteorological Centers. Real-time functions include preprocessing and postprocessing of data and the preparation of forecast products. Non-real-time functions include long-term storage of data and the preparation of products for climate-related analysis.

Increasingly, the World Weather Watch provides support for developing international programs related to global climate and other environmental issues, and sustainable development. The entire continent of Africa has only 1,150 World Weather Watch stations—one per 26,000 sq. km (10,038 sq. mi.)—even though the continent’s land mass is as large as North America, Europe, Australia, and Japan put together. 

This represents coverage eight times lower than the WMO’s recommended minimum level. The changing climate of Africa necessitates greater capacity building on the part of institutions prepared to address the likely crises that lie ahead. The World Weather Watch is vital in developing that capacity.

The World Weather Watch and its parent organization, the WMO, through the development of a permanent global weather data network, have proven critical to defining global warming as a given. As a consequence, the political and policymaking debates about climate change and its very real consequences, such as those facing Africa, have moved to a new arena. Although the World Weather Watch cannot compel individual governments to act on its findings, it has framed the issue of climate change on a truly global scale.

Westerlies Winds

The westerlies are the prevailing winds in the middle latitudes blowing from the subtropical high pressure toward the poles. The westerlies originate due to pressure differences between the subtropical high-pressure zone and the subpolar low-pressure zone. The westerlies curve to the east due to the Coriolis effect caused by the Earth’s rotation. In the Northern Hemisphere, the westerlies blow predominantly from the southwest, while in the Southern Hemisphere, they blow predominantly

from the northwest. The equatorward boundary is fairly well defined by the subtropical highpressure belts, while the poleward boundary is more variable. The westerlies can be quite strong, particularly in the Southern Hemisphere, where less land causes friction to slow them down. The strongest westerly winds typically occur between 40 and 50 degrees latitude.

Winds transport heat from warmer areas to cooler areas and help the Earth maintain equilibrium of its thermal environment. In the midlatitude, the westerlies play a large role in the weather and atmospheric circulation in the middle latitudes. They transport warm, moist air to polar fronts and are also responsible for the formation of extratropical cyclones. In winter, they collect warm, moist air from over the oceans, move it to the cooler continents, and bring heavy rainfall to areas like the northwest coast of the United States. In summer, they collect hot, drier air from over the continents and move it to the oceans.

• Global Warming and Westerlies

Does global warming influence westerlies? A recent study of westerlies in the Southern Hemisphere shows that the westerlies are shifting southward toward Antarctica. No conclusion has been made yet, however. Some scientists believe that recent observations are related to global warming, while others believe they are a part of natural variations. The North Atlantic Oscillation (NAO) is one indicator that shows the relationship between global warming and westerlies. NAO is calculated by the difference in pressure between the permanent low-pressure system located over Iceland (Icelandic Low) and the permanent high-pressure system located over the Azores (Azores High). Global warming can reduce the difference in pressure between two places. At a high NAO index, a large pressure difference between the two places induces stronger westerly flows. The storm track advances northward and Europe experiences milder winters but more frequent rainfall in central Europe and nearby. At a low NAO index with suppressed westerlies, the storm track moves more toward the Mediterranean and results in colder winters in Europe and southern Europe, and North Africa receives more storms and higher rainfall.

El Niño-Southern Oscillation (ENSO) is another indicator. In winters of El Niño years, the polar jet stream in the Northern Hemisphere moves further poleward and brings warmer winter weather to the northeastern part of the United States. In the winter of 2006–07, the warming induced was about 9 degrees F (5 degrees C), which was as much as five times the air temperature increase compared to warming in a typical El Niño year. Changes in both surface and upperlevel westerlies due to El Niño patterns can also influence the development, intensity, and track of hurricanes over the tropical Atlantic Ocean. In the fall of 2006, El Niño strengthened the upper-level westerlies, increased wind shear, and discouraged tropical cyclogenesis over the tropical Atlantic.

Whether or not global warming is behind these stronger El Niño patterns is still being researched. A 2007 climate model by Joellen L. Russell and colleagues indicates that westerlies influence the temperature of the Southern Ocean. According to the model, the southward movement of the Southern Hemisphere westerlies in recent years transfers more heat and carbon dioxide into the deeper waters of the Southern Ocean. This poleward shift of the westerlies has intensified the strength of the westerlies near Antarctica. The pattern could slow down global warming somewhat, but induce ocean levels to rise in Antarctica. How global warming influences the westerlies still remains in question. The recent observation, however, suggests that global warming brings noticeable change in the westerlies.

Easterlies Winds

Winds are defined by their origins. The “easterly” descriptor refers to winds with an easterly zonal component (coming from the east). These include northeasterly and southeasterly winds. Easterly winds occur at all atmospheric scales, including local and synoptic. However, the term easterly winds generally refers to large-scale belts of winds operating within the global circulation of the atmosphere.

In the atmosphere’s general circulation, two distinct bands of easterly winds exist: the trade winds and the polar easterlies. They are found at low and high latitudes, respectively, and arise from the dynamics of air flow among pressure systems.

• Trade Winds

Among the most consistent winds on Earth, the trade winds (or trades) are part of the Hadley cell circulation found from approximately 0 degrees to 30 degrees north and south of the equator. Air rises near the equator as a result of a combination of convection spurred by intense solar radiation and the low-level convergence of wind in a circumpolar zone called the Intertropical Convergence Zone. As the rising air approaches the tropopause, it turns. At approximately 30 degrees, the air subsides, or sinks, resulting in the belt of persistent subtropical highs. Air diverges out of these anticyclones and flows toward the equatorial low. This outflow of air gives rise to the trades. As the air moves toward the equatorial low, it is deflected as a consequence of the Coriolis force (or effect). This deflection results in northeasterly trade winds in the Northern Hemisphere and southeasterly trades in the Southern Hemisphere. The trade winds are also referred to as tropical easterlies, particularly when the associated vertical wind shear is large.

• Polar Easterlies

The polar easterlies, a belt of winds found from approximately 60 to 90 degrees in each hemisphere,

constitute another component of the general circulation. These winds originate from dynamics similar to those of the Hadley cell. Thermally driven high pressure exists at the poles; similarly, a circumpolar zone of low pressure is found near 60 degrees. As air flows from the polar high to the subpolar low (as a result of the presence of a pressure gradient), it is deflected. This deflection leads to a belt of easterly winds in both Northern and Southern Hemispheres .

Rising air over the western tropical Pacific, with sinking air over the eastern tropical Pacific, comprises the Walker air current. The trades, as part of the Walker circulation, push warm surface waters toward the west. Any weakening of the trades would disrupt this oceanic transport of water. This dynamic is similar to an El Niño, in which the trade winds slow and even break down. Recent

research suggests that climate changes, specifically global warming, would weaken or slow the trade winds. The Walker circulation has already diminished by 3.5 percent since the 1800s. This slowing is projected to continue, and much of this change is attributed to anthropogenic activity.

The culprit in the slowing trade winds is the balance between evaporation and precipitation. To maintain a balance, the atmospheric absorption of moisture must be balanced by its release by precipitation. Water vapor, transported west by the trade winds, is precipitated out over the western Pacific. Warmer temperatures, then, spur the absorption of additional water vapor by enhancing evaporation rates. However, precipitation rates increase more slowly. To balance these processes, wind flow must decrease.

Weakened trade winds would result in numerous consequences, including the disruption of normal weather and climate patterns, as well as suppressed oceanic upwelling and potentially reduced biological productivity. The latter would have important economic ramifications, particularly for the Pacific fishing industries.

Hurricane Belle

Eastern United States, August 8–10, 1976 

A relatively mild Category 1 hurricane whose 90-MPH (145-km/h) winds and torrential precipitation at landfall nonetheless claimed at least 12 lives and caused an estimated $23.5 million in property damage to communities in North Carolina, New Jersey, New York, and New England between August 9 and 10, 1976.

A midseason tropical cyclone that had developed off the northeast coast of the Bahamas during the afternoon of August 8, 1976, Belle briskly intensified as it first underwent recurvature and then sped up the United States’s eastern seaboard on August 9. Aerial reconnaissance flights conducted by the National Hurricane Center (NHC) indicated that Belle, with a central barometric pressure of 28.47 inches (964 mb), was packing sustained winds of 111 MPH (179 km/h) and gusts of 125 MPH (201 km/h) as it brushed past North Carolina’s Outer Banks shortly after dawn on August 9. The state was fortuitously spared a direct strike by Belle’s Category 3 winds and minimal four-foot (1.5-m) storm surge; however, five people were drowned when the van they were traveling in was washed into a shallow gorge by one of the hurricane’s rain-driven flash floods. Aside from a few isolated power outages and some minor beach erosion, material damage in North Carolina from Hurricane Belle’s glancing passage was on the whole considered fairly moderate.

Quickly progressing up the coast in a wavering north-northeasterly direction, Belle’s extensive spiral rain bands next grazed portions of New Jersey’s eastern shoreline. In Atlantic City, the hurricane’s showering winds and torrential rains brought with them a heavy surf, one that relentlessly pounded the resort community’s famed wooden Boardwalk, clientless hotels, and clifflike stone jetties. More than 150,000 homes in New Jersey, some as far inland as Trenton, were left without electrical power. Three people were reportedly killed in storm-related automobile accidents, and damage estimates in Atlantic City alone totaled some $4.5 million.

Shortly after midnight on August 10, 1976, Belle’s sprawling eye finally came ashore on the south coast of New York’s Long Island. Although the North Atlantic Ocean’s cooling waters had by this time reduced the hurricane to Category 1 status, its sustained 90-MPH (145-km/h) winds managed to uproot hundreds of trees, knock out traffic lights and road signs, smash windows, and demolish several beachside cottages on the heavily settled island. Downed power lines sapped more than 275,000 homes of electricity, while isolated flood action washed out a small bridge near the Fire Island Inlet. One fatality—a man who was struck by a falling tree—was reported on the island. Damage estimates ranged from $8–9 million, making it the most costly hurricane to strike Long Island since Hurricane Carol came ashore at Westhampton in August 1954.

With its wind and rain now sharply diminished, Belle spiraled inland, passing over southern and central New England during the evening hours of August 10. No casualties were reported in Connecticut, where the storm’s gale-force winds scattered nearly 20 percent of that year’s vital apple

harvest like grapeshot, causing upward of $4 million in agricultural losses to the state. In Vermont’s green hills, however, Belle’s lingering rains spawned serious flash floods: In one tragic instance, a mother and her seven-year-old son drowned when the footbridge they were traversing collapsed, pitching them into the raging torrent below. A further $2.5 million in property damage was tallied in Vermont, where the dissipating storm’s trampling effects on timber forests and dairy farms were judged to have been among the worst in living memory.