Precipitation

Precipitation is the primary factor that controls the hydrologic cycle. It takes different forms, such as rain, snow, hail, sleet, drizzle, dew, and fog. It supplies most of the freshwater on the Earth. Most precipitation starts from space as snow, as the upper space is cooler. If the temperature of the surface closer to the ground is below 32 degrees F (0 degrees C), then the precipitation falls on the ground in the form of snow. If the ground and closer surface temperature is above 32 degrees F (0 degrees C), the precipitation takes the form of rain. When the air at the ground is below freezing, the raindrops can freeze while hitting the ground, and that is known as freezing rain.

When a dust particle in the atmosphere attracts a moisture drop, hail is formed. Drizzle consists of very small raindrops, 1/1000 of a normal raindrop size. Sleet is a type of precipitation between rain and snow, but very distinct from hail. Dew is another form of precipitation that can be seen in the early morning on colder days. Water vapor in the atmosphere condenses on the surface of exposed objects at a greater rate than that at which it can evaporate, developing dew. Fog as such is not precipitation, but is considered one because of its low-altitude occurrence. This consists of a cloud in contact with the ground, and produces water droplets when intercepted with vegetation or other exposed objects. If the precipitation evaporates before reaching the ground, it is then known as virga.

Dynamic and adiabatic cooling causes precipitation. Because of this process, condensation of water vapor occurs and then falls to the Earth as rain, snow, or other forms of precipitation. Vertical air motion is the leading factor of all rainfall from clouds after condensation. Rising air in the tropics and midlatitudes (40 to 60 degrees N and S latitudes) causes more precipitation and descending air patterns in the subtropics (20 to 30 degrees N and S latitudes) and in the poles causes less. Precipitation is classified into different categories based on the conditions that generate vertical air motion: convective, orographic, and cyclonic.

Convective precipitation is mostly seen in the tropics. Heating up of the air at ground level, then moving upward and mixing with the water vapor in the atmosphere, with dynamic cooling in space, causes precipitation to fall on Earth. This is called convective precipitation. Orographic precipitation occurs because of the interception of moisture-laden air or clouds by mountain ranges. Cyclonic precipitation is caused by movement of moist air masses from high-pressure regions to low-pressure regions. In the hydrologic cycle, the total volume of precipitation onto land is measured at 42,471 sq. mi. (110,000 sq. km), while the total volume of precipitation on the ocean surface is 176,834 sq. mi. (458,000 sq. km).

Precipitation has greater ecological, geographical, and regional impact because of its characteristics, such as relative amount, seasonal timing, and most importantly the size and intensity. Lowintensity and well-distributed seasonal precipitation is good for agriculture. High-intensity, longduration precipitation creates more problems than good. Precipitation is governed primarily by atmospheric water vapor, but its variation depends upon other climatic factors such as temperature, wind, and atmospheric pressure at different locations and season. The amount of water vapor is very high in the atmosphere closer to water bodies. Therefore, coastal areas always have heavier precipitation (high-intensity, long-duration) than inland areas. Thunderstorms, hurricanes, typhoons, cyclones, blizzards, and hailstorms are high-intensity precipitations with damaging ability.

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.

Precipitation

As part of the hydrologic cycle, water vapor is transformed into clouds. Clouds are common and, although in constant motion, usually cover about half of the planet at a time. Even so, precipitation is occurring over a small portion of Earth because most clouds do not produce precipitation. How and why precipitation occurs is vital in understanding the physical geography of Earth, because the precipitated water affects the surface lithosphere, biosphere, and hydrosphere.

Precipitation is triggered by one of two processes. The first is known as the warm cloud process and involves only water droplets (including supercooled water) above tropical locations. Cloud droplets collide and some of them coalesce to become large enough to fall out of the sky as rain. The cold cloud process (also known as the Bergeron process) takes place in middle latitude and polar clouds. The temperatures are below freezing and supercooled droplets and ice crystals coexist at temperatures between 0° and −40°C. The saturation vapor pressure is a bit higher over liquid water than over ice crystals and the net effect is that ice crystals grow by adding mass from the nearby droplets. The enlarged ice crystals become large enough to fall as snow.

The general term for water precipitated out of the sky is “hydrometer.” There are several types of hydrometers and their occurrences are governed by moisture supply, rise of air, and the environmental lapse rate underneath the clouds. Snow starts as snowflakes in clouds and falls through below-freezing air to the surface.