Dangerous Semicircle

This term, now somewhat archaic, is given to that portion of a tropical cyclone in which the winds and rain are most intense. Primarily used by mariners, the term is derived from the practice of dividing a hurricane, typhoon, or cyclone into dangerous and navigable semicircles, or right and left halves, based upon the system’s forward motion. In the Northern Hemisphere, where tropical cyclones spin in a counterclockwise direction, an observer facing into the winds of an approaching hurricane will find the dangerous semicircle on the right, or eastern side, and the navigable semicircle on the left, or western side. The reverse is true, of course, in the Southern Hemisphere, where the clockwise spinning of a cyclone will yield a dangerous semicircle on the left, or western side, and a navigable semicircle on the right, or eastern half.

While in actuality both halves of a tropical cyclone are dangerous, the half that finds itself strengthened by both the forward speed of the system’s steering current and the storm’s own forward velocity will possess significantly faster winds and higher seas. For this reason, those mariners who have divided an oncoming hurricane, typhoon, or cyclone into dangerous and navigable semicircles have been better able to guide their vessels away from the storm’s most furious aspects, thus greatly improving their odds of surviving it.

Covalent Bonds

Recall that an atom is chemically stable when its outermost energy level is full. A state of stability is achieved by some elements by forming chemical bonds. A chemical bond is the force that holds together the elements in a compound. One way in which atoms fill their outermost energy levels is by sharing electrons. For example, individual atoms of hydrogen each have just one electron. Each atom becomes more stable when it shares its electron with another hydrogen atom so that each atom has two electrons in its outermost energy level.  How do these two atoms stay together? The nucleus of each atom has one proton with a positive charge, and the two positively charged protons attract the two negatively charged electrons. This attraction of two atoms for a shared pair of electrons that holds the atoms together is called a covalent bond.

Molecules A molecule is composed of two or more atoms held together by covalent bonds. Molecules have no overall electric charge because the total number of electrons equals the total number of protons. Water is an example of a compound whose atoms are held together by covalent bonds. The chemical formula for a water molecule is H2O because, in this molecule, two atoms of hydrogen, each of which need to gain an electron to become stable, are combined with one atom of oxygen, which needs to gain two electrons to become stable. A compound comprised of molecules is called a molecular compound. Polar molecules Although water molecules are held together by covalent bonds, the atoms do not share the electrons equally. The shared electrons in a water molecule are attracted more strongly by the oxygen atom than by the hydrogen atoms. As a result, the electrons spend more time near the oxygen atom than they do near the hydrogen atoms. This unequal sharing of electrons results in polar molecules. A polar molecule has a slightly positive end and a slightly negative end.

Ions

Sometimes atoms gain or lose electrons from their outermost energy levels. Recall that atoms are electrically neutral because the number of electrons, which have negative charges, balances the number of protons, which have positive charges. An atom that gains or loses an electron has a net electric charge and is called an ion. In general, an atom in which the outermost energy level is less than half-full — that is, it has fewer than four valence electrons — tends to lose its valence electrons.

When an atom loses valence electrons, it becomes positively charged. In chemistry, a positive ion is indicated by a superscript plus sign. For example, a sodium ion is represented by Na+. If more than one electron is lost, that number is placed before the plus sign. For example, a magnesium ion, which forms when a magnesium atom has lost two electrons, is represented by Mg2+.

An atom in which the outermost energy level is more than half-full — that is, it has more than four valence electrons — tends to fill its outermost energy level. Such an atom forms a negatively charged ion. Negative ions are indicated by a superscript minus sign. For example, a nitrogen atom that has gained three electrons is represented by N3‒. Some substances contain ions that are made up of groups of atoms—for example, silicate ions. These complex ions are important constituents of most rocks and minerals.

Isotopes

Recall that all atoms of an element have the same number of protons. However, the number of neutrons of an element’s atoms can vary. For example, all chlorine atoms have 17 protons in their nuclei, but they can have either 18 or 20 neutrons. This means that there are chlorine atoms with mass numbers of 35 (17 protons + 18 neutrons) and 37 (17 protons + 20 neutrons). Atoms of the same element that have different mass numbers are called isotopes. The element chlorine has two isotopes: Cl-35 and Cl-37. Because the number of electrons in an atom equals the number of protons, isotopes of an element have the same chemical properties.

Scientists have measured the mass of atoms of elements. The atomic mass of an element is the average of the mass numbers of the isotopes of an element. Most elements are mixtures of isotopes. The atomic mass of chlorine is 35.453. This number is the average of the mass numbers of the naturally occurring isotopes of chlorine-35 and chlorine-37.

Radioactive isotopes The nuclei of some isotopes are unstable and tend to break down. When this happens, the isotope also emits energy in the form of radiation. Radioactive decay is the spontaneous process through which unstable nuclei emit radiation. In the process of radioactive decay, a nucleus can lose protons and neutrons, change a proton to a neutron, or change a neutron to a proton. Because the number of protons in a nucleus identifies an element, decay changes the identity of an element. For example, the isotope polonium-218 decays at a steady rate over time into bismuth-214. The polonium originally present in a rock is gradually replaced by bismuth.

Littoral

THE WORD LITTORAL comes from the Latin root littus, or “seashore.” The littoral zone of a lake or ocean refers to the shallow waters closest to shore. In lakes, this is the zone dropping from the shoreline to roughly 10 ft (3 m) deep where there is enough sunlight for rooted plants to exist. Only in lakes with strong wave action will the littoral zone have algae instead of rooted plants.

Since the littoral zone is the interface between the lake and the surrounding watershed, it receives and accumulates sediment and nutrients that can support a wide variety of plants and animals. Plants include emergent wetland vegetation to submergent plants that may or may not reach the lake surface. This vegetation provides nutrients and habitat for fishes, birds, amphibians, invertebrates, and zooplankton.

Because of tides and wave action, the littoral zone of an ocean is subdivided into many parts. The supralittoral zone (spray zone) is found above the high tide mark up to the point that ocean spray cannot reach. This area usually receives only ocean spray, except for very high tides or storm surges that can inundate it. The intertidal littoral zone is bordered by the high and low tide marks. The sublittoral zone starts at the low tide mark and goes out to roughly 650 ft (200 m) deep, which is the average depth of the edge of the continental shelf. Numerous species live in these zones, each adapted to the abundance and presence of water they receive each day.

Littoral zones are the interface between land and water, and so they are very productive, meaning they support many plants and animals. However, these same areas are also easily altered by human development or pollution. For example, more people in the UNITED STATES are building homes on the remaining edges of lakes and rivers. Disturbing the soil to grade the lot and build the house causes erosion, which moves sediment into the water. Often, people will dump a load of sand on the littoral zone so that they don’t have to walk through plants to go swimming and so it will look neater. Piers built off of these lots can also damage the plants in the littoral zone. As houses age, septic systems start to fail and this nutrient-rich pollution moves laterally into the water, promoting eutrophication. Polluted runoff from driveways and lawns can also impair the waterways.

Coastlines in every country have been feeling additional pressures from growing cities. The more human activity near a coastline results in more damage to the littoral zone. In wealthier areas, beach replenishment (dumping new sand on the beach) buries the current plants and animals, and the building of seawalls and groins to protect one beach will disturb currents to more seriously erode the next unprotected beach downcurrent. The major problem, however, is pollution. Sewer pipes and polluted rivers both dump myriad pollutants into the littoral and sublittoral zones, damaging the life forms there.

Sometimes people hear about this type of pollution when local health departments close down shellfish beds after sewage pollution has been high (because of storm sewer overflows or treatment plant malfunctions). Similarly, each year more beachgoers arrive to find beaches closed where water quality monitoring has detected pathogens, particularly after rain events. Surfers on the west coast of the UNITED STATES have long complained of intestinal sickness and skin sores after spending time in the water near sewage and creek outfalls. There is increasing pressure on coastal cities to repair and upgrade their aging treatment facilities to solve this problem.

Other laws now prevent ocean liners from dumping their sewage (treated or not) near shore, and only to dump treated sewage many miles out to sea. In some countries, laws require that ships not dump their waste oil or oily bilgewater close to shore or that only double-hulled oil tankers may enter port. Degrading littoral zones, particularly along oceans, damages habitat that marine and shorebird animals need, thus reducing this common resource both regionally and globally. The LAW OF THE SEA expects that signatories respect and protect these areas, but oftentimes little or no protection exists.

Lithuania

THE REPUBLIC OF Lithuania in northern Europe is a lowland country that borders LATVIA to the north, BELARUS to the east and southeast, POLAND and the Russian enclave of Kaliningrad to the southwest, and the Baltic Sea to the west. Lithuania is a parliamentary democracy with the supreme council or Seimas serving as the legislature and the president serving as head of state. Its major cities are Vilnius, Kaunas, and Klaipedia. The Lithuanian countryside consists of lowlands and small hills and is dotted by 3,000 lakes. The climate is generally humid, with peak rainfall in August. Temperatures range from 23 degrees F (-5 degrees C) in January to 63 degrees F (17 degrees C) in June. The chief river is the Nemunas, which flows to the Baltic Sea.

Permanent human settlements in what is now Lithuania date to about 8000 B.C.E. In the 13th century, the Teutonic Knights were sent to Lithuania to convert the pagan population to Christianity. In 1385, Grand Duke Jogaila married Queen Jadwiga of POLAND, which began the Christianization of Lithuania and its special relationship with Poland. In 1569, the Treaty of Lublin joined the Grand Duchy of Lithuania and the Kingdom of Poland into a Polish-Lithuanian Commonwealth to balance the growing power of Moscow. Thereafter, Lithuania joined Poland’s decline, ultimately culminating in the partitions by AUSTRIA, Prussia, and RUSSIA between 1772 and 1795.

Lithuania was incorporated into the Russian Empire after the final Polish partition in 1795. The population was subjected to harsh policies forbidding use of the Lithuanian language, resulting in revolts in 1830 and 1863. In 1918, Lithuania declared its independence amid the destruction and chaos of World War I and the Russian Revolution. In August 1939, the Molotov-Ribbentrop Pact split Poland and the Baltic States between the Soviet Union and Nazi Germany. In 1941, the Nazis invaded Lithuania, only to be reoccupied by the Soviet Union. After World War II, Lithuania was incorporated into the Union of Soviet Socialist Republics.

In 1991, Lithuania regained its independence with the other Baltic States. The last Soviet troops withdrew from Lithuanian soil in 1993. Like its Baltic neighbors, Lithuania has taken steps to integrate into Europe by joining the NORTH ATLANTIC TREATY ORGANIZATION (NATO) in 2003 and the EUROPEAN UNION in 2004. 

Lithuanians make up 80 percent of the population, with the remainder consisting of Russians, Poles, and Belarussians. Roman Catholicism is the religion practiced by the majority of Lithuanians. After the demise of the Soviet Union, the state of Russians residing since the Soviet era poses a challenge for Lithuania. The Lithuanian economy has had to transform from a command economy under the control of Moscow to a free market economy. This process has been successful in bringing prosperity but has also created inequities in the standard of living. Lithuania’s chief exports are meat, milk, dairy products, and television parts, while remaining dependent on oil and natural gas from Russia.

Lisbon

WITH A metropolitan-area population of 2,682,687, Lisbon is the capital and largest city in PORTUGAL. It is located on the right bank of the Tagus River, where it forms a large estuary providing a natural safe harbor close to the ATLANTIC OCEAN. The city evolves over a series of hills and enjoys a Mediterranean climate, with mild winters and warm dry summers.

Lisbon was the capital of a worldwide empire for more than 400 years and is the main political, economic, and cultural center of Portugal. The origin of the settlement is remote. Occupied by the Romans in 205 b.c.e., the town was confined to old Castle Hill and the slope leading to the river. It was conquered by the Moors in 714 and recovered by the Crusades and the king of Portugal in 1147. Lisbon was promoted to capital of the kingdom in 1255, and thanks to commerce between the Mediterranean and northern Europe, it became a primary urban center during the 13th and 14th centuries.

Serving as base for the Portuguese expeditionary navies in the age of discoveries, the city benefited the most from this period and became a significant cultural and economic center: infrastructures and especially monuments were built, and the city expanded to the west. In 1527, Lisbon already stood out among Portuguese cities and gained importance at a European level. An earthquake in 1755 destroyed most of the city. The downtown area (Baixa) was rebuilt as a regular grid aligned with the river, replacing tortuous medieval streets and turning it into a hallmark of Lisbon for its unity and architectural value.

Until the development of railroad transportation, the river was the main thoroughfare for the transport of people and goods between the city and more interior areas of the country. During the 19th century industrialization, mass transit was introduced and larger factories were installed along the river, attracting peasants from the countryside and originating typical worker neighborhoods. At the end of that century, the opening of Avenida da Liberdade, a wide boulevard, changed the form of the city by directing growth toward the north. However, by 1940 Lisbon was still concentrated and close to the river. On the hill to the west, the 2,540-acre (1,028-hectare) Monsanto forest park was created on still undeveloped land.

After World War II, a sharp increase in urbanization pushed growth beyond the limits of the municipality, and toward the west and north along main transportation routes (especially railroads). Population of the city stabilized in the 1950s, and by 2000 only 20 percent of the population of the metropolitan area lived within city limits. In the 1950s and 1960s, new industrial areas were located in Cabo Ruivo and on the south bank, connected by a bridge in 1966. More recent transformations include the expanding network of expressways and construction of large peripheral shopping centers, underlining the ongoing strong suburbanization. Part of the east riverfront was rebuilt to replace heavy industry with an area of residential use.