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Acid rain and global warming are two of the most serious environmental issues related to large-scale fossil fuel combustion. Other environmental problems, such as land reclamation and oil spills, are also associated with the mining and transporting of fossil fuels. Acid Rain When fossil fuels are burned, sulfur, nitrogen, and carbon combine with oxygen to form compounds known as oxides. When these oxides are released into the air, they react chemically with atmospheric water vapor, forming sulfuric acid, nitric acid, and carbonic acid, respectively. These acid-containing water vapors—commonly known as acid rain—enter the water cycle and can subsequently harm the biological quality of forests, soils, lakes, and streams. Ash Particles Combustion of fossil fuels produces unburned fuel particles, known as ash. In the past, coal-fired power plants have emitted large amounts of ash into the atmosphere. However, government regulations also require that emissions containing ash be scrubbed o

Acid Rain, form of air pollution in which airborne acids produced by electric utility plants and other sources fall to Earth in distant regions. The corrosive nature of acid rain causes widespread damage to the environment . The problem begins with the production of sulfur dioxide and nitrogen oxides from the burning of fossil fuels, such as coal, natural gas, and oil, and from certain kinds of manufacturing. Sulfur dioxide and nitrogen oxides react with water and other chemicals in the air to form sulfuric acid, nitric acid, and other pollutants. These acid pollutants reach high into the atmosphere , travel with the wind for hundreds of miles, and eventually return to the ground by way of rain, snow, or fog, and as invisible “dry” forms. Damage from acid rain has been widespread in eastern North America and throughout Europe, and in Japan, China, and Southeast Asia. Acid rain leaches nutrients from soils, slows the growth of trees, and makes lakes uninhabitable for fish and other wil

The process that leads to acid rain begins with the burning of fossil fuels . Burning, or combustion, is a chemical reaction in which oxygen from the air combines with carbon, nitrogen, sulfur, and other elements in the substance being burned. The new compounds formed are gases called oxides. When sulfur and nitrogen are present in the fuel, their reaction with oxygen yields sulfur dioxide and various nitrogen oxide compounds. Nitrogen oxides enter the atmosphere from many sources, with motor vehicles emitting the largest share. Once in the atmosphere, sulfur dioxide and nitrogen oxides undergo complex reactions with water vapor and other chemicals to yield sulfuric acid, nitric acid, and other pollutants called nitrates and sulfates. The acid compounds are carried by air currents and the wind, sometimes over long distances. When clouds or fog form in acid-laden air, they too are acidic, and so is the rain or snow that falls from them. Acid pollutants also occur as dry particles an

The acids in acid rain react chemically with any object they contact. Acids are corrosive chemicals that react with other chemicals by giving up hydrogen atoms. The acidity of a substance comes from the abundance of free hydrogen atoms when the substance is dissolved in water. Acidity is measured using a pH scale with units from 0 to 14. Acidic substances have pH numbers from 1 to 6—the lower the pH number, the stronger, or more corrosive, the substance. Some nonacidic substances, called bases or alkalis, are like acids in reverse—they readily accept the hydrogen atoms that the acids offer. Bases have pH numbers from 8 to 14, with the higher values indicating increased alkalinity. Pure water has a neutral pH of 7—it is not acidic or basic. Rain, snow, or fog with a pH below 5.6 is considered acid rain. When bases mix with acids, the bases lessen the strength of an acid (see Acids and Bases ). This buffering action regularly occurs in nature. Rain, snow, and fog formed in regions free o

Acid rain can best be curtailed by reducing the amount of sulfur dioxide and nitrogen oxides released by power plants, motorized vehicles, and factories. The simplest way to cut these emissions is to use less energy from fossil fuels . Individuals can help. Every time a consumer buys an energy-efficient appliance, adds insulation to a house, or takes a bus to work, he or she conserves energy and, as a result, fights acid rain. Another way to cut emissions of sulfur dioxide and nitrogen oxides is by switching to cleaner-burning fuels. For instance, coal can be high or low in sulfur, and some coal contains sulfur in a form that can be washed out easily before burning. By using more of the low-sulfur or cleanable types of coal, electric utility companies and other industries can pollute less. The gasoline and diesel oil that run most motor vehicles can also be formulated to burn more cleanly, producing less nitrogen oxide pollution. Clean-burning fuels such as natural gas are being used

Endangered Species, plant and animal species that are in danger of extinction , the dying off of all individuals of a species. Over 34,000 plant species and 5,200 animal species around the globe are threatened with extinction, and many thousands more become extinct each year before biologists can identify them. The primary causes of species extinction or endangerment are habitat destruction, commercial exploitation (such as plant collecting, hunting, and trade in animal parts), damage caused by nonnative plants and animals introduced into an area, and pollution . Of these causes, direct habitat destruction threatens the most species. A steady rate of extinction is a normal process in the course of evolution , and is called the background rate of extinction. Species have slowly evolved and disappeared throughout geologic time because of climatic changes and the inability to adapt to survive competition and predation. Since the 1600s, however, the rate of extinction has accelerated rap

Extinction (biology), the end of existence of a group of organisms, caused by their inability to adapt to changing environmental conditions. Extinction affects individual species—that is, groups of interbreeding organisms—as well as collections of related species , such as members of the same family, order, or class (see Classification ). The dodo, for example, a species of flightless pigeon formerly living on the island of Mauritius, became extinct in 1665. About 10,000 to 12,000 years ago, the most of the woolly mammoths and the last of the mastodons , both members of the elephant family, died. And about 245 million years ago at the end of the Paleozoic Era, an entire class of primitive marine animals called trilobites disappeared forever. Fossils , the remains of prehistoric plants and animals buried and preserved in sedimentary rock or trapped in amber or other deposits of ancient organic matter, provide a record of the history of life on Earth. Scientists who study this fossi

Evolution, in biology, complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over the course of time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees that span millions of years. Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants , insects , birds , and mammals , share a common ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in

Coral Reef, ridge or elevated part of a relatively shallow area of the seafloor, approaching the sea’s surface. It is formed by a rocklike accumulation of calcareous (calcium-containing) exoskeletons of coral animals, calcareous red algae, and mollusks. Built up layer by layer by living corals growing on top of the skeletons of past generations, coral reefs grow upward at rates of 1 to 20 cm (0.4 to 7.8 in) per year. Coral reefs are tropical, extending to about 30° north and south of the equator and forming only where surface waters are never cooler than 20° C (68° F). Coral reefs are ecosystems with well-defined structures that involve both photosynthetic algae and consumers. The outer layer of a reef consists of living polyps of coral. Within the coral animals live single-celled, round algae called zooxanthellae. Below and surrounding the polyps is a calcareous skeleton, both living and dead, that contains filamentous green algae. Other species of algae, both fleshy and calcareous,

Sewage Disposal, or wastewater disposal, various processes involved in the collection, treatment, and sanitary disposal of liquid and water-carried wastes from households and industrial plants. The issue of sewage disposal assumed increasing importance in the early 1970s as a result of the general concern worldwide about the wider problem of pollution of the human environment, the contamination of the atmosphere, rivers, lakes, oceans, and groundwater by domestic, municipal, agricultural, and industrial waste. See Air Pollution ; Water Pollution . See also: Wastewater Treatment

The processes involved in municipal wastewater treatment plants are usually classified as being part of primary, secondary, or tertiary treatment. Primary Treatment The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. Such materials are removed by screens or vertical bars, and the debris is burned or buried after manual or mechanical removal. The wastewater then passes through a comminutor (grinder), where leaves and other organic materials are reduced in size for efficient treatment and removal later. Secondary Treatment Having removed 40 to 60 percent of the suspended solids and 20 to 40 percent of the BOD5 in primary treatment by physical means, the secondary treatment biologically reduces the organic material that remains in the liquid stream. Usually the microbial processes employed are aerobic—that is, the organisms function in the presence of dissolved oxygen. Secondary treatment actually involves harnessing and accelera

Solid Waste Disposal, disposal of normally solid or semisolid materials, resulting from human and animal activities, that are useless, unwanted, or hazardous. Solid wastes typically may be classified as follows: Garbage - decomposable wastes from food Rubbish - nondecomposable wastes, either combustible (such as paper, wood, and cloth) or noncombustible (such as metal, glass, and ceramics) Ashes - residues of the combustion of solid fuels Large wastes - demolition and construction debris and trees Dead animals Sewage-treatment solids - material retained on sewage-treatment screens, settled solids, and biomass sludge Industrial wastes - such materials as chemicals, paints, and sand Mining wastes - slag heaps and coal refuse piles Agricultural wastes - farm animal manure and crop residues. Learn more about: Waste Disposal Methods Recycling Hazardous Wastes

Disposal of solid wastes on land is by far the most common method and probably accounts for more than 90 percent of the nation's municipal refuse. Incineration accounts for most of the remainder, whereas composting of solid wastes accounts for only an insignificant amount. Selecting a disposal method depends almost entirely on costs, which in turn are likely to reflect local circumstances. Landfill Sanitary landfill is the cheapest satisfactory means of disposal, but only if suitable land is within economic range of the source of the wastes; typically, collection and transportation account for 75 percent of the total cost of solid waste management. In a modern landfill, refuse is spread in thin layers, each of which is compacted by a bulldozer before the next is spread. When about 3 m (about 10 ft) of refuse has been laid down, it is covered by a thin layer of clean earth, which also is compacted. Pollution of surface and groundwater is minimized by lining and contouring the fill,

The practice of recycling solid waste is an ancient one. Metal implements were melted down and recast in prehistoric times. Today, recyclable materials are recovered from municipal refuse by a number of methods, including shredding, magnetic separation of metals, air classification that separates light and heavy fractions, screening, and washing. Another method of recovery is the wet pulping process: Incoming refuse is mixed with water and ground into a slurry in the wet pulper, which resembles a large kitchen disposal unit. Large pieces of metal and other nonpulpable materials are pulled out by a magnetic device before the slurry from the pulper is loaded into a centrifuge called a liquid cyclone. Here the heavier noncombustibles, such as glass, metals, and ceramics, are separated out and sent on to a glass- and metal-recovery system; other, lighter materials go to a paper-fiber-recovery system. The final residue is either incinerated or is used as landfill. Increasingly, municipaliti

Hazardous wastes have been defined by the federal Environmental Protection Agency as wastes that pose a potential hazard to humans or other living organisms for one or more of the following reasons: (1) Such wastes are nondegradable or persistent in nature; (2) their effects can be magnified by organisms in the environment; (3) they can be lethal; or (4) they may cause detrimental cumulative effects. General categories of hazardous wastes include toxic chemicals and flammable, radioactive, or biological substances. These wastes can be in the form of sludge, liquid, or gas, and solid. Radioactive substances are hazardous because prolonged exposure to ionizing radiation often results in damage to living organisms, and the substances may persist over long periods of time. Management of radioactive and other hazardous wastes is subject to federal and state regulation, but no satisfactory method has yet been demonstrated for disposing permanently of radioactive wastes. See also Air Pollutio

Groundwater, water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, groundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earth's water, or about 20 times more than the total of surface waters on continents and islands. Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where th

Climate, the long-term effect of the sun's radiation on the rotating earth's varied surface and atmosphere . It can be understood most easily in terms of annual or seasonal averages of temperature and precipitation. Land and sea areas, being so variable, react in many different ways to the atmosphere, which is constantly circulating in a state of dynamic activity. Day-by-day variations in a given area constitute the weather , whereas climate is the long-term synthesis of such variations. Weather is measured by thermometers, rain gauges, barometers, and other instruments, but the study of climate relies on statistics. Today, such statistics are handled efficiently by computers. A simple, long-term summary of weather changes, however, is still not a true picture of climate. To obtain this requires the analysis of daily, monthly, and yearly patterns. Investigation of climate changes over geologic time is the province of paleoclimatology, which requires the tools and methods of g

Weather, state of the atmosphere at a particular time and place. The elements of weather include temperature , humidity , cloudiness , precipitation , wind , and pressure . These elements are organized into various weather systems, such as monsoons, areas of high and low pressure, thunderstorms, and tornadoes. All weather systems have well-defined cycles and structural features and are governed by the laws of heat and motion. These conditions are studied in meteorology, the science of weather and weather forecasting. Weather differs from climate, which is the weather that a particular region experiences over a long period of time. Climate includes the averages and variations of all weather elements. Learn more: Scales of Weather Causes of Weather Weather Systems

Temperature is a measure of the degree of hotness of the air. Three different scales are used for measuring temperature. Scientists use the Kelvin, or absolute, scale and the Celsius, or centigrade, scale. Most nations use the Celsius scale, although the United States continues to use the Fahrenheit scale. Temperature on earth averages 15° C (59° F) at sea level but varies according to latitude, elevation, season, and time of day, ranging from a record high of 58° C (140° F) to a record low of -88° C (-130° F). Temperature is generally highest in the Tropics and lowest near the poles. Each day it is usually warmest during midafternoon and coldest around dawn. Seasonal variations of temperature are generally more pronounced at higher latitudes. Along the equator , all months are equally warm, but away from the equator, it is generally warmest about a month after the summer solstice (around June 21 in the northern hemisphere and around December 21 in the southern hemisphere) and cold

Humidity is a measure of the amount of water vapor in the air. The air’s capacity to hold vapor is limited but increases dramatically as the air warms, roughly doubling for each temperature increase of 10° C (18° F). There are several different measures of humidity. The specific humidity is the fraction of the mass of air that consists of water vapor, usually given as parts per thousand. Even the warmest, most humid air seldom has a specific humidity greater than 20 parts per thousand. The most common measure of humidity is the relative humidity, or the amount of vapor in the air divided by the air’s vapor-holding capacity at that temperature. If the amount of water vapor in the air remains the same, the relative humidity decreases as the air is heated and increases as the air is cooled. As a result, relative humidity is usually highest around dawn, when the temperature is lowest, and lowest in midafternoon, when the temperature is highest.

Most clouds and almost all precipitation are produced by the cooling of air as it rises. When air temperature is reduced, excess water vapor in the air condenses into liquid droplets or ice crystals to form clouds or fog . A cloud can take any of several different forms—including cumulus, cirrus, and stratus—reflecting the pattern of air motions that formed it. Fluffy cumulus clouds form from rising masses of air, called thermals. A cumulus cloud often has a flat base, corresponding to the level at which the water vapor first condenses. If a cumulus cloud grows large, it transforms into a cumulonimbus cloud or a thunderstorm. Fibrous cirrus clouds consist of trails of falling ice crystals twisted by the winds. Cirrus clouds usually form high in the troposphere, and their crystals almost never reach the ground. Stratus clouds form when an entire layer of air cools or ascends obliquely. A stratus cloud often extends for hundreds of miles. Fog is a cloud that touches the ground. In dens

Precipitation is produced when the droplets and crystals in clouds grow large enough to fall to the ground. Clouds do not usually produce precipitation until they are more than 1 km (0.6 mi) thick. Precipitation takes a variety of forms, including rain , drizzle, freezing rain, snow , hail , and ice pellets, or sleet. Raindrops have diameters larger than 0.5 mm (0.02 in), whereas drizzle drops are smaller. Few raindrops are larger than about 6 mm (about 0.2 in), because such large drops are unstable and break up easily. Ice pellets are raindrops that have frozen in midair. Freezing rain is rain that freezes on contact with any surface. It often produces a layer of ice that can be very slippery. Snowflakes are either single ice crystals or clusters of ice crystals. Large snowflakes generally form when the temperature is near 0° C (32° F), because at this temperature the flakes are partly melted and stick together when they collide. Hailstones are balls of ice about 6 to 150 mm (about 0.

Wind, air in motion. The term is usually applied to the natural horizontal motion of the atmosphere; motion in a vertical, or nearly vertical, direction is called a current. Winds are produced by differences in atmospheric pressure, which are primarily attributable to differences in temperature. Variations in the distribution of pressure and temperature are caused largely by unequal distribution of heat from the sun, together with differences in the thermal properties of land and ocean surfaces. When the temperatures of adjacent regions become unequal, the warmer air tends to rise and flow over the colder, heavier air. Winds initiated in this way are usually greatly modified by the earth's rotation. Winds may be classified into four major types: the prevailing winds, the seasonal winds, the local winds, and the cyclonic and anticyclonic winds (see Cyclone ; Hurricane ; Tornado ).

Pressure plays a vital role in all weather systems. Pressure is the force of the air on a given surface divided by the area of that surface. In most weather systems the air pressure is equal to the weight of the air column divided by the area of the column. Pressure decreases rapidly with height, halving about every 5.5 km (3.4 mi). Sea-level pressure varies by only a few percent. Large regions in the atmosphere that have higher pressure than the surroundings are called high-pressure areas. Regions with lower pressure than the surroundings are called low-pressure areas. Most storms occur in low-pressure areas. Rapidly falling pressure usually means a storm is approaching, whereas rapidly rising pressure usually indicates that skies will clear.

Weather systems occur on a wide range of scales. Monsoons occur on a global scale and are among the largest weather systems, extending for thousands of miles. Thunderstorms are much smaller, typically 10 to 20 km (6 to 12 mi) across. Tornadoes, which extend from the bases of thunderstorms, range from less than 50 m (55 yd) across to as much as 2 km (1.2 mi) across. The vertical scale of weather systems is much more limited. Because pressure decreases so rapidly with height and because temperature stops decreasing in the stratosphere, weather systems are confined to the troposphere. Only the tallest thunderstorms reach the stratosphere, which is otherwise almost always clear.

All weather is due to heating from the sun. The sun emits energy at an almost constant rate, but a region receives more heat when the sun is higher in the sky and when there are more hours of sunlight in a day. The high sun of the Tropics makes this area much warmer than the poles, and in summer the high sun and long days make the region much warmer than in winter. In the northern hemisphere, the sun climbs high in the sky and the days are long in summer, around July, when the northern end of the earth’s axis is tilted toward the sun. At the same time, it is winter in the southern hemisphere. The southern end of the earth’s axis is tilted away from the sun, so the sun is low in the sky and the days are short. The temperature differences produced by inequalities in heating cause differences in air density and pressure that propel the winds. Vertical air motions are propelled by buoyancy: A region of air that is warmer and less dense than the surroundings is buoyant and rises. Air is als

In both hemispheres, the speed of the west wind increases with height up to the top of the troposphere . The core of most rapid winds at the top of the troposphere forms a wavy river of air called the jet stream. Near the ground, where the winds are slowed by friction , the air blows at an acute angle toward areas of low pressure, forming great gyres called cyclones and anticyclones. In the northern hemisphere, the Coriolis force causes air in low-pressure areas to spiral counterclockwise and inward, forming a cyclone, whereas air in high-pressure areas spirals clockwise and outward, forming an anticyclone. In the southern hemisphere, cyclones turn clockwise and anticyclones, counterclockwise. The air spreading from anticyclones is replaced by sinking air from above. As a result, skies in anticyclones are often fair, and large regions of air called air masses form; these have reasonably uniform temperature and humidity. In cyclones, on the other hand, as air converges to the cente

Monsoon (Arabic mauism, “season”), wind that changes direction with the change of seasons. The monsoon prevails mainly in the Indian Ocean. It blows from the southwest, generally from April to October, and from the opposite direction, the northeast, from October to April. The southwest, or summer, monsoon is usually accompanied by heavy rain in areas of India and the East Indies, constituting the dominant climate event of the area. The appearance of this wind pattern over geological time has been linked, through sedimentary evidence, to the uplift of the Himalayas and the Tibetan Plateau (Qing Zang Gaoyuan) as the Indian subcontinent began to collide with the Asian crustal plate about 20 million years ago. The northern land mass was high enough by about 6 million years ago to cause air rising from the southern land mass to be replaced by the monsoon, establishing this wind pattern. Monsoons, in weaker form, also occur in other parts of the world.

Halo, phenomenon of light refraction caused by ice crystals in the atmosphere between the observer and the sun or moon. The commonest form of halo is a circle of colored light surrounding the disk of the sun or moon. Light from the sun or moon is bent by the atmospheric ice crystals at a 22° angle toward the observer. Thus, the halo is a circle with a radius 22° from the center of the disk. Sometimes, a secondary halo caused by the refraction from ice crystals is seen outside the primary halo at a distance of 46° from the center of the sun or moon. Colored images resembling the disk of the sun may also be seen. Called parhelia, or sun dogs, they sometimes can be seen spaced 22° from the sun in a vertical or horizontal direction. Halos are larger in diameter than the coronas seen around the sun or moon in hazy weather. Coronas are caused by the diffraction of light by water particles in the atmosphere. A corona is similar to a rainbow and a fogbow. Fogbows occur when sunlight strikes

Rainbow, arch of light exhibiting the spectrum colors in their order, caused by drops of water falling through the air. It is seen usually in the sky opposite to the sun at the close of a shower and also in the spray of waterfalls. In the brightest or primary bow, often the only one seen, the colors are arranged with the red outside. Above the perfect bow is a secondary bow, in which the colors are arranged in reverse order; this bow is dimmer, because of a double reflection within the drops. When the sunlight enters a raindrop it is refracted, or bent, by and reflected from the drop in such a way that the light appears as a spectrum of colors. The colors can be seen, however, only when the angle of reflection between the sun, the drop of water, and the observer's line of vision is between 40° and 42°. When the sun is low in the sky the rainbow appears relatively high; as the sun rises higher, the rainbow appears lower in the sky, maintaining the critical 40°- to 42°-angle. When th

Rain, precipitation of liquid drops of water. Raindrops generally have a diameter greater than 0.5 mm (0.02 in). They range in size up to about 3 mm (about 0.13 in) in diameter, and their rate of fall increases, up to 7.6 m (25 ft) per sec with their size. Larger drops tend to be flattened and broken into smaller drops by rapid fall through the air. The precipitation of smaller drops, called drizzle, often severely restricts visibility but usually does not produce significant accumulations of water. Amount or volume of rainfall is expressed as the depth of water that collects on a flat surface, and is measured in a rain gauge to the nearest 0.25 mm (0.01 in). Rainfall is classified as light if not more than 2.5 mm (0.10 in) per hr, heavy if more than 7.50 mm (more than 0.30 in) per hr, and moderate if between these limits. Rain, precipitation of liquid drops of water. Raindrops generally have a diameter greater than 0.5 mm (0.02 in). They range in size up to about 3 mm (about 0.13 in)

Air masses acquire moisture on passing over warm bodies of water, or over wet land surfaces. The moisture, or water vapor, is carried upward into the air mass by turbulence and convection (see Heat Transfer ). The lifting required to cool and condense this water vapor results from several processes, and study of these processes provides a key for understanding the distribution of rainfall in various parts of the world. The phenomenon of lifting, associated with the convergence of the trade winds , results in a band of copious rains near the equator. This band, called the intertropical convergence zone (ITCZ), moves northward or southward with the seasons. In higher latitudes much of the lifting is associated with moving cyclones , often taking the form of the ascent of warm moist air, over a mass of colder air, along an interface called a front. Lifting on a smaller scale is associated with convection in air that is heated by a warm underlying surface, giving rise to showers and thunde

Despite the presence of moisture and lifting, clouds sometimes fail to precipitate rain. This circumstance has stimulated intensive study of precipitation processes, specifically of how single raindrops are produced out of a million or so minute droplets inside clouds. Two precipitation processes are recognized: (1) evaporation of water drops at subfreezing temperatures onto ice crystals that later fall into warmer layers and melt, and (2) the collection of smaller droplets upon larger drops that fall at a higher speed. Efforts to effect or stimulate these processes artificially have led to extensive weather modification operations within the last 20 years. These efforts have had only limited success, since most areas with deficient rainfall are dominated by air masses that have either inadequate moisture content or inadequate elevation, or both. Nevertheless, some promising results have been realized and much research is now being conducted in order to develop more effective methods o

Cyclone, in strict meteorological terminology, an area of low atmospheric pressure surrounded by a wind system blowing, in the northern hemisphere, in a counterclockwise direction. A corresponding high-pressure area with clockwise winds is known as an anticyclone. In the southern hemisphere these wind directions are reversed. Cyclones are commonly called lows and anticyclones highs. The term cyclone has often been more loosely applied to a storm and disturbance attending such pressure systems, particularly the violent tropical hurricane and the typhoon, which center on areas of unusually low pressure.

Hurricane, name given to violent storms that originate over the tropical or subtropical waters of the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or North Pacific Ocean east of the International Date Line. Such storms over the North Pacific west of the International Date Line are called typhoons; those elsewhere are known as tropical cyclones, which is the general name for all such storms including hurricanes and typhoons. These storms can cause great damage to property and loss of human life due to high winds, flooding, and large waves crashing against shorelines. See also Tropical Storm ; Cyclone .

Tropical Storm, weather system composed of a cluster of thunderstorms and of wind speeds near the surface of between 63 and 119 km/h (39 and 74 mph). Tropical storms develop out of storms called tropical depressions, in which wind speeds are less than 63 km/h (39 mph). If a tropical storm intensifies so that its wind speed reaches 119 km/h (74 mph), the storm becomes a hurricane. In contrast to a hurricane, a tropical storm typically does not have an eye, or calm area, at its center. Tropical storms form over large expanses of warm tropical ocean water. However, they do not form in the regions of the eastern Pacific or the Atlantic oceans near the equator or south of the equator. Tropical storms cause torrential rainfall and flooding, which pose the gravest threat to populated areas. For example, in 1994, tropical storms Alberto, Beryl, and Gordon caused nearly $1 billion worth of damage in the United States. The flooding caused by Alberto killed 30 people in Alabama and Georgia. In Ju

Thunderstorm, rain cloud or clouds that produce thunder and lightning . Thunderstorms are very tall clouds that extend from near the ground up to, and often slightly above, the top of the troposphere , the bottom layer of the atmosphere. A thunderstorm has a characteristic cylindrical or slight hour-glass shape with a puffy, cauliflower texture. Clouds with this texture are called cumulus, and clouds that produce rain are called nimbus. Because thunderstorms are a combination of these two, they are called cumulonimbus clouds. Many thunderstorms develop an anvil-shaped top as the top is sheared by high-altitude wind. Severe thunderstorms can produce hail , strong winds, and tornadoes . Weak thunderstorms are called thundershowers. Some thundershowers are so weak that they produce virga, which is rain falling from the cloud that evaporates before reaching the ground.

Tornado, violently rotating column of air extending from within a thundercloud down to ground level. The strongest tornadoes may sweep houses from their foundations, destroy brick buildings, toss cars and school buses through the air, and even lift railroad cars from their tracks. Tornadoes vary in diameter from tens of meters to nearly 2 km (1 mi), with an average diameter of about 50 m (160 ft). Most tornadoes in the northern hemisphere create winds that blow counterclockwise around a center of extremely low atmospheric pressure. In the southern hemisphere the winds generally blow clockwise. Peak wind speeds can range from near 120 km/h (75 mph) to almost 500 km/h (300 mph). The forward motion of a tornado can range from a near standstill to almost 110 km/h (70 mph). A tornado becomes visible when a condensation funnel made of water vapor (a funnel cloud) forms in extreme low pressures, or when the tornado lofts dust, dirt, and debris upward from the ground. A mature tornado may be

Hail, form of precipitation consisting of roughly spherical pellets of ice and snow usually combined in alternating layers. True hailstones occur only at the beginning of thunderstorms and never when the ground temperature is below freezing. Raindrops or snow pellets formed in cumulonimbus clouds are swept vertically in the turbulent air currents characteristic of thunderstorms. The hailstone grows by the repeated collisions of these particles with supercooled water, that is, water that is colder than its freezing point yet remains in liquid form. This water is suspended in the cloud through which the particle is traveling. When the particles of hail become too heavy to be supported by the air currents, they fall to earth. Hailstones range in diameter from 2 mm to 13 cm (w to 5 in); the larger ones are sometimes very destructive. Often several hailstones freeze together into a large, shapeless, heavy mass of ice and snow.

Ecosystem, organisms living in a particular environment, such as a forest or a coral reef, and the physical parts of the environment that affect them. The term ecosystem was coined in 1935 by the British ecologist Sir Arthur George Tansley, who described natural systems in “constant interchange” among their living and nonliving parts. The ecosystem concept fits into an ordered view of nature that was developed by scientists to simplify the study of the relationships between organisms and their physical environment, a field known as ecology . At the top of the hierarchy is the planet’s entire living environment, known as the biosphere . Within this biosphere are several large categories of living communities known as biomes that are usually characterized by their dominant vegetation, such as grasslands, tropical forests, or deserts. The biomes are in turn made up of ecosystems. The living, or biotic, parts of an ecosystem, such as the plants, animals, and bacteria found in soil, are kno

The living portion of an ecosystem is best described in terms of feeding levels known as trophic levels. Green plants make up the first trophic level and are known as primary producers. Plants are able to convert energy from the sun into food in a process known as photosynthesis. In the second trophic level, the primary consumers—known as herbivores—are animals and insects that obtain their energy solely by eating the green plants. The third trophic level is composed of the secondary consumers, flesh-eating or carnivorous animals that feed on herbivores. At the fourth level are the tertiary consumers, carnivores that feed on other carnivores. Finally, the fifth trophic level consists of the decomposers, organisms such as fungi and bacteria that break down dead or dying matter into nutrients that can be used again. Some or all of these trophic levels combine to form what is known as a food web , the ecosystem’s mechanism for circulating and recycling energy and materials. For example, i

Humans benefit from these smooth-functioning ecosystems in many ways. Healthy forests, streams, and wetlands contribute to clean air and clean water by trapping fast-moving air and water, enabling impurities to settle out or be converted to harmless compounds by plants or soil. The diversity of organisms, or biodiversity, in an ecosystem provides essential foods, medicines, and other materials. But as human populations increase and their encroachment on natural habitats expands, humans are having detrimental effects on the very ecosystems on which they depend. The survival of natural ecosystems around the world is threatened by many human activities: bulldozing wetlands and clear-cutting forests—the systematic cutting of all trees in a specific area—to make room for new housing and agricultural land; damming rivers to harness the energy for electricity and water for irrigation; and polluting the air, soil, and water. Many organizations and government agencies have adopted a new approac

Forest, plant community, predominantly of trees or other woody vegetation, occupying an extensive area of land. In its natural state, a forest remains in a relatively fixed, self-regulated condition over a long period of time. Climate , soil , and the topography of the region determine the characteristic trees of a forest. In local environments, dominant species of trees are characteristically associated with certain shrubs and herbs. The type of vegetation on the forest floor is influenced by the larger and taller plants, but because low vegetation affects the organic composition of the soil, the influence is reciprocal. Disturbances such as a forest fire or timber harvesting may result in a shift to another forest type. Left undisturbed, ecological succession will eventually result in a climax forest community (see Ecology ). Human intervention is practiced to maintain some desirable forest types. CLASSIFICATION Forests may be divided into the following eight general types on the ba

Savanna, also savannah, tropical grassland with a scattering of shrubs and small and large trees. Savannas may result from soil conditions, from periodic fires caused by lightning or set by humans, or from climatic influences. Climatically determined savannas, as found in western and southwestern Africa, develop in regions with marked wet and dry seasons, where rainfall ranges between 100 and 400 mm (4 and 16 in) a year. These savannas vary from open-canopied forests with a grassy understory to true savannas in which grasses are dominant. When the rainfall is 100 to 200 mm (4 to 8 in), generally only grasses can survive the dry season. When rainfall reaches 300 mm (12 in), the soil holds enough water to sustain shrubs through the dry season as well. When rainfall exceeds 300 mm, enough water is left to support solitary trees; and when rainfall exceeds 400 mm, enough moisture remains during the dry season to allow trees to grow more densely and to form a canopy, shading out the grasses

Chaparral, type of shrub-land community that is dominated by small-leaved evergreen vegetation. Such habitats are characteristic of the Mediterranean type of climate with warm, wet winters and long, dry summers. The name (Spanish chaparra,”scrub oak”) is applied to the shrub lands of California and Baja California that are dominated by scrub oak and by the dense shrubs chamiso and manzanita. Chaparral is fire dependent. Fire wipes out decadent growth, disposes of accumulated litter, recycles nutrients, and stimulates new, vigorous growth from seeds and sprouts. Other shrub lands in the American Southwest with similar vegetation are sometimes called chaparral, but they lack chamiso, and the summers are not as long and dry. In other areas with a Mediterranean climate, equivalent plant communities are given such local names as the tomillares of Spain, the macchia of the Mediterranean countries and South Africa, the phrygana of the Balkans, and the brigalow shrub of South Australia.

Ecological diversity is the intricate network of different species present in local ecosystems and the dynamic interplay between them. An ecosystem consists of organisms from many different species living together in a region that are connected by the flow of energy, nutrients, and matter that occurs as the organisms of different species interact with one another. The ultimate source of energy in nearly all ecosystems is the Sun. The Sun’s radiant energy is converted to chemical energy by plants. This energy flows through the systems when animals eat the plants and then are eaten, in turn, by other animals. Fungi derive energy by decomposing organisms, releasing nutrients back into the soil as they do so. An ecosystem, then, is a collection of living components—microbes, plants, animals, and fungi—and nonliving components—climate and chemicals—that are connected by energy flow. Removing just one species from an ecosystem damages the flow of energy of that system. For instance, in the l

Every species on Earth is related to every other species in a pattern every bit as complex as the patterns of energy flow within an ecosystem. In evolutionary diversity, the connection is not energy flow, but rather genetic connections that unite species. The more closely related any two species are, the more genetic information they will share, and the more similar they will appear. An ever-widening circle of evolutionary relatedness embraces every species on Earth. An organism’s closest relatives are members of its own species—that is, other organisms with which it has the potential to mate and produce offspring. Members of a species share genes, the bits of biochemical information that determine, in part, how the animals look, behave, and live. One eastern gray squirrel, for example, shares the vast majority of its genes with other eastern gray squirrels, whether they live in the same area or are separated by thousands of miles. Members of a species also share complex mating behavio

Air Pollution, addition of harmful substances to the atmosphere resulting in damage to the environment, human health, and quality of life. One of many forms of pollution , air pollution occurs inside homes, schools, and offices; in cities; across continents; and even globally. Air pollution makes people sick—it causes breathing problems and promotes cancer—and it harms plants, animals, and the ecosystems in which they live. Some air pollutants return to Earth in the form of acid rain and snow, which corrode statues and buildings, damage crops and forests, and make lakes and streams unsuitable for fish and other plant and animal life. Pollution is changing Earth’s atmosphere so that it lets in more harmful radiation from the Sun. At the same time, our polluted atmosphere is becoming a better insulator, preventing heat from escaping back into space and leading to a rise in global average temperatures. Scientists predict that the temperature increase, referred to as global warming , will

Most air pollution comes from one human activity: burning fossil fuels—natural gas, coal, and oil—to power industrial processes and motor vehicles. Among the harmful chemical compounds this burning puts into the atmosphere are carbon dioxide, carbon monoxide , nitrogen oxides, sulfur dioxide, and tiny solid particles—including lead from gasoline additives—called particulates. Pollutants also come from other sources. For instance, decomposing garbage in landfills and solid waste disposal sites emits methane gas, and many household products give off VOCs (volatile organic chemicals). Some of these pollutants also come from natural sources. For example, forest fires emit particulates and VOCs into the atmosphere. Ultrafine dust particles, dislodged by soil erosion when water and weather loosen layers of soil, increase airborne particulate levels. Volcanoes spew out sulfur dioxide and large amounts of pulverized lava rock known as volcanic ash. A big volcanic eruption can darken the sky