|
About Climates
The World's Climate
Climate is the weather of a place averaged over a length of time. The earth's climate varies from place to place, creating a variety of environments. Thus, in various parts of the earth, we find deserts; tropical rain forests; tundras (frozen, treeless plains); conifer forests, which consist of cone-bearing trees and bushes; prairies; and coverings of glacial ice.
Climate also changes with time. For example, a thousand years ago, northern latitudes were milder than they are today. The warmer climate enabled Vikings from Iceland to settle on the southern coast of Greenland. But the colder climate that developed over the following centuries wiped out the settlements.
One major environmental concern is that human activity may be changing the global climate. The burning of fossil fuels--coal, oil, and natural gas--to power motor vehicles, heat buildings, generate electric energy, and perform various industrial tasks is increasing the amount of carbon dioxide gas released into the atmosphere. Fossil fuels contain carbon, and burning them produces carbon dioxide. This gas slows the escape of heat released by the earth into space. Thus, an increase in atmospheric carbon dioxide may cause global warming--a rise in the temperature of the air next to the earth's surface.
Global warming could change rainfall patterns, leading to shifts in plant and animal populations. It could also melt enough polar ice to raise the sea level, and it could increase the frequency and severity of tropical storms.
Climates vary from place to place because of five main factors: (1) latitude (distance from the equator), (2) altitude (height above sea level), (3) topography (surface features), (4) distance from oceans and large lakes, and (5) the circulation of the atmosphere.
The Role of Latitude
The sun continually sends electromagnetic radiation into space. Most of the radiation is visible light, and it also includes infrared (heat) rays and ultraviolet rays. About 30 percent of the radiation that reaches the earth's atmosphere is reflected back into space, mostly by clouds. The remaining 70 percent is absorbed by the atmosphere and the earth's surface, heating them.
The intensity of the solar radiation reaching the atmosphere decreases with increasing latitude. The intensity depends on how high in the sky the sun climbs. The closer a place is to the equator, the higher the climb.
At latitudes between 231/2 degrees north and 231/2 degrees south, the sun is directly overhead at noon twice a year. In these cases, the sun's rays shine directly down toward the surface. The radiation that reaches the atmosphere is therefore at its most intense.
In all other cases, the rays arrive at an angle to the surface and are therefore less intense. The closer a place is to the poles, the smaller the angle and therefore the less intense the radiation. Due to decreases in the intensity of radiation, average temperatures decline from the equator to the poles. Seasonal changes in solar radiation and the number of hours of sunlight also vary with latitude.
In tropical latitudes (those near the equator), there is little difference in the amount of solar heating between summer and winter. Average monthly temperatures therefore do not change much during the year.
In middle latitudes, from the Tropic of Cancer to the Arctic Circle and from the Tropic of Capricorn to the Antarctic Circle, solar heating is considerably greater in summer than in winter. In these latitudes, summers are therefore warmer than winters.
In high latitudes, north of the Arctic Circle and south of the Antarctic Circle, the sun never rises during large portions of the year. Therefore, the contrast in solar heating between summer and winter is extreme. Summers are cool to mild, and winters are bitterly cold.
Terrain and Climate
The higher a place is, the colder it is. Air temperature drops an average of about 3.5 Fahrenheit degrees per 1,000 feet of altitude (6.5 Celsius degrees per 1,000 meters). The temperature of the air determines how much precipitation falls as snow, rather than rain. Even in the tropics, it is not unusual for mountaintops to be snow-covered.
The surface features of the earth influence the development of clouds and precipitation. As humid air sweeps up the slopes of a mountain range, the air cools, and so clouds form. Eventually, rain or snow falls from the clouds. Some of the rainiest places on earth are on windward slopes, those facing the wind.
As winds blow down the opposite slopes, known as the leeward slopes, the air warms, and clouds thin out or vanish. Leeward slopes of mountain ranges are therefore dry. In addition, a rain shadow (dry area) may stretch hundreds of kilometers downwind of a mountain range.
Oceans and large lakes make the air temperature less extreme in places downwind of them. An ocean or lake surface warms up and cools down more slowly than a land surface. Thus, between summer and winter, the temperature of the water varies less than the temperature of the land. The temperature of the water strongly influences the temperature of the air above it. Therefore, air temperatures over the ocean or a large lake also vary less than air temperatures over land. As a result, places that are immediately downwind of the water have milder winters and cooler summers than places at the same latitude but well inland.
San Francisco and St. Louis, for example, are at about the same latitude and therefore receive about the same amount of solar radiation during the year. But San Francisco is immediately downwind of the Pacific Ocean, and St. Louis is well inland. Consequently, San Francisco has milder winters and cooler summers.
The Influence of Wind
Atmospheric circulation influences climate by producing winds that distribute heat and moisture. Six belts of wind encircle the earth: (1) trade winds that blow between 30 degrees north latitude and the equator, (2) trade winds that blow between the equator and 30 degrees south latitude, (3) westerlies (winds from the west) that blow between 30 degrees and 60 degrees north of the equator, (4) westerlies blowing between 30 degrees and 60 degrees south of the equator, (5) polar winds north of 60 degrees north latitude, and (6) polar winds south of 60 degrees south latitude.
Trade winds north of the equator blow from the northeast. South of the equator, they blow from the southeast. The trade winds of the two hemispheres meet near the equator, causing air to rise. As the rising air cools, clouds and rain develop. The resulting band of cloudy and rainy weather near the equator is called the doldrums.
Westerlies blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. Westerlies steer storms from west to east across middle latitudes.
Westerlies and trade winds blow away from the 30 degrees latitude belt. Over broad regions centered at 30 degrees latitude, surface winds are light or calm. Air slowly descends to replace the air that blows away. Descending air warms and is dry. The tropical deserts, such as the Sahara of Africa and the Sonoran of Mexico, occur under these regions of descending air.
Polar winds blow from the northeast in the Arctic and from the southeast in the Antarctic. In the Northern Hemisphere, the boundary between the cold polar easterly winds and the mild westerly winds is known as the polar front. A front is a narrow zone of transition, usually between a mass of cold air and a mass of warm air. Where the air masses overlap, storms can develop and move along the polar front, bringing cloudy weather, rain, or snow.
As the seasons change, the global wind belts shift north and south. In the spring, they move toward the poles. In the fall, they shift toward the equator. These shifts help explain why some areas have distinct rainy seasons and dry seasons. Parts of Central America, North Africa, India, and Southeast Asia have wet summers and dry winters. Southern California and the Mediterranean coast have dry summers and wet winters.
Kinds of Climates
The earth's surface is a patchwork of climate zones. Climatologists (scientists who study the climate) have organized similar types of climates into groups. This article uses a modified version of a classification system introduced in 1918 by Wladimir Koppen, a German climatologist. Koppen based his system on a region's vegetation, average monthly and annual temperature, and average monthly and annual precipitation.
The modified version specifies 12 climate groups: (1) tropical wet, (2) tropical wet and dry, (3) semiarid, (4) desert, (5) subtropical dry summer, (6) humid subtropical, (7) humid oceanic, (8) humid continental, (9) subarctic, (10) tundra, (11) icecap, and (12) highland.
Warm Climates
Tropical wet climates
Tropical wet climates are hot and muggy the year around. They support dense tropical rain forests. Rainfall is heavy and occurs in frequent showers and thunderstorms throughout the year. Average annual rainfall varies from about 70 to 100 inches (175 to 250 centimeters).
Temperatures are high, and they change little during the year. The coolest month has an average temperature no lower than 64 degrees F (18 degrees C). The temperature difference between day and night is greater than the temperature difference between summer and winter. Frost and freezing temperatures do not occur. Plants grow all year.
Tropical wet and dry climates Tropical wet and dry climates occur in areas next to regions that have tropical wet climates. Temperatures in tropical wet and dry climates are similar to those in tropical wet climates, where they remain high throughout the year.
The main difference between the two climates lies in their rainfall. In tropical wet and dry climates, winters are dry, and summers are wet. Generally, the length of the rainy season and the average rainfall decrease with increasing latitude. Not enough rain falls in tropical wet and dry climates to support rain forests. Instead, they support savannas--grasslands with scattered trees.
Semiarid and desert climates
Semiarid and desert climates occur in regions with little precipitation. Desert climates are drier than semiarid climates. Semiarid climates, also called steppe climates, usually border desert climates. In both climate groups, the temperature change between day and night is considerable. One reason for the wide swings in temperature is that the the skies are clear and the air is dry. Clouds would reflect much of the sun's intense radiation during the day, slowing the rate of heating of the air near the surface. At night, clouds and water vapor would absorb much of the earth's radiation--most of which consists of infrared rays--slowing the rate of cooling.
Semiarid and desert climates occur over a greater land area than any other climate grouping. They occur in both tropical and middle latitudes. They cover broad east-west bands near 30 degrees north and south latitude.
Middle latitude semiarid and desert climates are in the rain shadows of mountain ranges. Winds that descend the leeward slopes of these ranges are warm and dry. Middle latitude semiarid areas and deserts differ from their tropical counterparts mainly in their seasonal temperature changes. Winters are much colder in middle latitude semiarid areas and deserts.
Subtropical dry summer climates
Subtropical dry summer climates feature warm to hot, dry summers and mild, rainy winters. These climates, sometimes called Mediterranean climates, occur on the west side of continents roughly between 30 degrees and 45 degrees latitude. The closer to the coast the area is, the more moderate the temperatures and the less the contrast between summer and winter temperatures.
Humid subtropical climates
Humid subtropical climates are characterized by warm to hot summers and cool winters. Rainfall is distributed fairly evenly throughout the year. Winter rainfall--and sometimes snowfall--is associated with large storm systems that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and an occasional tropical storm or hurricane. Humid subtropical climates lie on the southeast side of continents, roughly between 25 degrees and 40 degrees latitude.
Humid oceanic climates
Humid oceanic climates are found only on the western sides of continents where prevailing winds blow from sea to land. The moderating influence of the ocean reduces the seasonal temperature contrast so that winters are cool to mild and summers are warm. Moderate precipitation occurs throughout the year. Low clouds, fog, and drizzle are common. Thunderstorms, cold waves, heat waves, and droughts are rare.
Humid continental climates Humid continental climates feature mild to warm summers and cold winters. The temperature difference between the warmest and coldest months of the year in-creases inland. The difference is as great as 45 to 63 Fahrenheit degrees (25 to 35 Celsius degrees). Precipitation is distributed fairly evenly throughout the year, though many locations well inland have more precipitation in the summer.
Snow is a major element in humid continental climates. Winter temperatures are so low that snowfall can be substantial and snow cover persistent. Snow cover has a chilling effect on climate. Snow strongly reflects solar radiation back into space, lowering daytime temperatures. Snow also efficiently sends out infrared radiation, lowering nighttime temperatures.
Cool Climates
Subarctic climates
Subarctic climates have short, cool summers and long, bitterly cold winters. Freezes can occur even in midsummer. Most precipitation falls in the summer. Snow comes early in the fall and lasts on the ground into early summer.
Tundra climates
Tundra climates are dry, with a brief, chilly summer and a bitterly cold winter. Continuous permafrost (permanently frozen ground) lies under much of the treeless tundra regions.
Icecap climates
Icecap climates are the coldest on earth. Summer temperatures rarely rise above the freezing point. Temperatures are extremely low during the long, dark winter. Precipitation is meager and is almost always in the form of snow.
Highland climates
Highland climates occur in mountainous regions. A highland climate zone is composed of several areas whose climates are like those found in flat terrain. Because air temperature decreases with increasing elevation in the mountains, each climate area is restricted to a certain range of altitude.
A mountain climber may encounter the same sequence of climates in several thousand meters of elevation as he or she would encounter traveling northward several thousand kilometers. For example, the climate at the base of a mountain might be humid subtropical, and the climate at the summit might be tundra.
return to top
|
