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A tropical Hawaiian rainforest.
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Michelle Malven/iStockphoto
Climates of the World
From Grolier's New Book of Popular Science
Autumn colors in Massachusetts.
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Denis Jr. Tangney/iStockphoto
Icebergs drift in the summer sun in Iceland.
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Alexander Hafemann/iStockphoto

General patterns of weather vary from place to place around the world. It is well-known, for instance, that warm tropical breezes blow year-round in the Caribbean. It is also common knowledge that winter in Chicago, Illinois, is cold, windy, and often snowy. Indeed, everyplace on the globe has a characteristic pattern of weather conditions that persists over many years. This pattern is the region's climate. So, while a region's weather can be quite unpredictable from day to day, and is sure to vary from season to season, its climate remains relatively stable.

The existence of so many different types of climate makes the science of climatology an exciting and multifaceted pursuit. Climatologists directly observe the climate by systematically collecting such weather data as temperature, humidity, and precipitation over many years. They also study the ways in which the natural world is affected by climate. Some climatologists examine the methods by which humans have adapted to climate. Others analyze short-term climatic anomalies (deviations from the norm), as well as long-term changes in climate. Finally, climatologists try to predict how anomalies and changes may affect society.

THE IMPORTANCE OF CLIMATE

The particular climate of an area profoundly affects the way local people lead their daily lives. It determines, for instance, how they dress for school or work, and what kinds of outdoor recreation they pursue. But its impact goes much further. Climate determines what types of plants dominate the landscape and the types of crops and flowers that can be grown. A farmer in the Midwest, for example, would never consider planting an orange grove. The growing season is too short for the fruit to mature before winter, when subfreezing temperatures would likely kill the trees. So Midwestern farmers plant wheat, corn, and other crops more suitable to their climate, and leave citrus fruits to Florida and California farmers.

Climatic conditions such as humidity can also affect human health. People with severe respiratory problems, for instance, often benefit from moving to a place where the climate is drier. Practically every other aspect of human life is touched by climate as well. The climate of a region controls the availability of water, determining where people obtain it and how much they have to pay for it. Climate governs the way people construct buildings, including the types of materials they use and the kind of heating and cooling systems they install.

CAUSES OF DIFFERENT CLIMATES

A great many factors work "behind the scenes" to shape the climate of a locality. The energy that drives Earth's weather and climates comes from the Sun.

The distance between Earth and the Sun is so great that each ray of sunlight streaming toward the planet is parallel to the others. Because Earth is more or less round, the amount of energy that strikes the surface is greatest in the region of the equator, and decreases to a minimum at the poles. This uneven heating of Earth's surface is the most important factor behind the generation of world climates. But it is not the whole story.

As sunlight passes through the atmosphere and strikes Earth, a large part of the solar energy is reradiated from the surface as heat. That is, light energy hits the ground or water, is absorbed, and then is converted to heat. The heat radiates back through the atmosphere, thus warming it. The lower region of the atmosphere, called the troposphere, is primarily heated in this way.

There is a surplus of heat near the equator because the planet's surface receives more energy per square foot there than elsewhere on the planet. The opposite is true at the poles, which experience a deficit of heat, or cooling. This uneven heating sets up currents in the atmosphere. Warm air rises, and cool air sinks, causing the atmosphere to circulate.

Further influencing global climate is the tilt of Earth on its rotational axis with respect to its orbit around the Sun. This tilt produces seasons. When Earth's Northern Hemisphere is tilted toward the Sun (March through September), the areas north of the equatorial zone experience warming. During these months, the Sun's rays strike the Northern Hemisphere at a nearly vertical angle. The Southern Hemisphere, meanwhile, is tilted away from the Sun, and experiences cooling.

At the autumnal equinox (about September 23 in the Northern Hemisphere), Earth's axis is neither tilted toward nor away from the Sun. After this date, the Northern Hemisphere begins a general cooling trend that lasts through the fall and winter. At the same time, the Southern Hemisphere begins to warm.

Earth's daily rotation on its axis and the friction that exists between the atmosphere and the planet's surface also shape general trends in weather and climate. If Earth did not rotate, surface winds would blow in a straight line from the poles to the equator. Instead, the planet's rotation deflects the winds into various belts such as the trade winds on either side of the equator, the westerlies of the midlatitudes, and the polar easterlies.

Other factors that influence the weather include the composition of atmospheric gases, land topography, and temperature differences between land and sea. Atmospheric gases, such as carbon dioxide and water vapor, help retain heat, acting like a blanket around the planet. Dust particles high in the atmosphere have a cooling effect on the atmosphere.

Lofty mountains, such as the Rockies of western North America, tend to divert prevailing winds. In addition, they encourage winds to drop their moisture on the windward side of the mountains, leaving the leeward side with a much drier climate.

High plateaus, such as the Tibetan plateau in Central Asia, also alter wind patterns. In addition, they regulate global atmospheric pressure by the way their surface heats and cools. For example, during the summer, the monsoon winds of India blow from the sea toward an area of low pressure over the Tibetan plateau. This pocket, or cell, of low pressure is created when hot air rises from the overheated surface of the plateau.

Oceans and other large bodies of water have a moderating effect on climate. Water heats and cools more slowly than does the land. As a result, water can have a cooling effect on coastal land areas in summer, and a warming effect in winter. In addition, the difference in temperature between water and land generates coastal winds. Water also tends to humidify adjacent land areas.

Besides water's generalized effects on climate, specific ocean currents carry warmth or coolness into an area. The Gulf Stream in the northeast Atlantic, for example, carries its warmth to high latitudes, as does the Japan Current in the Pacific. The cool California Current is in part responsible for the fogs that roll into San Francisco Bay.

The many factors mentioned above work together to produce local climate. There are other variables, to be sure. Indeed, every square mile of Earth's surface can be said to have its own distinct microclimate based on its particular geographic and its prevailing atmospheric conditions.

CLIMATE CLASSIFICATION

Throughout history, many attempts have been made to classify the climates of the world. The most straightforward way is to divide Earth's geographic regions into latitudinal climatic zones based on variations in temperature. Using temperature alone, Earth's climate may be divided into two polar zones (one centered around the Arctic, and the other around the Antarctic); a tropical zone (between approximately 30 degrees north latitude and 30 degrees south latitude); and two midlatitude zones sandwiched between them in each hemisphere.

At their extreme, the polar zones include essentially cold, summerless areas. Conversely, the tropical zone has essentially hot, winterless areas. Midlatitude zones include areas with both summer and winter seasons. This simple type of zonation is a start, but it fails to account for many notable climatic differences.

Different locations at the same latitude do not necessarily have similar climates. Little Rock, Arkansas, for example, is at the same latitude as Santa Barbara, California, and Kabul, Afghanistan—about 35 degrees north. Little Rock has a humid climate and mild winters with occasional light snow. Santa Barbara is drier and warmer than Little Rock, experiences less seasonal variation in temperature, and rarely sees snow. In contrast to either American city, Kabul, high in the Hindu Kush Mountains, is both cool and dry. The differences in climate between these cities reflect differences in local topography and their relative distances from the ocean. Any classification system must account for such differences.

Köppen's Classification System. The most widely used classification of climate was initially developed by Vladimir Köppen (1846–1940), a German botanist and meteorologist, and it has been refined and improved over the years. Köppen's classification system is based on yearly patterns of temperature and precipitation.

Being a botanist, Köppen was understandably focused on the way that climate affects vegetation. Thus, he used differences in temperature and precipitation—which certainly affect the vegetation—to define boundaries between different climates. For example, according to Köppen's classification, tropical climates must average at least 64.4° F (18° C). This is the temperature limit that stops several tropical plants from growing at higher latitudes.

Since plants are also limited by the availability of water, Köppen expanded his climate classification to include such subdivisions as tropical savanna and tropical rain forest. Both climate types are equally hot, but the savanna is characterized by a dry winter. As a result, its vegetation includes abundant grass and scattered, stunted trees, rather than lush forest. Although Köppen's system clearly delineates major climatic zones, the boundaries between the zones constitute transitional regions. That is, the climate of one zone blends gradually into the climates of adjacent zones.

Climatic Zones of the World. Köppen's climate-classification system recognizes six main categories of climate: tropical; subtropical, or humid mesothermal; temperate, or humid microthermal; polar; arid; and highland. The two general types of climate found in the middle latitudes of the world are subtropical, which is characterized by mild winters, and temperate, which is characterized by cold winters.

With the exceptions of arid and highland regions, each major climatic zone of Köppen's climate-classification system is defined by latitudinal changes in temperature. Departures from a latitudinal climate zonation can generally be explained by local differences in geography, such as elevation and proximity to large bodies of water, and by patterns of global circulation and atmospheric-pressure systems. All these factors affect prevailing temperature and precipitation conditions.

Tropical Climates. Tropical climates are found near the equator, between the tropic of Cancer (23.5 degrees north latitude) and the tropic of Capricorn (23.5 degrees south latitude). They are hot, with monthly average temperatures above 64.4° F (18° C). Also, all tropical climates are wet, receiving large annual totals of rainfall. The seasonal distribution of rain varies from place to place; climatologists therefore recognize three major tropical-climate divisions:

tropical rain forest (tropical wet);  tropical savanna (tropical wet-dry); and  tropical monsoon.

Of the three main subdivisions of tropical climate, tropical-rain-forest climates are always moist, with each month averaging at least 2.4 inches (6 centimeters) of rain. There may be slight variations in total monthly rainfall, but the change is not as great as that seen in the other two subdivisions of tropical climates. A significant feature of rain-forest climates is the fact that the daily temperature range of nearly 20° F (11° C) is much larger than the annual temperature range, which is quite small—approximately 5° F (3° C).

Tropical-savanna climates are generally found north and south of the rain-forest zone that straddles the equator. Large areas of tropical savanna are found in Africa and South America. Savannas are distinguished from the tropical-rain-forest climates by distinctly seasonal patterns in precipitation: namely, dry winters and wet summers.

Tropical-monsoon climates share traits with both rain forest and savanna. Similar to rain-forest climates, monsoon climates receive high rainfall. However, as in the savanna, the rainfall is seasonal. Monsoon climates usually occur in coastal regions, where onshore winds bring excessive moisture during the summer rainy season. Monsoon climates are found in South America and Africa. But the most famous occur in India and Southeast Asia, where the success of agriculture is directly controlled by the summer monsoons.

Subtropical Climates. Subtropical, or mesothermal, climates are midlatitude humid climates that have mild winters. Precipitation is moderate. Subtropical climates are subdivided by scientists based on variations in rain patterns and differences in summer temperature.

Among these subdivisions are Mediterranean climates, which are typically found on the western coasts of continents. Mediterranean climates experience warm, dry summers (droughts sometimes occur); mild, moist winters; and lots of sunshine. Coastal Mediterranean climates have slightly cooler summers than do those farther inland, due to the moderating effect of the water. The region around the Mediterranean Sea is an obvious example; the California coast is another.

Subtropical-humid climates are found inland and on the eastern coast of continents. They occur over large areas of the southeastern United States, Brazil, Uruguay, Argentina, southeast China, and eastern Australia. Similar to Mediterranean climates, subtropical-humid climates experience warm summers and mild winters. But summer weather is humid and rainy rather than dry, due to unstable air masses and a summer influx of moist tropical air. Midlatitude storms dominate winter weather.

The coastal subtropical climates known as marine-west-coast climates are found at much higher latitudes than other subtropical climates, thanks to the climate-moderating effect of the ocean. Most notable are the marine-west-coast climates found in the Pacific Northwest and northern Europe. There rain falls year-round. Winters tend to be foggy and mild, and snowy days are surprisingly few.

Temperate Climates. Temperate, or microthermal, climates extend across North America and Eurasia. No true temperate climates exist in the Southern Hemisphere. By definition, the warmest month averages over 50° F (10° C). But winters can be quite cold, with at least one month averaging well below freezing (26.6° F, or −3° C). Climatologists recognize several subcategories of temperate climate, divided by differences in temperature and precipitation patterns. The coldest temperate climates have at least one winter month in which temperatures drop below −36.4° F (−38° C). Within the temperate climates, seasonal temperatures gradually shift from warmer to colder with rising latitude and distance from oceans.

Continental-humid climates cover large areas of the middle latitudes. Typically, winters are cold and summers are warm. The southernmost temperate climates have hot, humid summers. Farther poleward, subarctic climates are dry, with cool summers and very long, cold winters. Much of Alaska and northern Canada fall into the subarctic category.

Polar Climates. Polar climates are bitterly cold—by definition, the average temperature of the warmest month is less than 50° F (10° C). They are found essentially poleward of the Arctic and Antarctic circles. Polar climates are quite dry, although water is abundant in the form of solid ice. Precipitation remains low throughout the year, averaging about 5 inches (13 centimeters), usually in the form of snow. Winters are long and dark; summers are short and cool.

There are two main types of polar climate: tundra climates, where the average temperature of the warmest month is above freezing; and ice-cap, or ice-sheet, climates, where average monthly temperatures never reach above freezing. Tundra climates are found in the northernmost parts of Canada, Alaska, Scandinavia, and Russia, as well as coastal Greenland. This type of climate supports very little vegetation because soil remains frozen much of the year.

Areas with ice-cap climates, which are found in the interior of Greenland and most of Antarctica, are completely snow- and ice-covered, and support absolutely no vegetation.

Arid Climates. The arid, or dry, climate is the only climate in the Köppen scheme where temperature is not the primary defining characteristic. An arid climate is one in which evaporation exceeds precipitation. Rainfall is generally sporadic at best.

There are two main subcategories of arid climates: steppe, which is semiarid, and true desert. Steppes are transitional to true deserts, receiving slightly more precipitation. But evaporation is still greater than the levels of rainfall or other precipitation. Steppe climates, found on all continents except Antarctica, support grasslands where drought conditions are a constant threat to the local wildlife.

Contrary to popular belief, not all desert climates feature scorchingly hot weather. In fact, deserts may get quite cold. Winter temperatures in the Gobi Desert of Asia often drop below freezing. Climatologists distinguish between relatively hot and cold arid climates, hot desert being the general term for those areas where the mean annual temperature is greater than 64.4° F (18° C), while cold desert refers to those areas where the mean annual temperature is less than 64.4° F.

Rather than being wastelands, many deserts support an abundance of life. The plant life consists largely of xerophytic vegetation—cacti and succulents that have evolved special adaptations to aid in their survival during times of drought.

Highland Climates. Highland climate, as the term implies, refers to climates found in mountainous terrain. In general, temperature decreases as altitude increases. The average atmospheric lapse rate (temperature change with altitude) is approximately 3.6° F per 1,000 feet (6° C per kilometer). Because of this, mountains in low latitudes display a range of climates. In moving from the foothills to the summits, one may pass through tropical, subtropical, temperate, even polar climates. The Andes Mountains in Peru are a good example. The elevation of the tree line roughly marks the transition from subtropical to temperate climate, while the snow line marks the transition from temperate to polar climate.

Air temperature is not the only factor governing the climate in mountainous regions. Highland climate is also affected by exposure. Consider the difference between a northward-facing slope and a southward-facing slope. Over the period of any day, the southern exposure will probably receive more sunlight; thus, the southern slope will experience warmer daytime temperatures. Additionally, windward sides of mountains are usually wetter than leeward sides, because moist air tends to release precipitation before passing over the peaks (the so-called rain-shadow effect).

THE CHANGING CLIMATE

In recent years, people have learned that the relationship between climate and society goes both ways. Humans can and have altered local climates, and may be doing so on a global scale as well.

The building of cities, for example, raises average temperatures and alters wind patterns over the local area. Deforestation and overgrazing by livestock alter vegetation in a way that disrupts the delicate balance between evaporation and precipitation. The result has been the desertification of many semiarid lands. On a global scale, human-made pollutants in the atmosphere appear to help trap heat around the planet, causing a generalized heating known as the greenhouse effect.

The climates on Earth have changed dramatically over the ages. Scientists see the clear evidence in tree rings, rocks, ocean and lake sediment, and polar ice caps. During the most-recent ice ages, for instance, many areas in North America that now display temperate climates were covered with ice.

Scientists are trying to determine if the climate will substantially shift in the future and to what extent human activities will ultimately affect global climate change. Climatologists are attempting to answer these questions and others with computer models of Earth and its climates. Global warming is a factor that has started to manifest itself in recent years. Many researchers throughout the world agree that a warming trend has begun. But what the next 1,000 years will bring is open to speculation.

Joanne M. Alexandrovich