Climate is defined as average long-term weather patterns, and it describes the state of the atmosphere's behavior and its variation from place to place over time. Climatology is concerned with both the description of climate and the analysis of the causes of climate differences, climate changes, and their conse

Photograph of Hurricane Fran taken from a NOAA/National Weather Service satellite, September 4, 1996 (NASA)

quences. These factors are controlled by latitude, irregular distribution of land and water, prevailing winds, jet stream position, belts of high and low pressure, ocean currents, terrestrial altitude and relief, and the difference between highest and lowest elevations.

Climatology is often considered a branch of meteorology, the study of the atmosphere and all its phenomena, ranging from global patterns to the long-term heat and wind effects of a house or building in an urban setting, a pond in a farmer's field, or the consequences of microclimate on an ant colony.

On a global scale, climate is dependent upon the amount of solar energy intercepted by the atmosphere, which in turn depends on the earth-sun distance and the angle of the earth's inclination at any given place, which changes daily with the earth's rotation. Thus climate study includes the physical laws that determine how solar energy is converted to heat, and other factors such as air pressure, wind, and the distribution of land and sea.

In the equatorial regions the earth heats up more than it cools, while in polar regions it loses more heat than it receives. If it were not for the transfer of heat from equatorial zones to polar zones by wind and ocean currents, the polar regions would get colder and colder and the equatorial regions hotter and hotter.

Climatology is concerned not only with fluctuations in climate caused by variations in solar radiation and changes in the earth's orbital parameters, but also with movement of continents, distribution of land and sea, and volcanic activity. Over time the rise or erosion of mountains causes changes in wind direction and precipitation that affect plant and animal distribution and alter climatic zones. Oceans, because they are warmer in winter and cooler in summer than the adjacent continents, help to moderate climate. This explains why large landmasses such as North America are hotter during the summer and colder in the winter in their interiors, which are far from the sea.

Climate is also a very important factor in terrestrial ecology. Maps presenting vegetation patterns and climatic zones show a close correlation, and plants affect the distribution and abundance of the animals that depend on them. Plants are not only the source of food but also provide habitats for many animals.

Mountaintops at the equator are very cold, and the temperature range between a mountain's base and summit is much like the changes between the warm latitudes and the polar regions. For example, it is not uncommon for tall peaks to have a deciduous forest at their base, evergreen forest on their upper slopes, and tundra above treeline. Snow and even glaciers may be found at a mountain's crest. Mountains may also alter rainfall patterns. As air rises along the mountain front, it cools and therefore cannot hold has much moisture as it did when it was warmer. The moisture is precipitated out, and as dry air flows down the leeward side of the mountain it causes moisture in the soil to evaporate. This resulting climatic feature, the rain shadow, may cause a desert to form at the base of a mountain.

Compared with the past, continents today stand high above sea level, and there is probably more land today than during most of

Phanerozoic time. As a result there is a steep temperature gradient between the poles and the equator: the equator is tropical, while the average temperature at the poles is below freezing. In the past, as shown by fossils and sediments, polar regions were warmer than they are today, and there weren't as many climatic belts.

Waldimir Koppen, a botanist, recognized climate as a major factor in the distribution of vegetation, and he used the relationship as an indicator for major climatic gradations. Koppen produced his first map in 1918 and named his climates, in part, according to vegetation formations—tropical rain forest climate, tropical savanna climate, and so forth. There have been several attempts to improve on his early classification of climate by using such criteria as global radiation, precipitation, and temperature.

Human activity such as forest clearing and other large-scale land use changes, as well as the production of carbon dioxide and other greenhouse gases, affect and alter weather and climate. Climate also affects human activity, such as agriculture, which is influenced by the length of the growing season, amount and distribution of rainfall, and temperature. Climate affects transportation, restricting it with storms, fog, ice in rivers, and heavy snowfalls. It also determines the amount of fuel needed to heat and cool buildings.

—Sidney Horenstein

See also: Atmosphere; Atmospheric Cycles; Carbon Cycle; Global Climate Change; Hole in the Ozone; Layer; Hydrologic Cycle; Meteorology; Oxygen, History of Presence in the Atmosphere


Akin, Wallace. 1990. Global Patterns: Climate, Vegetation, and Soils. Norman: University of Oklahoma Press; Bryant, Edward. 1997. Climate, Process and Change. New York: Cambridge University Press; Huggett, Richard John. 1991. Climate, Earth Processes, and Earth History. New York: Springer Verlag; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown.

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