Global Climate

The Earth's climate zones are classified according to their average temperature and rainfall accumulation, and, in general, form latitudinal, east-west oriented bands on the Earth's surface. Average temperatures increase with latitude and decrease with altitude; temperatures are highest near the equator and near sea level. This pattern of uneven heating drives convection, or heat-driven circulation, of the oceans and atmosphere. Warm, moisture-laden air at the equator rises and flows toward the poles, cooling, releasing precipitation, and sinking as it flows. The tropical zone, which extends about 15° north and south of the equator, is extremely warm and wet. The Earth's hot semi-arid and arid zones lie beneath dry, sinking air between about 15° and 30° North and South. This vertical convection cycle of rising warm, wet air and sinking cool, dry air is called a Hadley cell. The Earth's has six Hadley cells that are responsible for the Earth's alternating wet and dry climate bands. The temperate zones, between 30° and 60° North and South, lie beneath Hadley cells with rising limbs at 60° and a sinking limbs at 30°. The polar climates form beneath sinking, dry, very cold air at the north and south poles.

The Earth's rotation, the global distribution of ocean basins and continents, and the location of high mountain ranges add complexity to the pattern of latitudinal climate bands. The Coriolis effect, a phenomenon that deflects air and water currents to the right in the northern hemisphere, and to the left in southern hemisphere, is a consequence of the Earth's eastward spin. For example, surface air flowing south in the northern equatorial Hadley cell creates the southwesterly Trade Winds instead of a direct, southerly wind. (Winds are named for the direction from which they originate; a nor'easter, for instance, blows from the northeast toward the southwest.) Belts of alternating easterly and westerly winds drive corresponding west- and east-flowing ocean currents, and distribute heat and moisture east and west within the climate bands. Air flowing across a continent loses moisture the farther it travels from the ocean. Consequently, the windward side of a continent is often wetter than its leeward side, and the interior a large continent is dryer than its coasts. When flowing air reaches a mountain front, it rises, cools, and releases its moisture as precipitation. Large mountain ranges thus receive heavy rain and snowfall on their upwind flank, and arid deserts and semi-arid grasslands form in their leeside rainshadows.

The German climatologist, Wladimir Köppen, developed the most common classification nomenclature for climatic zones in the early 1900s. The Köppen system recognizes five general types of regional climate based on average temperature and precipitation: humid tropical, dry, humid mid-latitude with mild winters, humid mid-latitude with cold winters, and polar. The system further divides the general categories into sub-types. Dry regions, for example, can be arid deserts or semi-arid steppes, and polar regions contain frozen tundra as well as ice sheets. The Köppen system has been modified over the years to include finer sub-divisions, and a sixth category for alpine environments was added, but the system remains a valuable and widely used tool for general climatic mapping.

Ecosystems of specifically adapted plants and animals inhabit each climatic zone. The climatic zones delineated by the Köppen system generally correspond to characteristic networks of species that have evolved to survive the region's seasonal temperature changes, precipitation fluctuations, and weather events. Desert plants, for example, have waxy leaves and stems that reduce the amount of water lost by transpiration, and many desert animals are nocturnal, an adaptation that has allowed them to survive in some of the hottest regions on the planet. Biologically productive rainforests and corral reefs flourish in the warmth and humidity of tropical zones. Arctic plants and animals are adapted to take advantage of the short polar summer season by reproducing and storing nutrients quickly before the long, dark, cold polar winter.