The Climate

United States Geography

Climate is the aggregate of day-to-day weather conditions over a period of many years. It is the result of the interaction of many different elements, the most important of which are temperature and precipitation.

Climatic patterns are a result of the interaction of three geographic controls. The first is latitude. The earth is tilted on its axis with reference to the plane of its orbit around the sun. As it makes its annual revolution around the sun, first the Northern Hemisphere and then the Southern are exposed to the more direct rays of the sun. During the Northern Hemisphere's summer, higher latitude locations have longer days, with far northern points experiencing a period of continuous daylight. Daylight periods during the winter months are shorter at higher latitudes, whereas more southerly locations have both longer days and exposure to more direct rays of the sun.

The second control is based on the relationship between land and water. Land tends to heat and cool more rapidly than water. In a tendency called continentality, places far from large bodies of water experience greater seasonal extremes of temperature than do coastal communities. Parts of the northern Great Plains experience annual temperature ranges close to 65C; annual differences of as much as 100C (from 50C to -50C) have been recorded in some locations.

The converse effect occurs at maritime locations, especially on the western coast of continents in the mid-latitudes. These locations have smaller temperature ranges as a result of what is called a maritime influence. Summer and winter extremes are moderated by the movement onshore of prevailing westerly wind systems from the ocean. Horizontal and vertical ocean currents minimize seasonal variations in the surface temperature of the water. The moderated water temperature serves to curb temperature extremes in the air mass above the surface.

Proximity to large water bodies also tends to have a positive influence on precipitation levels, with coastal locations receiving generally higher amounts. The reason for this should be obvious; large water bodies provide greater levels of evaporation and thus increase the amount of moisture in the atmosphere. That, in turn, increases the possibility of precipitation. There are, however, notable exceptions to this rule, including the dry coast of southern California and the Arctic coastline of Alaska.

The third prime geographic influence on climate is topography. Most obvious is the relationship between elevation and temperature, with higher elevations cooler than lower elevations. The influence of topography can be broader, however, because of its effect on wind flow. If a major mountain chain lies astride a normal wind direction, the mountains force the air to rise and cool. As the air mass cools, the amount of moisture that it can hold is reduced. Precipitation results if the cooling causes the relative humidity to reach 100 percent. Moisture falls on the windward side, and the lee is dry. The wettest area in North America is along the Pacific coast from Oregon to southern Alaska, where moisture-laden winds strike mountains along the shore. Average annual precipitation is more than 200 centimeters throughout the area, and in some places exceeds 300 centimeters.

Mountains also can reduce the moderating effects of maritime conditions on temperature, as happens in the interior of the Pacific Northwest. The Western Cordillera (mountain mass) confines West Coast maritime climatic conditions to that coast. Some of the greatest variations in both precipitation and temperature to be found across a small distance anywhere in America exist between the west and east sides of parts of the Coast Ranges. The aridity of the central and northern interior West is due in large part to the barrier effect of the north-south-trending mountain ranges of the West.

East of the Rockies, the topographic effect on precipitation eventually disappears, partly because the eastern mountains are lower and thus pose less of a barrier to moving air, and partly because much of the weather of the interior is a result of conflict between two huge air masses that are unimpeded, one flowing northward from the Gulf of Mexico, and the other flowing southward out of Canada. The contact of these air masses creates what are often violent displays of weather in the region.

This illustrates a fourth major and complex influence on climate, the impact of air mass characteristics and wind systems. America's weather is affected markedly by the confrontation between polar continental air masses (usually cold, dry, and stable) and tropical maritime air masses (warm, moist, and unstable). The former push farthest south in winter, whereas the latter extend farthest north in summer. Most parts of America are subject to a generally westerly wind flow that tends to move weather systems eastward. The continental climate of the interior is thus pushed onto the East Coast.

The interaction of these climatic controls creates a pattern of climatic regionalization. In the East, the principal element in climatic variation is temperature; in the West, it is precipitation. In the East, the divisions between the climate regions are based largely on the length of the growing season--the period from the average date of the last frost in spring to the first frost in fall--and on the average summer maximum temperature or winter minimum temperature. In the West, average annual precipitation is the key, although moderated temperatures are an important aspect of the marine West Coast climate. In the East, the more northerly areas are generally drier; in the West, they are colder. In the East, the major influence on climatic variation is latitude; in the West, it is topography.

Average temperatures and rainfall in US cities

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Source: U.S. Department of State