Climate and the General Circulation

What Factors Affect a Region's Climate?

A region's climate is shaped by influences operating on all scales of space and time from microscale to global. Many factors work together to shape the climate of any locality.

Controls of climate consist of:

Latitude - most important climatic control, due to the effect is has on the amount of solar radiation reaching the Earth's surface. The seasonal changes in incoming solar radiation, as well as the length of the day, vary with latitude.

As the Earth revolves around the sun, it is tilted on its axis by an angle of 23½°. Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather rather than in December, when the Northern Hemisphere is tipped away from the sun.

The angle of incidence between the sun's rays and the surface of the earth varies with latitude.

Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly.

In the example above, Seattle, lying between the Pacific Ocean and the Cascade Mountains, has a climate that reflects stronger maritime influence (greater precipitation, less temperature variation) that Spokane's.

Also, topographic barriers trigger orographic rainfall on their windward slopes, often leaving a dry rain shadow on the leeward side. In the figure on the left, orographic lifting leads to precipitation on the windward slopes. By the time the air reaches the leeward side of the mountains, much of the moisture has been lost.

Elevation - Influences air temperature (since temperatures usually decrease with height by about 6.5°C per 1000 meters or 3.6°F per 1000 ft) and whether precipitation falls in the form of rain or snow.

A local example is provided by the differences in temperature and precipitation between Tucson and Mount Lemon. During the last storm, while the precipitation in the valley was in the form of rain, over the mountains it was in the form of snow.

Topography - mountain ranges can affect the climate for considerable distances by stimulating cyclone formation and inducing orographic clouds and precipitation.

Proximity to large bodies of water - The ocean's

Specific Heat of Various Substances.

Substance	Specific Heat
Water	1.00
Air	0.24
Granite	0.19
Sand	0.19
Iron	0.11

moderating "influence" is an important determinant of a region's climate.

Water has a high specific heat . It takes a great deal more heat to raise the temperature of 1 gram of water 1°C than in does to raise the temperature of 1 gram of soil or rock by 1°C. Thus, land heats more rapidly and to higher temperatures than water, and it cools more rapidly and to lower temperatures than water.

The circulation of the ocean is a key factor in air temperature distribution.

Ocean currents that have a northward or southward component, such as the warm Gulf Stream in the North Atlantic or the cold Humboldt Current off South America, effectively exchange heat between low and high latitudes.

The circulation of the ocean.

A good example of the effect of a warm current is that of the Gulf Stream in January, which causes a strong east-west gradient in temperatures across the eastern edge of the North American continent.

Notice the vegetation patterns on the mountain pictured above.

The relative warmth of the Gulf Stream affects air temperatures all the way across the Atlantic, and prevailing westerlies extend the warming effect deep into northern Europe. As a result, January temperatures of Tromsø, Norway (69°40'N), for example, average 24°C above the mean for that latitude.

Local topography - Variations in the slope of the ground affects the absorbed sunlight, exposure to winds, and runoff.

Prevailing atmospheric circulation - This last control encompasses the combined influence of all weather systems.

Other Variable Factors:

Nature of the surface - Local surface conditions might shape climate to a considerable extent. Consider the where would you choose to sit (or even stand) on a sunny day in July... Have you noticed differences in temperature as you drive out of Tucson city limits on a summer evening?

Anthropogenic effects - Agriculture and other human induced changes in the amount of vegetation might have variable effects on local climate. With less vegetation, the surface has less moisture available for evaporation. As a result, it experiences higher temperatures than the forested region.