Micro Climates and Local Climates

Local climates are influenced by special factors that operate within comparatively small areas - various local winds, for example.

The Foehn winds of the northern foothills of the Alps blow when low-pressure systems over northern Europe suck in winds from the south. As they descend, the Foehn winds become warm, causing rapid temperature increases in the areas through which they pass.

The Chinook, a similar wind which occurs in late winter and early spring on the eastern slopes of the Rockies, can raise air tempera­tures by 25°C in less than an hour.

Another local influence is the albedo (the percentage of solar radiation reflected or, more precisely, defused by the surface) of the ground conditions. Newly-fallen snow for instance has an albedo of about 90 per cent, which explains why it may not melt in bright sunlight.

Dry, sandy soils have a higher albedo than dark, clayey soils. Forests have low albedos, but forest floors tend to remain cool even on hot days because much of the Sun's radiation is absorbed by the trees and comparatively little penetrates to ground level.

Some local climatic factors are man-made. For example, air pollution over cities reduces the sun­shine, and therefore the heat, that reaches the ground. But this effect is counteracted by the warm air that blankets many urban areas.