Surface and local winds
Winds are very much influenced by the ground surface at altitudes up to 100 m. The wind will be slowed down by the earth's surface roughness
, as will be explained in next pages. Wind directions near the surface will be slightly different from the direction of the geostrophic
wind because of the earth's rotation Coriolis
When dealing with wind energy, the surface winds are of interest and how to calculate the usable energy content of the wind.
winds are important in determining the prevailing winds in a given area, local climatic conditions may wield an influence on the most common wind directions.
Local winds are always superimposed upon the larger scale wind systems, i.e. the wind direction is influenced by the sum of global and local effects.
Land masses are heated by the sun more quickly than the sea in the daytime. The air rises, flows out to the sea and creates a low pressure at ground level which attracts the cool air from the sea. This is called a sea breeze. At nightfall there is often a period of calm when land and sea temperatures are equal. At night the wind blows in the opposite direction. The land breeze at night generally has lower wind speeds, because the temperature difference between land and sea is smaller then.
The monsoon known from South-East Asia is in reality a large-scale form of the sea breeze and land breeze, varying in its direction between seasons, because land masses are heated or cooled more quickly than the sea. Other examples are the Mistral flowing down the Rhone valley into the Mediterranean Sea and the Scirocco, a southerly wind from Sahara blowing into the Mediterranean sea.
Mountain regions display many interesting weather patterns. One example is the valley wind which originates on south-facing slopes (north-facing in the southern hemisphere). When the slopes and the neighbouring air are heated the density of the air decreases and the air ascends towards the top following the surface of the slope. At night the wind direction is reversed and turns into a downslope wind. If the valley floor is sloped, the air may move down or up the valley, as a canyon wind. Winds flowing down the leeward sides of mountains can be quite powerful.
Examples of mountain winds are the Foehn in the Alps in Europe, the Chinook in the Rocky Mountains and the Zonda in the Andes.
- Local wind systems: land sea systems, mountain passes