ARTICLE INDEX:
Small scale refers to distances of some tens of kilometers and a little more. In both cases, for large scale and for small scale, the geographic features of the landscape, the shape and the surface, combine with the forces in forming the winds. Some of the same forces that were encountered in understanding the origins of the large scale global winds are found again ie.
1. Gravitational force
2. Pressure gradient force
3. Coriolis force
4. Centrifugal force
5. Friction force
Principal Drivers of Wind at Small Scale
1. Pressure Gradient Force
This is part of the same force as for the global wind drivers
2. Frictional Force
"The main type of friction in a fluid such as the atmosphere is actually turbulence caused by swirling eddies of different sizes. These eddies are stirred up by the sun, by blowing around obstacles, and by the atmosphere itself." (7)
3. Gravitational Force
At this scale the coriolis or centrifugal forces do not play a significant role.
Land Sea Breezes
Differential heating between land and sea surface, and different rates of cooling of sea and land during day and night are the causes. Of the two the day one is a stronger effect. They would be superimposed on the more regional pressure driven effects.
Mountain/Valley Breezes and Windstorms
During the day, the thin air above a high mountain warms quickly. The warm air rises and creates an upslope breeze that becomes strongest around noon. This is a valley breeze or anabatic wind.
At night, the high mountain slopes cool very quickly. This cold, sense air forms a local high-pressure area. The pressure gradient drives a gentle breeze down the slope into the valley. It is strongest just before sunrise. This the mountain breeze.(7)
Katabatic Winds
In mountainous areas with steep sided-sided snow covered large plateaus, the cool mountain breeze can be quite strong. These are called katabatic winds. associated with mountains in high latitudes. Well known ones are the Bora, in Adriatic coast of Europe, and the Mistral, in the French Riviera. The winds are formed as a result of the force of gravity giving rise to a down slope wind.(7)
Gravity Waves
Another down slope wind occurs in some parts of the world due to process other than gravitational cold air drainage. These are due to stationary gravity waves. The gravity waves create a downward momentum flux which brings strong upper air flow down to the surface. This is found in the westerly airflow on the eastern slopes of the Rocky Moumtains(3), but they can be found in non-mountainous areas as well. They are found in the U.S. in the MidWest and the South.
Night Time Jet
Another phenomenon of interest effecting winds in the US in the Great Plains and down into Colorado and Texas is the nocturnal low-level jet(8). A jet phenomenon has also been observed in China (9). This is most evident in the spring and summer. In the US it has been observed that this has the effect of generating higher winds and steeper vertical wind profiles in the layer of the atmosphere in which wind turbines operate at night time.
Gap Winds
An effect sometimes observed in mountainous regions are gap winds. Examples of this would be the Altamont Pass in California , in the Tehachapi Mountains south of Bakerfield in California , the Colombia Gorge, and the Straits of Gibralter. With a strong pressure difference present on either side of the feature, the wind seeks out a gap or pass and is funneled strongly through.
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