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Current Category » Agricultural Meteorology

Forces Acting to Produce Wind

Wind is motion of air in response to unbalanced forces acting in horizontal direction. The different forces involved in the flow of the wind are described below:-

1. Horizontal pressure gradient (PG) force:
The rate of change in atmospheric pressure between two points at the Same elevation is called the pressure gradient or isobaric slope. It is proportional to the difference in pressure and is the immediate Cause of horizontal air movement. The direction of air flow is from high to Low pressure and the speed of flow is directly related to the pressure Gradient. The pressure gradient is said tube steep when the rate of change Is great and the gradient the more rapid will be the flow of air. The direction of the pressure gradient is perpendicular to the isobars and pointing towards low pressure.

PG = 1/P* dp/dn
Where P= air density
Dp/dn = rate of change in pressure with distance.

2. The earth’s rotational deflective force [Coriolis force]:
This force comes into play due to rotation of the earth on it axis. It has most potent influences upon wind direction. The Coriolis force effect Causes all winds in the nor them hemisphere to move or deflect toward the Right and those of the sot herm hemisphere to move to the left with respect to the rotating earth. At the equator the effect has a value of zero (Sin 90=1).and it increases regularly towards the poles (and becomes Sin 90=1). The Coriolis effect changes wind direction but does not change wind speed.

3. Centrifugal force :
This force tends to throw the air particles outward from the centre of small circle path on which the particle is moving.  The centrifugal force works against the gravitational attraction directed forwards the earth centre.

4. Frictional force:

The roughness of the surface provides frictional resistance to the air motion.  It is the retarding effect of trees, buildings and other irregularities in the topography; It is always opposed to the direction of the air motion and therefore fends to decrease the wind speed.  Friction causes a movement of air across the isobars towards low pressure.

5. Geotropic winds:
When a wind flows in a straight line with no acceleration or frictional force on it, the only forces acting are the carioles and pressure gradient foresee.  The wind that blows under these conditions is called as geotropic wind.

 

Current Category » Agricultural Meteorology