About wind speed at different heights above ground...?

Question

What exactly causes variation in wind speed at different heights above ground?

Answer 1

Pressure gradient,coriolis force, friction, gravity,turbulence,convection currents are all responsible for change in wind speed and direction particularly in the lower atmosphere.In the upper part of the troposphere and stratosphere,the wind is generally westerlies in the northern hemisphere,the speed increasing with height.

Pressure gradient:-Due to pressure gradient,the wind flows from high pressure area to low pressure areas.If the pressure gradient is steep,winds are stronger.

Coriolis force:- This force deflects the wind to the right in the northern hemisphere.

Friction:- This is caused by the obstruction to wind by tall buildings, trees etc.,near the surface.However,above 2000 feet(approximately),this force decreases rapidly.

Convection:-wind speed is affected by forced convection when a mountain range obstructs the wind flow.Free convection of warm air due to buoyancy also affects the horizontal flow of air at the bottom (near the surface).

Turbulence:- If the free convection becomes strong, turbulence(vertical  rotating parcels of air) are created leading to lulls and gusts in the horizontal flow of air.

Gravity:-Gravity pulls the heavier air below(this is what happens in a high pressure area where cold air ,being heavier, sinks) and causes a density distribution of air with heavier air at the bottom and lighter air above, which also affects the horizontal flow of air.

The temperature gradient between the equator and the poles increases with height and hence the pressure gradient between these two places also become more leading to strong westerlies.In the northern hemisphere,wind flows keeping the low pressure areas(poles) on the left and hence the winds are westerlies.So,in the upper troposphere and stratosphere the winds are usually westerlies with the speed increasing with height.