Why does the wind gust instead of blowing at an even speed?

Answer 1

The wind at a place is seldom steady.There are rapid fuctuations in wind speed which are known as gusts and lulls.Gusts are sudden increase in wind speed and lulls are sudden decrease in wind speed.Gusts and lulls in airflow are caused by turbulence in the atmosphere.
Atmosphere becomes turbulent when there are mechanical obstruction to the flow of air or when there is marked convection in the atmosphere.The former is called mechanical turbulence and the latter is called thermal turbulence.
The mechanical turbulence occurs when windflow is obstructed by tall buildings,hanger in an aerodrome,small hills etc which results in the formation of rotating parcels of air called eddies whose movement causes gusts and lulls.
Thermal turbulence occurs when the air layers near the ground are subjected to intense isolational heating by the sun's radiation.These air layers rise due to buoyancy leading to convection currents resulting in turbulence.This turbulence is responsible for the gusts and lulls in the horizontal wind particularly on hot summer days especially during afternoons.

Answer 2

Yes, the gusts are due to turbulence, which can be caused by obstructions. I just wanted to add that turbulence can also be caused by instability. When unstable, the air near the surface will tend to rise. A circulation called an eddy forms. It is more or less circular in shape, so the air flow affects the winds near the ground.

Answer 3

Turbulence caused by landforms, obstructions etc.. are inevitable in the free flow of a relatively low viscosity gas (air).

The relative velocities and dimensions of flow that become turbulent are given by the Reynolds Number. Google it.

Answer 4

I would say, (unscientifically), that hot air and cold air meet, and then they dance!