Q: What mechanisms are responsible for causing the wind to blow in
gusts?
A: Near the surface of the Earth, friction slows the wind. Turbulence is
almost always created by layers of air moving at different velocities and this
enhances or reduces the surface wind. The enhancements are the gusts. Strong
turbulence is also created by obstructions such as buildings which is why city
centres are notoriously gusty.
If the surface is sufficiently warmer than the air above, then convection
will produce columns or walls of warm air called 鈥渢hermals鈥�. These will rise
from the surface, and draw in currents of air to the base of the rising
column. These currents can add to the mean wind to produce gusts that are
longer lived than the usual turbulent gust.
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In addition, if the convection is strong enough, it may produce shower
clouds by condensation of moisture in the thermal as it rises and cools.
Subsequent evaporation can then result in columns of cold air rapidly
descending from these clouds to produce violent gusts at the surface. These
are sometimes called squalls.