What are the primary forces that affect wind?

The three primary forces that affect wind are pressure gradient force, Coriolis force, and friction.

The atmosphere has different areas of pressure.  Wind is created as the pressure systems try to equalize.  The airflow moves from high pressure to low pressure.

The Coriolis force causes the wind to deflect to the right in the Northern hemisphere.  he Coriolis force is directly proportional to the speed of the wind.  The greater the wind speed, the greater the Coriolis force at a given latitude.  The Coriolis force is zero at the equator, and more pronounced at middle and higher latitudes.  The Coriolis force affects all moving objects.  This is the same Coriolis force that affects the rotor system, such as when coning.

Wind is slowed down near the surface due to friction with the earth.  The rougher the terrain, the greater the frictional effect.  Also, the stronger the wind speed, the greater the friction.  The frictional drag of the ground normally decreases with height and becomes insignificant above a few thousand feet.

Local winds are small-scale wind systems driven by heating or cooling of the ground.  Air temperature differences develop over adjacent surfaces.  Air in contact with the ground heats during the day and cools at night.  Local winds are a significant factor for helicopters because their flight is generally local and often close to the ground.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 7-1, 9-1

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