Wind is the movement of air. Moving air is a fluid, behaving in
many ways like water. However, air is 1000 times less dense than
water and is limited to moving much smaller materials.
People often refer to
wind as if it has a constant speed and direction (e.g. a west wind
at 30 mph), but most wind flow is not that uniform. The usual condition
is turbulence, with eddies and pockets of air moving up or down
or sideways or backwards at different speeds. The reported wind
speed and direction are statistical averages of the wind conditions.
Wind direction is always reported as the direction the wind is coming
from (a west wind is moving from west to east).
Friction between the
moving air and a surface slows down the wind. Thus, winds are generally
stronger with height above the ground. In the United States and
worldwide, the standard height for wind measurements is 10 m (~33
ft) above the ground. Sand movement on beaches and dunes depends
on wind conditions near the surface.
These anemometers at a study site measure wind speed at five different
heights above the dune surface.
(Hoffmaster State Park in December 2002.)