NOTE: We've guessed that
you're not using a client that supports colored tables and have tried
to compensate. Low graphics mode looks much better on clients that
do... we recommend switching to Netscape 3.0 or Microsoft Internet
Winds near the surface
Winds affected by friction
Geostrophic wind blows parallel to the
isobars because the
Coriolis force and
pressure gradient force are in balance.
However it should be realized that the actual wind is not always
geostrophic -- especially near the surface.
The surface of the Earth exerts a frictional drag on the air blowing
just above it. This friction can act to change the wind's
direction and slow it down -- keeping it from blowing as fast
as the wind aloft. Actually,
the difference in terrain conditions directly affects how much
friction is exerted. For example, a calm ocean surface is pretty
smooth, so the wind blowing over it does not move up, down,
and around any features. By contrast, hills and forests force the
wind to slow down and/or change direction much more.
As we move higher, surface features affect the wind less until
the wind is indeed geostrophic. This level is
considered the top of the boundary (or friction) layer. The height of
the boundary layer can vary depending on the type of terrain,
wind, and vertical temperature profile. The time of day and season of
the year also affect the height of the boundary layer.
However, usually the boundary layer exists from the surface
to about 1-2 km above it.