"Lawrence D¹Oliveiro" wrote in message
...
| I keep seeing descriptions (e.g.
| http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/fw/fric.rxml) of winds
| blowing parallel to isobars. Yet surely this is not physically possible?
| The only force driving the wind is the pressure gradient force. The
| Coriolis force always acts perpendicular to the direction of movement,
| so it cannot do work--that is, change the kinetic energy of the wind in
| in any way.
|
| Look at it this way: the pressure gradient force is always perpendicular
| to the isobars. The Coriolis force is always perpendicular to the
| direction of the wind. So when the wind is moving parallel to the
| isobars, you have both forces acting perpendicular to its motion,
| leaving _no_ force acting in the direction of its motion. So what's
| keeping the wind moving against the drag of the ground?

The geostrophic wind is not a "real" wind. It is an "ideal" wind based on
certain assumptions. One of these is that there is no friction against the
ground, which is why it does not appear in the formulae used to calculate
this wind.

In the absence of ground friction, (which applies very nearly in the free
atmosphere a kilometre or two up or more), the geostrophic wind is a very
close approximation, providing the systems are not moving too quickly or are
developing/decaying rapidly. These circumstances also produce effects not
included in the geostrophic wind calculation.

What happens with ground friction is that the wind speed is reduced from the
geostrophic and so the coriolis force term drops. The pressure gradient
force is then left unbalanced and the wind is deflected to blow at an angle
towards low pressure, effectively filling it up. Just as well, really, for
this limits the practical depth to which depressions can get and causes them
to fill when the processes generating them are finished. Otherwise there is
no telling how strong a hurricane could get (the same argument applies with
cyclostrophic balance, which is when the pressure gradient force provides
the acceleration required to maintain the moving air in a circular path - it
balances the "centrigugal force" which also drops with windspeed). That
ground friction drag prevents things being much worse than they actually are
in the real world.
--
- Yokel -
oo oo
OOO OOO
OO 0 OO
) ( I ) (
) ( /\ ) (

"Yokel" now posts via a spam-trap account.
Replace my alias with stevejudd to reply.