Geostrophic winds cannot be exactly parallel to isobars
 

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?

Geostrophic winds cannot be exactly parallel to isobars
 

On Fri, 20 Aug 2004 22:57:41 +1200,
Lawrence D¹Oliveiro , in
wrote:
+ 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?

Geostrophic winds happen rarely in the real world. And they are, by
definition, the component parallel to isobars. The ageostrophic
component is perpendicular to the isobars. So, in reality, the winds
look like this:

wind = geostrophic + ageostrophic

Now that's a bit of a simplification since those are vectors
(direction and speed).

James
--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
(I R A Darth Aggie) wrote:

On Fri, 20 Aug 2004 22:57:41 +1200,
Lawrence D¹Oliveiro , in
wrote:
+ 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?

Geostrophic winds happen rarely in the real world. And they are, by
definition, the component parallel to isobars.

I thought the definition
http://ww2010.atmos.uiuc.edu/(Gl)/ww...hret=/guides/m
tr/fw/fric.rxml was that the Coriolis force was in balance with the
pressure gradient force. Which I took to mean, that component of the
pressure gradient force perpendicular to the direction of the wind.

As long as the drag is nonzero, there must be a component of the
pressure gradient force in the direction of the motion of the wind, to
offset the drag. So the wind can never be exactly parallel to the
isobars.

Geostrophic winds cannot be exactly parallel to isobars
 

"Lawrence D¹Oliveiro" wrote in message
...
... Which I took to mean, that component of the
pressure gradient force perpendicular to the direction of the wind.

As long as the drag is nonzero, there must be a component of the
pressure gradient force in the direction of the motion of the wind, to
offset the drag. So the wind can never be exactly parallel to the
isobars.

Why?

The fact that the air is moving at all, is determined by the pressure
gradient force (PGF). The fact that it's direction is 90 degrees offset to
the PGF is a function of the coriolis effect... you cannot ask for *more*
PGF in that direction.

Geostrophic winds cannot be exactly parallel to isobars
 

"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.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ers.com,
"Icebound" wrote:

"Lawrence D¹Oliveiro" wrote in message
...
... Which I took to mean, that component of the
pressure gradient force perpendicular to the direction of the wind.

As long as the drag is nonzero, there must be a component of the
pressure gradient force in the direction of the motion of the wind, to
offset the drag. So the wind can never be exactly parallel to the
isobars.

Why?

The fact that the air is moving at all, is determined by the pressure
gradient force (PGF). The fact that it's direction is 90 degrees offset to
the PGF is a function of the coriolis effect... you cannot ask for *more*
PGF in that direction.

The work done by a force is the dot product of the force and the
displacement of the object it's pushing on. In particular, if the two
vectors (force and displacement) are at right angles, then the dot
product is zero. Which means if the wind is moving at right angles to
the pressure gradient force, then the pressure gradient force cannot
transfer any kinetic energy to the wind, so it cannot cause the wind to
blow.

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
"Yokel" wrote:

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.

Another thing to keep in mind is that the Coriolis force increases with
latitude. This means that, even in the complete absence of drag, the
wind cannot follow a closed path (which is what an isobar is), as that
would cause a pressure build-up at some point, which would stop the wind
flowing.

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:
In article ,
"Yokel" wrote:


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.


Another thing to keep in mind is that the Coriolis force increases with
latitude. This means that, even in the complete absence of drag, the
wind cannot follow a closed path (which is what an isobar is), as that
would cause a pressure build-up at some point, which would stop the wind
flowing.

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

Scott

Geostrophic winds cannot be exactly parallel to isobars
 

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:
In article ,
"Yokel" wrote:


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.


Another thing to keep in mind is that the Coriolis force increases with
latitude. This means that, even in the complete absence of drag, the
wind cannot follow a closed path (which is what an isobar is), as that
would cause a pressure build-up at some point, which would stop the wind
flowing.

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

Geostrophic winds cannot be exactly parallel to isobars
 

Lawrence D¹Oliveiro wrote:

In article ,
Scott wrote:

Lawrence D¹Oliveiro wrote:
In article ,
"Yokel" wrote:


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.


Another thing to keep in mind is that the Coriolis force increases with
latitude. This means that, even in the complete absence of drag, the
wind cannot follow a closed path (which is what an isobar is), as that
would cause a pressure build-up at some point, which would stop the wind
flowing.

Thus, even neglecting drag, the idea of winds flowing parallel to
isobars is still unrealistic.

I guess someone oughta tell the 500-mb flow that, then ;)

You have some empirical evidence contradicting my claim?

If you're talking *exactly* parallel to the isobars at all times, no.
But the 500 mb winds do flow generally approximately parallel to the
isobars, to within a good enough approximation for many purposes,
especially in areas of respectable gradients. A look at a couple of
500 mb synoptic charts will show that. For instance see
http://www.hpc.ncep.noaa.gov/dailywx..._20040101.html .
Note that the contours are isoheights of the 500 mb surface, not
isobars, but it makes no difference to Scott's point. If you want
to dig out a meteorology text from the 1940s or 1950s, when they
still used isobars on a constant height surface, be my guest.

Cheers,
Russell
--
All too often the study of data requires care.