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Re: Hot air rising and automobile thermometers



On Mon, 26 Jul 1999 17:09:22 -0700 Leigh Palmer <palmer@SFU.CA> writes:
But Leigh, surely a steady wind experiences more than one force.
While the
net-force should be zero, the current question really addresses the
'driving' force, the one that keeps the air moving in spite of the
'frictional-type' forces which must also be present. Indeed, if
there were no such driving force the motion of the air would soon
cease.

There is more than one force acting, but there is no frictional
force. Given a piece of air in a steady wind the immediately adjacent
air is
moving at the same speed. By "to first order" I mean considering no
wind shear. The conventional explanation of geostrophic winds is of
first order. In this case the net force is zero. The force due to the
horizontal component of the pressure gradient is exactly balanced by
the horizontal component of the Coriolis force. Of course more forces
are present in real situations, but those are the two large forces.

When you have internalized this idea you will be one step farther
from the seductive Aristotelean construct, that force impels motion,
which seems to follow us around for years after we learn that it is
fundamentally incorrect. I have the same problem.

Leigh

It would really be appreciated if you would rephrase your
explanations a little to help me (and possibly some others)
clarify the basic concepts that are involved in your discussion
in this thread concerning winds and air currents.

1. Most earth science and meteorology textbooks refer to
"wind" as a horizontal movement of air, either close to the
earth or aloft. What do you mean by calling wind a "horizontal
component of a pressure gradient"?


2. If a force "due to the horizontal component of the pressure
gradient is exactly balanced by the horizontal component of
the Coriolis force" what causes the Coriolis acceleration ?


3. If, as you say, there is no frictional force to stop the
movement of air once it is in motion, why doesn't
the wind continue blowing forever once it has been
started by a pressure gradient?

Herb Gottlieb from New York City
(Where air moves horizontally in our demonstration convection
apparatus only when the candle under one of the chimneys is
still burning)