Archived posting to the Leica Users Group, 2000/12/23
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Greg Locke jotted down the following:
> Another affect of air being forced through a constricted place (ala thumb
> over garden hose ... or between to two fat people) such as a venturi, is an
> increase in velocity as is does so.
>
> As the velocity of the air increases, it's tempeture decreases. This is
> why pilots get carburator ice while the ambient tempeture may well be
> above freezing. Yes, I know, the dew point has to be within a certain
> range also to get the condensation which freezes and turns to ice in the
> carburator venturi.... the only place it is cold enough to freeze as this
> is where the fast moving air is cold enough.
Just come back from a rather poor movie ("Dracula:2000" -- guess he had a
few bugfixes and an update or two) which gave me plenty of time to think
about this.
Basically, I don't think that the Venturi effect can apply to atmospheric
windchill. I think you should have prefixed "as the velocity of air
increases, its temperature decreases" with the conditions under which this
is true: namely, air being forced through a small opening. I'm guessing
that the decreased temperature under those conditions comes from the
necessity to maintain the energy equation: kinetic energy increases, so
thermal energy decreases.
If you stand nekkid on the Ohio plains at around four in the morning with
the wind gusting up your lallies, airspeed isn't increasing by air being
thrust through a narrow opening, but I'm willing to bet a penny or two that
you'd be having a pretty vivid experience of windchill. Wind gets its
kinetic energy from the rotation of the Earth and from effect of the Sun
heating the Earth, not by being forced through a small opening.
Besides -- if the Venturi effect explained windchill, then how come
windchill doesn't affect thermometers, causing them to read lower
temperatures? Or make water freeze above 0C when the wind is blowing like
mad?
(OK -- I'm guessing that about now, there are 700 people on this list who
are wishing that Hollywood produced better movies ;)
M.
- --
Martin Howard |
Visiting Scholar, CSEL, OSU | It is essentially contestable.
email: howard.390@osu.edu |
www: http://mvhoward.i.am/ +---------------------------------------