warm and cold winds

[Peter Gilberd]

Hi all

The following website quantifies Francis's statement that the KE of the air particles due to the wind is small compared with the KE due to their random motion:

http://www.rwc.uc.edu/koehler/biophys.2ed/diffusion.html

Wind velocity might be ~10 m / s. Random velocity of air particles is ~ 500 m / s.

There is also rotational and vibrational energy stored in molecules; for a diatomic molecule such as nitrogen this accounts for approximately half the molecule's energy.

Cheers
Peter
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Peter Gilberd
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The Royal Society of New Zealand
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-----Original Message-----
From: phys-teach-talk-bounces at nzip.org.nz [mailto:phys-teach-talk-bounces at nzip.org.nz] On Behalf Of Bryden, Francis
Sent: Friday, 9 May 2008 8:22 a.m.
To: David Housden; phys-teach-talk at nzip.org.nz
Subject: RE: warm and cold winds

Hi David

The way I think of it is..........

The temperature of the air (and the thermal energy your skin loses to it ) depends on the kinetic energy of the randomly moving particles.

In warm air the particles have greater KE and so the skin loses less thermal energy because there is a smaller temperature gradient.

I suspect the KE of the airparticles due to the wind is small compared with the KE due to their normal random motion.

(Wind has no effect on the temperature of a thermometer. Point a fan at a thermometer and the effect is negligible.)

 Wind has two effects on us.

 We are usually warmer than the air, and there is a layer of warm air near our skin that we have heated up, this will keep us warm. Any wind will move this warm layer away.

Because we sweat, wind will also cool us as it increases the rate of evaporation, taking thermal energy from our skin



Thanks ................ Francis

Francis Bryden
HoD Physics
St Cuthbert's College
122 Market Rd               or Box 26 020
Epsom                              Epsom
Auckland 1051                   Auckland 1344
New Zealand                      New Zealand

ph: 63 9 5204159 ext 7808

fbryden at stcuthberts.school.nz<mailto:fbryden at stcuthberts.school.nz>
-----Original Message-----
From: phys-teach-talk-bounces at nzip.org.nz [mailto:phys-teach-talk-bounces at nzip.org.nz] On Behalf Of David Housden
Sent: Friday, 9 May 2008 6:59 a.m.
To: phys-teach-talk at nzip.org.nz
Subject: FW: warm and cold winds

Hi all
I'm not sure this actually got sent to everyone so I will resend on David King's behalf.
Cheers
David Housden

From: phys-teach-talk-bounces at nzip.org.nz [mailto:phys-teach-talk-bounces at nzip.org.nz] On Behalf Of David King
Sent: Thursday, 8 May 2008 11:01 p.m.
To: PHYSICS TALK
Subject: warm and cold winds

Can someone help?

I got myself in a bit of a confuddle yesterday trying to answer a pupils question.

"Sir, why, if temperature is to do with Kinetic Energy is wind cold?"

I started on evaporation from the skin etc and the idea of us 'feeling' cold, when he asked

"Why do we have warm winds? What's the difference?"

I started to answer then stopped. What makes air particles travelling at whatever kph warm or cold?

Cheers


David King
Physics HoD
Christ's College
Private Bag 4900
Christchurch
New Zealand






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