cc: m.hulme@uea
date: Wed, 01 Nov 2000 13:59:05 +0000
from: Tim Mitchell <t.mitchell@uea.ac.uk>
subject: Re: quick questions
to: Sari Kovats <Sari.Kovats@lshtm.ac.uk>

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>How is relative humidity calculated?
>And is wetbulb temperature a good measure of humidity?

Sari,

The questions are quick, but the answers ain't necessarily so!

Moisture is a complicated animal. It can be expressed in a number of 
different ways, and the way that you express it tells you something 
different about what is happening in he atmosphere. I'm not sure what 
exactly you are aiming at. Rather than my simply explaining all the 
conceivable moisture indices, it would probably be better if you told me 
what you wanted a moisture index for. Then I might be able to tell you what 
the best measure might be. For example, wet-bulb temperature tells you 
something valuable about moisture, but usually only in conjunction with 
other variables.

Relative humidity (r) expresses the actual moisture content of a sample of 
air as a percentage of what that sample would contain at that temperature 
if it were saturated. It is defined in terms of the mixing ratio. The 
mixing ratio (x) is the mass of water vapour (g per kg of dry air). The 
definition is thus:
r = 100 * x (actual) / x (saturated)

So tell me what you are trying to get information about, and the 
meteorological variables you have in your possession, and I'll get back to 
you...

Tim



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Tim Mitchell

post:   CRU, UEA, Norwich, NR4 7TJ, UK
email:  t.mitchell@uea.ac.uk
web:    http://www.uea.ac.uk/~f709762
phone:  +44 (0)1603 593161 (direct)
fax:    +44 (0)1603 507784 (office)
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