cc: m.hulme@uea date: Wed, 01 Nov 2000 13:59:05 +0000 from: Tim Mitchell subject: Re: quick questions to: Sari Kovats >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 /// \\\ ( 0 0 ) ____oOO (_) OOo_________________________ 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) __________________________________________