It wasn't apparent to me from the BAWUG list whether you ever got an answer to your original question. It is, for f<100 GHz
n = 77.6 P/T + 3.7E5 e/T^2
where P is atmospheric pressure in mbar, T is in Kelvin (thus P/T is a density measure), and e is the partial pressure of water vapor in mbar. A bar is 1.01 standard atmospheres (760 Torr = 760 mm of mercury) or 1E5 Pascals, and a mbar is of course 1/1000 of a bar.
This formula comes from the very useful book Microwave Radio Transmission Guide, by Trevor Manning, Artech House 1999. (Note that the formula is unlikely to be accurate at frequencies where strong absorbtions exist, i.e. around 24 GHz or 60 GHz, etc.)
Daniel M. Dobkin Enigmatics 1-408-314-2769 [EMAIL PROTECTED]
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