In message , Will
writes

Norman Lynagh wrote in message ...
In message , Will
writes

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This posting expresses the personal view and opinions of the author.
Something which everyone on this planet should be able to do.
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Norman, I use Teten's formula in my excel spreadsheets to derive
dewpoint from
T/RH
http://www.paroscientific.com/dewpoint.htm

You'll have to a little bit of schoolboy maths to transpose the equation to
get
RH from T/Td !

Will.

Many thanks, Will. That takes me part of the way but it doesn't cope
with dew points below 0c. Many of the values I am dealing with are below
0c.

Norman.
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OK Norman. back to basics.

If you want high accuracy for -35 to +35 C
use our old friend Clausius Clapeyron for calculating saturated and vapour
pressu

Es = 0.6112 x exp(17.67T/(T+243.5)) T in deg C
E = 0.6112 x exp(17.67Td/(Td+243.5)) Td in deg C
RH = E/Es

Will.

Excellent, Will. Many thanks.

Norman.
(delete "thisbit" twice to e-mail)
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Norman Lynagh Weather Consultancy
Chalfont St Giles
England