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