Cumulus Support

Hi Steve, I sometimes notice that the 3 weather programs I am running, Cumulus and 2 others, give different dew point readings. This only happens when "Calculate dewpoint" is selected in Cumulus.

As an example, just before it was 25.3°C at 55% humidity - Cumulus said DP 15.2, the other 2 programs said 15.6. I looked up 2 online calculators which both gave me 15.6.

Seems a bit strange, shouldn't there be only one formula for dewpoint? Im very curious about this...

There are dozens of different formulae for dew point. Cumulus uses this very simple one:

dewpoint := TempinC + ((0.13 * TempinC) + 13.6) * Ln(humidity / 100);

Mine is current showing minus 1.8C with a temp of 1.6C and an RH of 78%. I switched over to the Davis value and it showed the same; it probably depends on the range of input data.

steve wrote: it probably depends on the range of input data.

Yes it does. There are conditions when the difference is minimal, and sometimes up to 0.5C difference.
Thanks for the reply.

I've found the formula used by Davis. According to them, it's an approximation to the formula recommended by the WMO, so I'll switch to using that.