Calculate humidity for METAR data

[bergie]

Needed for the forecaster. Formula:

Tc = Temperature in degrees C
Tdc is dewpoint in degrees C

First calcutate saturation vapor pressure (Es) and actual vapor pressure
(E):

Es=6.11*10.0**(7.5*Td/(237.7+Td))
E=6.11*10.0**(7.5*Tdc/(237.7+Tdc))

Relative humidity then becomes (E/Es)*100

https://groups.google.com/forum/#!topic/uk.sci.weather/sIh6-ZV9GjI

[bergie]

Example https://github.com/c-base/c-flo/blob/master/components/AirportWeather.coffee