(Temperature - Dew point) x 400 has already been mentioned.
There is also a simple approximation for T-DP = (100-Relative Humidity) / 5
Dew point - Wikipedia, the free encyclopedia
So with the answer in feet you can use 80 x (100 - RH)
Some weather forecasts (eg the BBC for the UK) give RH estimates several days ahead, so this can provide a rough and ready longer forecast. Eg should I cancel something four days from now because I might finally do some night flying.
Sometimes it even works!
