Topic: Calculating Relative Humidity from dry and wet bulb temps and BP

[SLOweather]

For my weather station project, I've been looking for the equation for this and found several references. Here's one:

From http://maxwellsci.com/print/rjaset/v6-2984-2987.pdf



the expanded formula containing all of the intermediate equations is:



For the intermediate stuff, see the source PDF.

That will be fun to program!

Interestingly, a lot of sources use the mean barometric pressure of 1013.25024 mb instead of the current local reading. I think I'll build that equation in Excel and then see how much difference the BP makes.

[nincehelser]

Interestingly, a lot of sources use the mean barometric pressure of 1013.25024 mb instead of the current local reading. I think I'll build that equation in Excel and then see how much difference the BP makes.

Also, might the pressure be "absolute" rather than a sea-level pressure?

[SLOweather]

Also, might the pressure be "absolute" rather than a sea-level pressure?

The reference cited says:

When P is the mean atmospheric pressure (assumed to be 1013.25024 mb)...

     

Emphasis mine... I have seen other references with the same assumption, including one, I believe, from the NWS.

[nincehelser]

I think what they're doing there with that assumption is to reduce it down to a simpler calculation.

In physics the absolute pressure at the location would be used, but perhaps it doesn't make much difference in the meteorological world.