### Author Topic: Calculating Relative Humidity from dry and wet bulb temps and BP  (Read 3303 times)

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#### SLOweather

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##### Calculating Relative Humidity from dry and wet bulb temps and BP
« on: March 22, 2016, 02:09:30 PM »
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

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##### Re: Calculating Relative Humidity from dry and wet bulb temps and BP
« Reply #1 on: March 22, 2016, 02:42:25 PM »
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

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##### Re: Calculating Relative Humidity from dry and wet bulb temps and BP
« Reply #2 on: March 22, 2016, 02:55:47 PM »

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

The reference cited says:

Quote

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

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##### Re: Calculating Relative Humidity from dry and wet bulb temps and BP
« Reply #3 on: March 22, 2016, 03:27:31 PM »
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.

anything