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I am using MetPy to calculate relative humidity from dewpoint and temperature. The resulting data looks pretty good, with values between 0.23 and 0.95 (intuitively seems to be ok).

My problem is, there is no accurate information about which formula is used to calculate it in MetPy documentation. I only found this: https://unidata.github.io/MetPy/latest/api/generated/metpy.calc.relative_humidity_from_dewpoint.html
It says that the formula used is

e(Td)/e(T)

It is, actual vapor pressure divided by saturation vapor pressure.
The inputs of the function are temperature and dewpoint (not vapor pressure).

So, which formula is using MetPy to calculate it?

Thanks in advance! ^o^)b

Hanna Beatriz Wollmeister Muoz
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2 Answers2

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This is the correct formula. The e(Td) and e(T) represent the actual vapor pressure and the saturation vapor pressure respectively.

To calcualte the vapor pressure from temperature they use the following formula:

e(T) =  6.112 e^(17.67T/(T + 243.5))

where T is the temperature in degree celsius.

Mohammad
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The "See also" section of the doc page for relative_humidity_from_dewpoint points to the docs for saturation_vapor_pressure, which gives the proper formula from Bolton (1980):

Formula used by MetPy for saturation vapor pressure

DopplerShift
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