Topic: Estimating RH from Temp and Dewpoint

[Old Tele man]

When you suspect that the RH numbers from your outdoor sensor are not what you think they should be, here is a fairly reliable method for estimating what the relative humidity (RH) value SHOULD BE using: (a) your outside ambient air temperature (T) and (b) known-good dewpoint temperature (T_dp) from your nearest NOAA Weather Station (within 25-30 miles):

RH = 100*[(173 - 0.1·T + T_dp)/(173 + 0.9·T)]⁸

This is a simple approximation equation that is quite accurate. It "works" because dewpoint temperature does not vary with air temperature as relative humidity does (humidity roughly halves with each 20ºF increase in temperature). Thus, dewpoint temperature is relatively stable across large geographic areas.

Using YOUR ambient air temperature and the known-good NWS dewpoint temperature enables you to estimate what the RH should be at your location. This "check" process enables you to learn how accurate (or not) your weather stations outdoor RH-sensor is, because many older RH-sensors tend to "flatten-out/compress" their high (>80%) and low (<30%) relative humidity readings.

CREDIT - this approximation equation was developed by Mr. Julius F. Bosen of the National Weather Center, NC, and was first published in the December 1958-issue of MONTHLY WEATHER REVIEW, Vol. 86, No. 12, page 486:
• http://docs.lib.noaa.gov/rescue/mwr/086/mwr-086-12-0486.pdf
• https://pdfs.semanticscholar.org/d46b/8d55db12fc734ed01092636149fe059c8a92.pdf?_ga=2.216343957.2086198372.1569365026-1660109167.1565985494

NOTE: For temperatures in ºC, change the two 173 constant values to 112 values.

[galfert]

I wrote a little program in Python:

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Install Python and then run this code. I saved it as RH.test.py

Code: [Select]

print('RH.test version 0.6')
print('This program helps you determine if your humidity sensor is reasonably okay.\n')

T=float(input('What is your temperature in °F?: '))   #Your temperature
Tdp=float(input('What is local METAR dewpoint in °F?: '))   #Dewpoint of METAR

print('\nRH=' + str(round(100*((173-0.1*T+Tdp)/(173+0.9*T))**8,0)) + "%")
print('If your humidity sensor differs too much from this value then it may be bad.\n')

print('Brought to you by galfert and Old Tele man\n')

print('CREDIT - this approximation equation was developed by Mr. Julius F. Bosen of the National Weather Center, NC, and was first published in the December 1958-issue of MONTHLY WEATHER REVIEW, Vol. 86, No. 12, page 486:')
print('http://docs.lib.noaa.gov/rescue/mwr/086/mwr-086-12-0486.pdf\n')

input('Press Enter to close...')


See attached, download and rename to remove the .txt so that it ends in .py
(yes you need to download and install Python from python.org for this to work)

[ValentineWeather]

http://www.dpcalc.org/  has a handy online calculator too, for the mathematically challenged.