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 (
Tdp) from your
nearest NOAA Weather Station (within 25-30 miles):
RH = 100*[(173 - 0.1·T + Tdp)/(173 + 0.9·T)]8This 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=2.216343957.2086198372.1569365026-1660109167.1565985494NOTE: For temperatures in
ºC, change the two
173 constant values to
112 values.