Relative humidity fall crocus
Thu, 12 Aug 2004 12:54:05 PDT
Jane McGary wrote:
>True; but the nights, at least at my home (which is about 1000 feet
>higher in elevation than Dave's) are in the fifties F (10-15 degrees C),
>and the humidity has mostly been below 20%

Hi All,

This is not an attempt to pick on Jane at all, just a chance to bring forward
a better way to express humidity.

When Jane says 20% humidity, I ask myself at what temperature is this
occcuring?  Relative humidity varies throughout the day, 100% or close if
dew is formed overnight and dropping as the temperature rises during the
day, assuming no airmass change.

Dew point is a better measure of humidity (absolute) than RH and is not
expressed as a percentage, but rather the temperature at which the
condensation rate exceeds the evaporation rate.

The case for 100% or near 100% relative humidity overnight is common for many
locations when winds are light and cloud cover is not extensive. Without the sun
to heat it up, the air temperature in many areas areas cools until it reaches
the dew point. At this point further cooling is difficult, because a whole
lot of heat is released by the water vapor condensing into liquid (dew), or ice (frost) if it is cold enough.

With overnight lows in the 50's, Jane's dew point is probably in the 50's
and with a daily maximum RH of 100%.

For more information see:…

E-mail me if you want a demostration.


Jay Yourch

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