Here's how it works. On sunny days with clear skies, there's
nothing to prevent any moisture at the surface of the earth
from being evaporated quickly and easily into the open air.
This is why, in Arizona, where I live, we say "it's a dry
heat", because, although the temperatures are very high,
your perspiration evaporates so quickly that you hardly
know you're sweating (but of course, you are, which is what
helps you feel cooler in spite of all that heat).
This all changes during our monsoon season. Here's a good
definition of monsoon from The Dictionary Page in the UK:
"Heavy winds characterized by a pronounced seasonal change
in direction. Winds usually blow from land to sea in the
winter, while in the summer, the flow reverses and
precipitation is more common. Monsoons are most typical
in India and southern Asia."
This is often combined with the effects of a temperature
inversion, also well-defined on The Dictionary Page:
"The abnormal reversal of temperature in the troposphere
caused by the entrapment of urban air pollution. Under
normal circumstances, air in the troposphere is cold at
high altitudes and warm at low altitudes, or near the
earth's surface. When air pollutants, such as SO2 or
NOx, in urban areas get too concentrated, smog is
produced. Since cold air underlies the warmer air at
high altitudes, the air does not mix well which can
lead to human health disturbances in people with
respiratory illnesses such as asthma."
The result is a period of cloudy days where the
temperature is still fairly high, but the water which
is evaporating from the surface can't dissipate into
open air. Rather it collects in the air beneath the
clouds and gathers in the clouds, as well. This makes
for very humid air, but not as humid as when it finally
Before it rains, the temperature is still high enough to
promote some evaporation. Once it rains, the temperature
drops considerably. Combine the low temperature with the
huge amount of water on the surface and in the air, and
you have almost no evaporation, and the humidity is as
close as it can get to 100%. Since the temperature has
dropped, you may still feel that it is cooler, but it
will be very easy to perspire, and very hard for that
perspiration to evaporate.
Here's a simple explanation of relative humidity on a page
from the State of Utah's Office of Education online that
even teaches you how to make a hygrometer of your own to
measure relative humidity. Knowing this will help you to
understand what evaporation has to do with relative humidity:
DX radio enthusiasts, who get a thrill at contacting
far away AM radio stations, get excited by this higher
humidity, because it assists them in contacting stations
that are further away, per this cached page from CCrane:
"If you?ve ever kept your radio on through a rain storm,
and then kept on listening after the clouds subsided,
and the leaves stopped fluttering, you might have
noticed a sudden increase in radio reception. DX radio
fans, who thrill at the challenge of pulling in distant
radio signals, look forward to the end of a rain storm
when lightening static stops. That?s when the recent
rainfall increases the relative humidity at the surface,
and the temperature drops several degrees. The combination
of an increase in humidity and a decrease in temperature
at the surface results in a phenomenon called tropospheric
You can learn more about the troposphere, stratosphere and
ionosphere on that page, as well as distant radio reception.
Those who have told you that nothing ends a humid spell
better than a good storm aren't entirely incorrect. It's
just that the relief from the high humidity usually comes
a day or so after the rain stops. If the clouds have spent
themselves or the wind has moved the remainder away, the
sky will once again be clear, and the water on the surface
will quickly evaporate, producing a period of lower humidity
until the next clouds appear on the horizon.
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A user's guide on this topic is on skermit-ga's site, here:
Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.
Searches done, via Google:
humidity * after a rainstorm
monsoon "temperature inversion"