Does humidity increase or decrease after a heavy rainfall? If both are
possible, what determines the result? I would magine that rain,
especially in hot summers, creates humidity but others have told me
that nothing ends a humid spell better than a good rain storm.

nicolaig...

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."
http://chemistry.f.dictonarypage.co.uk/monsoon/

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."
http://chemistry.f.dictonarypage.co.uk/temperature+inversion/

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
rains. 

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:
http://www.usoe.k12.ut.us/curr/Science/core/4th/4thSciber/weather/html/humidity.htm


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
 ducting."
http://64.233.179.104/search?q=cache:xYblnBxl7WMJ:ccrane.com/news/distant-radio.08.20.01.aspx

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.


Please do not rate this answer until you are satisfied that  
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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
://www.google.com/search?q=humidity+*+after+a+rainstorm

monsoon "temperature inversion"
://www.google.com/search?q=monsoon+%22temperature+inversion%22

Thanks very much for the rating and the tip!

I wanted to expand a little on the answer you received.  Rain cools
the air as it falls, on a muggy day.  However, the addition of water
to the ground does indeed provide added humidity to the air, in stable
conditions.  For instance, we had storms tonight in Seattle, after
which the weather cleared and clear skies appeared.  However, the
water left on the ground attempts to evaporate, and combined with the
increased cooling of a clear sky at night, will induce fog.  Anyone
who lives in a humid climate will know what I'm talking about.

However, to the spirit of your question.  Rain does add humidity to
the ground, if it comes as isolated thunderstorms within a humid
airmass.  In the case of a common summer storm, water cooled by the
upper troposphere will rain down upon the hot land, lowering the
temperature of the land rapidly.  Now, if there is no wind afterward,
this water will seek to re-evaporate, and the dewpoint will rise to at
least the level it was before the storm, as the added rain increases
the water budget of the airmass.  So it will be cooler and feel
muggier.  However, if the airmass is unstable, everything changes. 
Say that instead of a garden variety thunderstorm, a line of storms
forms along a cold front.  The denser air coming from the north sweeps
up hot humid air as it pushes south, causing uplift, and thus the
storms.  In *this* case, the storm is an effect of changing airmasses.
 After the storms, the wind direction can be expected to change from
roughly southerly to northerly, the barometer will rise, and the wind
from the high pressure in the new airmass will quickly dry out the
humidity that the rain provided.  In this case, the storm truly does
bring relief, as opposed to simply lowering the temperature.

So, the short answer is that rain itself does little to affect
humidity within a certain airmass, but when it comes as a harbinger of
a cold front, you can expect the dewpoint to take a dive.

I should also point out that in cooler times of year, a warm front
preceding a low pressure system will rapidly raise the dewpoint,
contributing to the effects of the rain on humidity, except you won't
be able to feel it.  Warm fronts in summer are by and large dry
events...but this shows that frontal systems exhibit both kinds of
influence.