Apparent Temperature and Humidity

name         Katlyn
status       student
grade        9-12
location     KY

Question -   Why does it feel hotter when it is more humid?
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Katlyn -

Have you ever notice that land that is near a large body of water 
tends to not get as hot or a cold as similar areas that are 
inland?  That is part of the answer to your question.  Water is a 
great "dampener" of temperature change (and you judge the 
temperature of air by how much it cools you).  The dampening effect 
is because water can take in and give off a great deal of energy 
(heat is energy) without changing temperature much.  Air that blows 
across your skin will change temperature (i.e. make you feel cooler) 
when it changes temperature faster.  Dry air will change temperature 
faster.  If you want to know more about this read about specific heat.

Perhaps part of your answer is related to how hard you work to take 
in the oxygen you need.  The more space in the air that is taken up 
by water, the less space is available for oxygen.  You need to work 
harder to breath in humid air.  We even sometime talk about humid 
air as being heavy, because it requires more work to breath in humid 
conditions.

In either case, it is not actually hotter, but rather it is how you sense it.

Larry Krengel
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From a physical point of view, the rate of evaporation of perspiration
(water) is slower the higher the humidity, so the body's "cooling" 
mechanism (evaporation) is slowed. Since the heat of evaporation of 
water is roughly 500 cal / gm (which is quite large) the lower the 
humidity the more cooling takes place on the surface of the skin.

Vince Calder
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Katlyn,

Sweating helps to keep your skin temperature from getting
too high. The water that you sweat from your skin surface
evaporates.  It takes energy to evaporate water, therefore
sweating takes heat away from your skin, transferring it
into the air.  However, the rate at which the sweat can
evaporate into the air depends on how much water vapor
there is in the air (water vapor density, an absolute measure
of water content) and how close to saturated with water
the air is (relative humidity).  When the water vapor
density and/or relative humidity are high, sweat can
be evaporated from your skin less rapidly than if the
air is "dry" (lower water vapor density and/or relative
humidity).  If the relative humidity is high, less sweat
evaporates, less heat is taken away from your skin, and
so the air temperature appears to be higher than on a
"dry" day.


David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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