Ask A Scientist Weather Archive

name         Leah
status       educator
age          20s

Question -   I have heard conflicting reports on the effect of
humidity on low temperatures.  I have heard both that humidity at cold
temperatures can make it feel colder (because humid air conducts heat
way from your skin) and also that humidity has nothing to do with how
old it feels (which seems to make more sense to me).  Can you help?
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Leah,

To answer your question, let me first say something about heat indices.

The effects of higher relative or absolute humidity on temperatures of
80 degrees F and above have been estimated with several indices, alternately
called the "Apparent Temperature" or "Heat Index".  Many variations of this
index have been devised, with a more realistic and scientifically based
version having been recently adopted by the National Weather Service.

During cold temperatures the "Wind Chill" has been used, but humidity has
normally been ignored when trying to determine how cold it feels.

In our training at Penn State, one of our professors declared that high
humidity makes it seem colder when the temperature is below 53 degrees F and
warmer when the temperature is above 53 degrees F. This makes sense if you
extrapolate the heat index chart downward to cooler temperatures.  I also can
attest to the cooling effect of high humidity at cool temperatures from 
working
outside in a wide variety of humidity conditions during cold weather
(including Barrow, Alaska).  Because of this experience, I have always said
that the most uncomfortable conditions to work in outside are with the
temperature right around freezing and the relative humidity at near 
100%.  However, this has
always been a subjective thing and not scientifically well determined.

AccuWeather has done us a great service by coming up with an index that
determines the effect of humidity (as well as other parameters) for both cold
and warm temperatures, called "RealFeel".  It also takes into 
consideration other
effects, such as cloudiness, radiation intensity, wind speed, etc.
Unfortunately, because this is a patented product, RealFeel is only 
available by subscribing to
AccuWeather's premier products service, which is well worth it if you need
detailed information for planning purposes on a routine basis.


David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Dear Leah-

   Humidity "might" have some noticeable effect on the sensation of cold, but
not NEARLY the effect that wind has on sensible temperatures.  An increase
in wind speed at low temperatures can make it "feel" MUCH colder than the
actual temperature.

   Humidity has a much more noticeable effect at high temperatures, because
high humidity with high temperatures, retards evaporation of perspiration
from our bodies, and it "feels" much warmer than the actual temperature
suggests.

As a rule, wind is the major factor in sensible temperature at low readings,
and humidity is the major factor at high temperature values.

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
=============================================================
"Feeling colder" is sensory perception that may or may not have any relation
to measurable variables. In addition, it is important to specify "How cold
is cold?"-- 30 F vs. 50 F or -30 F vs. +30 F. It makes a big difference.
Moisture produced by the body permeates clothing from the inside out. If the
relative humidity is high that moisture will condense in the fibers of the
sweater, coat, parka, etc. This would compromise the insulating value of the
covering, because liquid water certainly conducts heat better than vapor and
insulating clothes depend upon "dead air spaces" to provide insulation. This
is also related to the use of hydrophobic fibers like polypropylene in
insulating garments next to the skin. These fibers do not absorb moisture and
so allow the protective garments to "breathe". And of course, wind speed
plays an important role that is often convoluted with relative humidity. I
realize that this response may not answer your question, but I am not sure
that the humidity variable can be isolated from others that are occurring
simultaneously.

Vince Calder
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