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Name: Leah
Status: educator
Age:  20s
Location: N/A
Country: N/A
Date: Tuesday, May 21, 2002


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?


Replies:



Replies:
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


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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