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Temperature Measurement
Name: Martha
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: N/A
Question:
Why is climatic temperature measured in the shade? I
live in the
desert and when they say it is 115 degrees out, it is acutally much
hotter! (but it doesn't matter because it's a DRY heat!!) Wouldn't
temperature measured in the sun be more accurate? Thank you!
Replies:
The reason you're hotter in the sun is that your clothes and skin are
absorbing energy directly from the sunlight. Temperatures measured in the
shade measure the air temperature. Measuring temperatures in the sun,
different thermometers would give different readings, depending on what
color they are, what their heat capacity is, etc. Basically, you'd need to
define a standard thermometer with specific light-absorbing properties (to
be similar to human skin? What shade?) in order for temperatures measured
in the sun to really mean anything. Perhaps a more meaningful report would
be to give both the temperature in the shade and some measure of solar
intensity, such as watts per square meter.
Richard Barrans Jr., Ph.D.
Chemical Separations Group
Chemistry Division CHM/200
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
Dear Martha-
Your question about how we measure the air temperature is a good one. And
you are right in that when you are outside in the summertime in the
sunshine, it certainly "feels" warmer than the air temperature. But, in
order for temperature data to be useful to weather forecasters, it must be
"standardized" as much as possible, so the values can be compared with
adjacent locations, or other locations around the world.
The ideal or "standard" temperature observing site has the thermometers
located about 2 meters above the ground, in a shelter box that allows free
circulation of the air, situated over a grassy area. This type of exposure
will provide the "truest" temperature of the air. Now, many of our
observations sites do not fit the standard, and this can. and does in many
instances, skew the data.
If we took the temperature in the sunshine, the actual reading would be
something other than the true air temperature, because the sunshine would
heat the thermometer as well as the air heating it. And the intensity of the
sunshine is constantly changing, whether by clouds in the sky, or the time
of the year. And temperature readings made at night would be different also.
Any sort of standardization or comparison would be impossible.
I hope this explanation has answered your question. Thanks for visiting
NEWTON, and for your question.
Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
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Update: June 2012
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