Ask A Scientist Weather Archive

name         Craig S.
status       other
age          30s

Question -   My children of 6 and 7 asked me why a thermometer shows
a higher reading in direct sun light and lower in the shade. Also they ask
me on how the true outside temperature is determined if the sun or shade
effects its reading? And now they have me wondering.
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A thermometer in sunlight absorbs infrared radiation which is a component
of sunlight. Infrared radiation is "heat" radiation. It is what makes you
feel warmer when you stand in sunlight compared to standing in the shade. In
addition, the thermometer absorbs some visible light a portion of which is
converted to heat by the thermometer material. The thermometer is "feeling"
the same effect that you do when standing in sunlight compared to standing
in shade. There really is no "true" temperature, because, as you have
observed it depends upon where you put the thermometer. I think that
meteorologists (people who study and try to predict weather) put their
thermometer in a "box" that has baffles so that air can circulate freely,
but blocks any wind, which also can affect the temperature reading of the
thermometer. The "box" is constructed so that heat cannot build up, which
will make the temperature very high -- think about how hot it feels when you
get into a car in the summer when it has been standing in the sun!

Vince Calder
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Craig,

When the thermometer is in the shade, it is reading the temperature of the
air around it. When it's in direct sunlight, in addition to responding to
the air temperature, it is soaking up radiant energy from the light that
falls upon it. That part of the light which is absorbed by the thermometer
will be converted into heat in the thermometer itself. That heat energy,
plus that gained from the nearby air results in the higher direct sunlight
reading.

Try an experiment: Measure the temperature in sun and shade with a
thermometer that can be (alternately) left as is and then covered with black
cardboard -- or better yet, painted black at its sensing end. The unclad
thermometer should always provide the lower temperature.

Most weather-station temperature sensors are housed in ventilated enclosures
to shade their sensor from direct sunlight, thereby allowing them to measure
the temperature in the shade.

Regards,
ProfHoff 449
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Craig,

The true outside temperature is always taken in the shade.

The thermometer used for official records is also aspirated; air is passed 
over
it at a specific speed and in a shield to prevent sunlight from heating the
thermometer.  Solar radiation from the Sun heats up anything, including a
thermometer that is in the Sun, just as it heats up your body.  The Sun's 
energy
is absorbed by the thermometer, causing it to heat up, thus showing a higher
temperature than the true air temperature.

Try standing in the sunlight as opposed to standing in the shade and you'll
experience why thermometers read higher when exposed to the Sun.


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

   As you have observed, the thermometer does not directly display the
temperature of the air at all, rather it shows the temperature of the
sensing element, usually metal or glass. When the sensing element is placed
in the sunlight, it is warmed considerably above the ambient air
temperature. When the sensor is placed in the shade, it is only warmed by
the air. That is why the air temperature is measured by thermometers placed
in the shade.

   Thermometers in the shade be warmed by structures close to the sensor that
radiate heat to the sensor. Thermometers should be placed over grass
surfaces, not over concrete or asphalt, and away from buildings and brick or
stone walls.

   Here is a link to an interesting, and non-technical explanation of how
thermometers work.  There are instructions on how you and your children can
build a thermometer from items you have in your house.

   http://www.howstuffworks.com/therm.htm

That was a VERY good question from your children...!  We might have some
future meteorologists there..!

Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
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