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Name: Craig S.
Status: other
Age:  30s
Location: N/A
Country: N/A
Date: Wednesday, July 24, 2002


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.


Replies:
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


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


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


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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