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Ask Eric: Are Temperatures Predicted For Sun Or Shade?

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(Photo credit:  FREDERIC J. BROWN/AFP/Getty Images)

(Photo credit: FREDERIC J. BROWN/AFP/Getty Images)

WBZ-TV's Eric Fisher Eric Fisher
Eric Fisher is Chief Meteorologist for CBS Boston’s WBZ-TV News and...
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Are forecast or posted temperatures for what has occurred or is predicted to occur in the sun or in the shade? What is usually the difference between the two?
Gina in Sterling  

Gina – When we talk about temperatures on a map or in a forecast, we’re always referring to the temperature of the air in the shade. We do that for a couple of reasons. The first is that you want it to be standardized, so that 50 degrees here is the same as 50 degrees anywhere else. The other reason is so that we can use it in computer model data to help forecast the weather. If the surface observation isn’t precise, then we’ll get bad data when trying to compare it to the mid/upper levels of the atmosphere.

Old fashioned weather stations were set up in white huts for backyard weather spotters (white so that the sun’s light would reflect off of it and little would be absorbed, to avoid contaminating the readings). Nowadays, most reporting stations are automated (ASOS – automated surface observing system) but include shields to protect the instruments from having the sun beat down on them.

The main difference between a shade temperature and a sun temperature is that when you are measuring in the sun, you’re not really measuring the air. You’re measuring the temperature of something that’s absorbing the sun’s energy. For instance, if you put your thermometer on deck of your house and attached to a wall, you will be measuring the temperature of the wall/thermometer itself which is absorbing the sun’s radiation. If it’s on the surface of, say, a NASCAR track, you’re measuring all that heat being absorbed by the black asphalt. These numbers are also important (the NASCAR driver will want to know how hot the track is so he/she will be able to figure out gas mileage, wear on the tires, pressure, etc). But for weather purposes, it’s air only!

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