Published: 2007-03-12 13:28:37
Updated: 2007-03-12 13:28:37
Posted March 12, 2007
By Sandy Demeree
MIKE MOSS SAYS: Sandy, You've run into a nice demonstration of the reason "official" temperature readings are taken in a well-ventilated shelter designed to prevent direct sunlight from shining on the sensor. The idea with a thermometer for weather purposes is to measure as accurately as possible the temperature of the air, ideally at about 4-5 feet above a grassy surface and in the shade. In those conditions, the thermometer will take on the same temperature as the air around it. However, in direct sunlight, the solar radiation striking the thermometer will be absorbed by the thermometer itself and can easily become much warmer than the surrounding air due to the radiant energy it is receiving. If you've ever been out on a cool, sunny day and found it comfortably warm in the sun but uncomfortably cool in a shady spot, you've experienced the same effect, as the air at both locations was probably about the same temperature, but your body was being heated to a notable degree while you were in sunlight - same goes for standing by an active fireplace in a chilly room. It makes a big difference if someone steps in front of you and blocks the radiant heat.
How much warmer than the air a thermometer will become in direct sunlight depends on the type of thermometer and what kind of housing it's in, and can vary from just several degrees up to several tens of degrees, as you experienced.
Incidentally, the opposite effect can happen at night. An unsheltered thermometer is capable of losing energy by way of long wave infrared radiation, and by that means can become considerably cooler than the air around it if it is exposed to open sky, especially on a clear night with light winds. For this reason, it is almost as important to have a properly sheltered sensor at night as it is in the daytime.