Effect of Radiation on Thermometers
The air temperature in sunshine is practically the same as that in the shade; however, the thermometers with which we measure the air temperature must be carefully protected from radiation. Thermometers absorb a considerable amount of radiation. For this reason, a thermometer exposed to the air and to sunlight will give a temperature reading that is higher than the air temperature. The amount depends on the thermometer.
Take two or more thermometers of different types, such as a dial thermometer (bimetal), an ordinary thermometer mounted on metal or plastic, or one of each, and a window thermometer in a glass or plastic tube. First expose the thermometers in the shade. Avoid radiation reflected from walls or the ground, and record the temperatures of each thermometer. The readings should be all the same, but, because of inaccuracies in calibration, there may be slight differences.
Expose all thermometers fully and equally to sunshine for a few minutes and read again. If there is a wind, make sure that all thermometers receive the same amount of ventilation. In the event no sunshine is available, use a 150 to 200 watt light bulb as a radiation source. Lay the thermometers on a table propped up in such a way that the thermometer bulbs are about 2" from each other and the table surface. The light bulb is placed about a foot above the thermometer bulbs so that its distance from each is the same. Take readings before turning on the light and again 5 minutes after turning it on.
Material: Thermometers of different types.
One 150 to 200 watt light bulb as radiation source if needed.