WeatherTalk: The temperature in sunshine is not what you think it is
Air is not heated up very much by sunlight.
FARGO — It seems simple enough to hold a thermometer out in the sun in order to get the temperature “in the sun,” but this idea is all wrong. Air is not heated up very much by sunlight. The rays pass right through. Air warms up in sunlight mainly because the sun’s rays heat the ground and that heat rises up into the air via conduction and convection. A thermometer bulb in the sun will heat up dramatically past the ambient air temperature because it is a solid object and is warmed directly by the sun’s rays. You always have to measure air temperature in the shade.
If you want to measure the temperature of the ground or pavement, then you have to stick a thermometer into the ground and out of the sun or measure the infra-red radiation emanating from the ground with a hand-held infra-red thermometer. A standard thermometer in the sun is only measuring the temperature of the thermometer bulb.