Measuring weather temperature

[matt_london1]

On saturday I put my thermometer in the garden to see how warm it was. I left it in the sun for 5 mins and when I checked it, it was on the maximum - about 48C!!!
Now, I know it wasn't that warm so why did my thermometer show that?

[New Judge]

Because the temperatures you are provided with in weather forecasts and reports are shade air temperatures.

They are taken in a carefully controlled shaded environment where the effect of the sun's direct radiation cannot have an effect.

[Clanad]

As a segue into answering your question; gardener's are often warned against watering their plants during later daylight hours, especially by sprinkling. One of the reasons is that droplets of water can cling to the plant's leaves and actually burn them. This is caused by the sun's rays being magnified just as we did when we were kids using Uncle Filbert" magnifying glass. Same holds true with any thermometer that has a reflective base to which it's attached. An accurate air temperature thermometer should be placed in the shade in a free flowing air environment. The little white shacks with louvered sides one sees at weather reporting stations (the old fashioned kind) contain both the dry and wet bulb thermometers enabling them to aslo give you the dew point...

[Clanad]

Apologies, New Judge. ... you weren't there when I started...

[matt_london1]

So why does the reading show higher than the actual air temperature?
Why isn't the air as hot as the reading?

[Teddio]

The thermometer bulb absorbs infrared radiation from the Sun and its temperature rises.
Eventually the thermometer bulb will reach an equilibrium temperature where the infrared received from the Sun equals the amount of infrared radiated by the thermometer bulb.

[mibn2cweus]

The atmosphere is relatively transparent to the Sun's radiation and is actually heated mostly from re-radiation at longer wavelengths of energy absorbed by the Earth's surface.

atmospheric heating