How hard can it be to measure the temperature
of the air? Surely it's just a case of holding a thermometer out in the open
and reading off a number?
In fact, it is surprisingly difficult to get a
consistent, accurate measure of air temperature. Put a thermometer in the shade
and it will give one figure. Move it into the sunshine and it will rocket up
several degrees. The reading will change if you move it closer to a wall, high
above your head or down on the ground.
Met Office
If you want to compare
temperature records over a long period of time or in different places then it
is essential to standardise the way the readings are taken, so that you are
comparing like with like. Weather stations around the world keep their
thermometers (mostly digital rather than mercury and glass these days) in a
white-painted box with slatted sides, designed to allow the air to flow through
but keep direct sunshine out. The most widely used design is known as a
Stevenson screen, which is mounted at an agreed standard height of between 1.25m
and 2m, in an open site well away from trees and buildings.
So when temperatures
are soaring and the car thermometer is going off the scale, that's telling you
a lot about the heat capacity of sun-baked metal but you won't be able to claim
a new record.
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