The thermometer in the sun

Weather stations keep their thermometers inside a white box on legs, with slatted sides, standing at a fixed height above short grass. The box looks fussy. Every part of it is solving a measurement problem.

A thermometer in direct sun does not report the air temperature. It reports its own temperature, which is higher, because it is absorbing sunlight. The white paint reflects most of that sunlight and the slats keep direct rays off the instrument while letting air pass through, so the thermometer stays at the temperature of the moving air around it rather than its own.

The height matters for a different reason. Air close to the ground on a sunny day can be several degrees warmer than air at chest height, and on a clear night it can be several degrees colder. A reading taken at ankle level and one taken at head level are both real and they are not comparable. The standard height exists so that a measurement in one country means the same thing as a measurement in another.

The grass matters too. Concrete stores heat and releases it slowly into the evening, which is why a city is warmer at night than the fields outside it. A station over tarmac would measure the tarmac.

None of this makes the reading more accurate in a simple sense. It makes it comparable, which is a different and more valuable property. A hundred years of records are worth something only if every one of them was taken the same way.