Weather instruments such as the rain gauge and wind vane have to be sitting out in the weather in order to measure it properly, but the thermometers are kept inside the Stevenson Screen. Why?
To give useful measurements thermometers need to be sheltered from the rain and shaded from direct sunlight. If the thermometer was sitting outside, then on a partially cloudy day, the measurement would swing up and down as the sun moved in and out from behind the clouds. That wouldn’t be very helpful. What is needed is the measurement of the shade temperature, and the shade is provided by the Stevenson screen.
However, the screen itself is sitting out in the sun.
Let’s consider its design.
Firstly, it’s white. And they always are. No pinks or yellows or other striking colours to help you see them, nor green or brown to blend in with the environment and definitely no black. Why?
White is the colour used for all Stevenson screens since white does the best job at reflecting sunlight. Other colours would absorb greater amounts of the sun’s rays, causing the box to heat up. To keep a cool box, you need a white box.
But that isn’t all. The roof actually has a double layer. Between the two layers is an air gap so that if the outer roof heats up, the heat cannot easily be passed to the lower roof.
But although the thermometers are inside the box, they want to be measuring the outside temperature. In order for that to happen, the sides are not solid but made of angled slats. These allow the outside air to flow in, while at the same time keeping the rain out.
Thus, the Stevenson screen is carefully designed so that as far as possible the temperature inside is the same as the temperature outside.
By using the same standard design everywhere, readings can be compared, in the knowledge that differences are due to real differences in temperature and not irregularities in the siting of the instruments.