Inside the Stevenson Screen on Cairngorm Mountain

We had a peek inside the Stevenson Screen by the ski station on Cairngorm. The screen is designed to ensure that air conditions inside are the same as those outside minus wind and sun. Inside was a wet and dry thermometer. The bulb of one thermometer is wrapped in fabric which is dipped into a pot of water. This wicks up the water to keep the bulb wet. As water evaporates from the wick, it takes energy from the thermometer thus lowering its temperature. Evaporation happens best in dry air and is more difficult in humid air because the air is able to carry less extra vapour. So by comparing the temperatures on the two thermometers, you can tell the humidity. In really humid conditions the temperatures will be the same but in really dry conditions the wet bulb thermometer will have a lower temperature.