VIRTUAL TOUR OF THE WEATHER STATION
THE STEVENSON SCREEN
The Stevenson screen contains instruments for determining temperature (current, maximum and minimum), relative humidity and evaporation. Positioned centrally are 4 thermometers - two vertically (the dry bulb thermometer on the left to record current air temperature, and the wet bulb thermometer with its bulb kept moist by a muslin wick on the right used to calculate relative humidity) and two horizontally (the maximum thermometer on top to record the highest temperature since the last observation, and the minimum thermometer below to similarly record the lowest temperature). On the far left is a hygrograph (for continuous monitoring of relative humidity) and on the far right a thermograph (recording temperature). In front of the thermograph on the right is piché evaporation tube (read each day to determine water loss due to evaporation).
The Stevenson screen is opened once a day at 0900 GMT for observations of temperature, humidity and evaporation to be made. To prevent direct sunshine from affecting the readings when the screen is opened, the door faces due north. The whole assembly is painted white to reflect incoming solar radiation from the sun and the sides are louvred to allow the air to circulate over the thermometers. The screen itself is raised on a tall stand to reduce the impact of radiation from the surface.