Stevenson Screen and instruments housed within
The Stevenson Screen is a standard housing for meteorological thermometers. The screen is naturally ventilated with double-louvred sides which serve to shield the instruments from direct sunlight and precipitation but allow an airflow over the sensors. The screen is painted white to reduce the effects of heating by solar radiation. The screen is located in as open a location as possible such that the thermometer sensors are 1.25m above the ground. As far as possible the ground cover beneath the screen should be short grass.
The screen was devised in 1864 by Thomas Stevenson (1818-1887), father of the writer Robert Louis Stevenson.
Inside the screen are a number of instruments and sensors
Starting top right in the right-hand picture and going clockwise
Temperature sensor (wireless) sending signal to receiver in top room of house
Davis Weather Monitor temperature and humidity sensor(cabled) sending signal display indoors and then to PC
Kestrel 4000 hand held weather tracker. This measures a range of weather parameters and is used for checking the accuracy of the other sensors.
Casella thermohygrograph. Something of an old timer this, having been overtaken by the flood of digital sensors now in use. The thermohygrograph records temperature and relative humidity. The temperature is measured by means of a bimetallic strip and humidity is measured by utilising the changes in the length of strands of human hair as humidity varies. All these changes are transferred by means of levers and pen arms to two fiber tipped pens on pen-arms which record the data on a chart wrapped round a rotating drum. The drum is geared to make a complete rotation in 7 days.
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