Stevenson screen

A Stevenson screen or instrument shelter is a shelter or an enclosure to shield meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard weather station. The Stevenson screen holds instruments that may include thermometers (ordinary, maximum/minimum), a hygrometer, a psychrometer, a dewcell, a barometer and a thermograph. Stevenson screens may also be known as a cotton region shelter, an instrument shelter, a thermometer shelter, a thermoscreen or a thermometer screen. Its purpose is to provide a standardised environment in which to measure temperature, humidity, dewpoint and atmospheric pressure.

It was designed by Thomas Stevenson (1818–1887), a British civil engineer who designed many lighthouses, and was father of the author Robert Louis Stevenson. Thomas Stevenson’s development of his small thermometer screen with double louvered walls on all sides and no floor was reported in 1864. After comparisons with other screens in the UK, Stevenson’s original design was modified. The modifications by Mawley of the Royal Meteorological Society in 1884 included a double roof, a floor with slanted boards, and a modification of the double louvers. This design was adopted by the British Meteorological Office and eventually other national services such as Canada. The national services developed their own variations such as the single louvered Cotton Region design in the United States.

The traditional Stevenson Screen is a box shape, constructed of wood, in a double-louvered design. However, it is possible to construct a screen using other materials and shapes, such as a pyramid. The World Meteorological Organization (WMO) agreed standard for the height of the thermometers is between 1.25 m (4 ft 1 in) and 2 m (6 ft 7 in) above the ground. The Stevenson Screen is always painted white, to reflect direct solar radiations.

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