A Stevenson screen or instrument shelter is 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.
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 interior size of the screen will depend on the number of instruments that are to be used. A single screen may measure 765 mm high by 610 mm wide by 593 mm deep (30.1 in by 24.0 in by 23.3 in) and a double screen 765 mm high by 1050 mm wide x 593 mm deep (30.1 in by 41.3 in by 23.3 in). The unit is either supported by four metal or wooden legs or a wooden post. The top of the screen was originally composed of two asbestos boards with an air space between them. These asbestos boards have generally been replaced by a laminate due to health and safety reasons. The whole screen is painted with several coats of white to reflect sunlight radiation and will usually require repainting every two years.
The siting of the screen is very important to minimise the effects of buildings and trees. Environment Canada, for example, recommends that the screen be placed at least twice the distance of the height of the object, e.g., 20 m from any tree that is 10 m high, or 40 ft from one 20 ft high. In the northern hemisphere, the door of the screen should always face north so as to prevent direct sunlight on the thermometers. In polar regions with twenty-four hour sunlight the observer must take care to shield the thermometers from the sun and at the same time avoiding a rise in temperature being caused by the observer's body heat. A special type of Stevenson screen with an eye bolt on the roof is used on a ship. The unit is hung from above and remains vertical despite the movement of the vessel. The use of a standard screen theoretically allows temperatures to be compared accurately with those measured in earlier years and at different places. However, theory is not borne out in fact. A 2007 survey of a quarter of the 1,221 stations that make up the U.S. Historical Climatology Network, performed by volunteers coordinated by meteorologist Anthony Watts, showed that over half of the stations fall short of U.S. federal guidelines for optimum placement. Some stations never met the guidelines from the start and other stations that at one time may have met the guidelines were compromised by artificial heat sources introduced since their erection, causing them to report abnormally high temperatures. The relevance of this to derived long-term real temperature trends has been questioned, however.
In some areas the use of single unit automatic weather stations is supplanting the Stevenson screen (and other stand-alone meteorological equipment).
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