Stevenson screen
A Stevenson screen or instrument shelter is a shelter or an enclosure used to protect meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them.[1] It forms part of a standard weather station and 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 is white in color to reflect direct solar radiation.
History
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It was designed by Thomas Stevenson (1818–1887), a Scottish civil engineer who designed many lighthouses, and was the father of author Robert Louis Stevenson. The development of his small thermometer screen with double-louvered walls on all sides and no floor was reported in 1864.[2] After comparisons with other screens in the United Kingdom, Stevenson's original design was modified.[3]
The modifications by Edward Mawley of the Royal Meteorological Society in 1884 included a double roof, a floor with slanted boards, and a modification of the double louvers.[4] 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.[5]
Composition
[edit]The traditional Stevenson screen is a box shape, constructed of wood, in a double-louvered design.[6] 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 and 2 m (4 ft 1 in and 6 ft 7 in) above the ground.
Size
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The interior size of the screen will depend on the number of instruments that are to be used. A single screen may measure 76.5 by 61 by 59.3 cm (30.1 by 24.0 by 23.3 in) and a double screen 76.5 by 105 by 59.3 cm (30.1 by 41.3 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 for health and safety reasons. The whole screen is painted with several coats of white to reflect sunlight radiation, and usually requires repainting every two years.
Siting
[edit]The siting of the screen is very important to avoid data degradation by the effects of ground cover, buildings and trees: WMO 2010 recommendations, if incomplete, are a sound basis.[7] In addition, 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 (66 ft) from any tree that is 10 m (33 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.
Future
[edit]In some areas the use of single-unit automatic weather stations is supplanting the Stevenson screen and other standalone meteorological equipment.[citation needed]
References
[edit]- ^ "Stevenson Screen". Environment Canada. Archived from the original on 5 March 2008. Retrieved 4 December 2007.
- ^ Thomas C.E. Stevenson (June 1864). "New Description of Box for Holding Thermometers". Journal of Scottish Meteorological Society. 1: 122. hdl:2027/nyp.33433110019654.
- ^ Middleton, W.E. Knowles (1966). A History of the Thermometer and Its Uses in Meteorology. Baltimore: Johns Hopkins Press. ISBN 9780801871535.
- ^ Edward Mawley (January 1884). "Report on Temperatures in Two Different Patterns of Stevenson Screens". Quarterly Journal of the Royal Meteorological Society. X (49): 1–7. Bibcode:1884QJRMS..10....1M. doi:10.1002/qj.4970104902.
- ^ Sparks, W.R. (1972). "The Effect of Thermometer Screen Design on the Observed Temperature". Wmo-315. Geneva: World Meteorological Society.
- ^ "High May Temperatures in London". Symons's Meteorological Magazine. Vol. XLVIII, no. 569. June 1913. pp. 88–90. (See p. 89.)
- ^ "Commission for Instruments and Methods of Observation". 2–8 September 2010.
External links
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