Stevenson screen

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Exterior of a Stevenson screen

A Stevenson screen or instrument shelter is a shelter or an enclosure to 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. The common type of Stevenson screen has a maximum and minimum thermometer for daily readings.

History[edit]

American variant
(Cotton Region shelter)

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 156 years ago 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. 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.

Size[edit]

Interior of a Stevenson screen

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.[6] 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.

References[edit]

  1. ^ "Stevenson Screen". Environment Canada. Archived from the original on 2008-03-05. Retrieved 2007-12-04.
  2. ^ Thomas C.E. Stevenson (June 1864). "New Description of Box for Holding Thermometers". Journal of Scottish Meteorological Society. 1: 122.
  3. ^ A History of the Thermometer and Its Uses in Meteorology, W.E. Knowles Middleton, Johns Hopkins Press, Baltimore, 1966
  4. ^ 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.
  5. ^ The Effect of Thermometer Screen Design on the Observed Temperature, W.R. Sparks, WMO-315, World Meteorological Society, Geneva, 1972
  6. ^ [1] World Meteorological Organization Commission for Instruments and Methods of Observation, Fifteenth session, (CIMO-XV, 2010) WMO publication Number 1064

External links[edit]