A
Stevenson screen or
instrument shelter is an enclosure to shield meteorological instruments against
precipitationIn meteorology, precipitation is any product of the condensation of atmospheric water vapor that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel...
and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard
weather stationA weather station is a facility with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind direction, and...
. The Stevenson screen holds instruments that may include
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
s (ordinary, maximum/minimum), a
hygrometerHygrometers are instruments used for measuring relative humidity. A simple form of a hygrometer is specifically known as a psychrometer and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature...
, a psychrometer, a
dewcellDew cells are instruments used for determining the dew point. They consist of a small heating element surrounded by a solution of lithium chloride. As the LiCl absorbs moisture from the air, conduction across the heating element increases, current in it increases, and heat increases, evaporating...
, a
barometerA barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather...
and a thermograph.
A
Stevenson screen or
instrument shelter is an enclosure to shield meteorological instruments against
precipitationIn meteorology, precipitation is any product of the condensation of atmospheric water vapor that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel...
and direct heat radiation from outside sources, while still allowing air to circulate freely around them. It forms part of a standard
weather stationA weather station is a facility with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind direction, and...
. The Stevenson screen holds instruments that may include
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
s (ordinary, maximum/minimum), a
hygrometerHygrometers are instruments used for measuring relative humidity. A simple form of a hygrometer is specifically known as a psychrometer and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature...
, a psychrometer, a
dewcellDew cells are instruments used for determining the dew point. They consist of a small heating element surrounded by a solution of lithium chloride. As the LiCl absorbs moisture from the air, conduction across the heating element increases, current in it increases, and heat increases, evaporating...
, a
barometerA barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather...
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.
History
It was designed by
Thomas StevensonThomas Stevenson was a pioneering Scottish lighthouse designer, who designed over thirty lighthouses in and around Scotland, as well as the Stevenson screen used in meteorology...
(1818-1887), a
BritishThe United Kingdom of Great Britain and Ireland was the formal name and the state form of the United Kingdom from 1 January 1801 until 12 April 1927...
civil engineerA civil engineer is a person who practices civil engineering, one of the many professions of engineering. Originally a civil engineer worked on public works projects and was contrasted with the military engineer, who worked on armaments and defenses...
and father of the author
Robert Louis StevensonRobert Louis Balfour Stevenson was a Scottish novelist, poet, essayist and travel writer. Stevenson was greatly admired by many authors, including Jorge Luis Borges, Ernest Hemingway, Rudyard Kipling, Marcel Schwob, Vladimir Nabokov, J. M. Barrie, and G. K...
.
Composition
The traditional Stevenson Screen is a box shape, constructed of wood, in a double-
louverA louver or louvre , from the French l'ouvert; "the open one") is a window, blind or shutter with horizontal or, less often, vertical slats, that are angled to admit light and air, but to keep out rain, direct sunshine, and noise...
ed design. However, it is possible to construct a screen using other materials and shapes, such as a triangle. The
World Meteorological OrganizationThe World Meteorological Organization is an intergovernmental organization with a membership of 188 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
(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
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
asbestosAsbestos is a naturally occurring silicate mineral with long, thin fibrous crystals. The word asbestos is a borrowed Greek adjective meaning inextinguishable. The Greeks termed asbestos the miracle mineral because of its soft and pliant properties, as well as its ability to withstand...
boards with an air space between them. These asbestos boards have generally been replaced by a
laminateA laminate is a material constructed by uniting two or more layers of material together. The process of creating a laminate is lamination, which in common parlance refers to the placing of something between layers of plastic and sealing them with heat and/or pressure, usually with an adhesive...
due to health and safety reasons. The whole screen is painted with several coats of white to reflect sunlight
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
and will usually require repainting every two years.
Siting
The siting of the screen is very important to minimise the effects of buildings and trees.
Environment CanadaEnvironment Canada , legally incorporated as the Department of the Environment under the Department of the Environment Act Environment Canada (EC), legally incorporated as the Department of the Environment under the Department of the Environment Act Environment Canada (EC), legally incorporated as...
, 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. However, in
polar regionEarth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
s 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.
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.
A special type of Stevenson screen with an eye on the roof is used on a ship. The unit is hung from the ceiling and remains vertical despite the movement of the vessel.
Future
In some areas the use of single unit
automatic weather stationAn automatic weather station is an automated version of the traditional weather station, either to save human labour or to enable measurements from remote areas...
s is supplanting the Stevenson screen (and other stand-alone meteorological equipment).
External links