Met Office - Electrical resistance thermometers (ERT or PRT)
The instrument measures the resistance of platinum which depends on temperature according to a quadratic relationship.
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More Information (under review)
Screen level temperature
The standard exposure for thermometers for measuring the dry, wet-bulb, maximum and minimum temperatures is at 1.25m above the ground in a louvered white screen of wooden construction. This design allows a free circulation of air around the thermometers while shielding them from precipitation and external radiation.
Grass and concrete minimum
The grass minimum temperature is the lowest temperature reached overnight by a thermometer freely exposed to the sky with its bulb just touching the tips of short grass (25 to 50 mm above the ground). Normally the thermometer is exposed at the last hour before sunset and the reading is taken next morning. However, at stations where an observer is not available near sunset, such as at Ordinary Climatological Stations, the thermometer is often exposed throughout the day. When snow covers the ground the thermometer should be supported immediately above the surface of the snow without actually touching it, though this is only possible at manned stations. Doubtful readings, such as might occur where snow falls overnight, are not normally reported. Long grass and other characteristics of a poorly maintained site will cause inaccurate measurements.
The concrete minimum thermometer is exposed at the centre of, and in contact with, a concrete slab which should be cleared of any snow. At some climate stations in the first half of this century readings were taken over bare soil but these values have never been stored in MIDAS.
At many stations temperatures below the surface are measured at various depths. The depths used today are 5, 10, 20, 30 and 100cm, although measurements are not necessarily made at all these depths at a station and exceptionally measurements may be made at other depths. When imperial units were in general use, typically before 1961, the normal depths of measurement were 4, 8, 12, 24 and 48 inches.