Understanding how temperature is measured is fundamental to the accuracy of weather forecasts and climate monitoring.
At the Met Office, we use thermometers and a range of specialist techniques tailored to the purpose of the measurement. These instruments are carefully calibrated, checked and maintained to ensure consistency and reliability across our network of stations and importantly, linked to national and international standards.
Measuring air temperature
Air temperature is the most frequent reported figure by the Met Office in forecasts and observations. Unless otherwise stated, air temperature is the figure given in Met Office forecasts.
Air temperature is measured using platinum resistance thermometers (PRTs) at all synoptic stations. These thermometers are housed within Stevenson screens, which are white, louvered enclosures designed to shield instruments from direct sunlight and precipitation while allowing air to circulate freely.
The PRTs are positioned 1.25 metres above the ground and ventilated naturally through the screen’s side louvers. The PRTs operate by measuring the electrical resistance of a platinum wire, which varies predictably with temperature. This resistance is recorded using high-precision equipment located near the screen.
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Measuring ground/grass minimum temperature
Ground minimum temperature refers to the lowest overnight temperature recorded just above the surface. Traditionally, this was measured using a thermometer suspended horizontally in contact with grass blades and fully exposed to the sky. However, with the shift to automated systems and reduced manual oversight, natural grass surfaces have often been replaced with artificial equivalents.
At automatic stations, ground minimum temperature is measured using PRTs. In weather forecasts, the term “ground frost” indicates that the grass minimum temperature has fallen below 0 °C.
Measuring concrete minimum temperature
Concrete minimum temperature is measured using a thermometer placed in direct contact with a concrete slab. The slab is laid horizontally, exposed to the sky, and flush with the surrounding ground. This measurement is particularly relevant for assessing the risk of ice formation on runways and roads.
Measuring soil temperature
Soil temperature is monitored at depths of 10 cm, 30 cm, and 100 cm below the surface. At automatic stations, PRTs are used for these measurements. For the 10 cm and 30 cm depths, thermometers are inserted into the vertical wall of a trench, which is then back-filled. The 100 cm measurement is taken by suspending the thermometer inside a tube with its tip at the correct depth.
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Challenges can arise when the tube becomes flooded due to waterlogged conditions or heavy rainfall. To maintain consistency across sites, the ground above the 10 cm thermometer is kept as bare soil.
The Met Office employs a robust and diverse set of techniques to measure temperature across different surfaces and depths. From air and grass to concrete and soil, each method is carefully designed to ensure precision and consistency. They are a crucial part of the many measurements that form the backbone of our forecasting and climate analysis, helping us deliver accurate and reliable information.
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