At the Met Office we report weather extremes, such as the maximum monthly temperatures, as well as long-term weather averages. This post is in response to questions posed by our Twitter followers, who have been asking how maximum temperatures are measured and why observations from some thermometers are not valid for official records.
How are official temperatures measured?
Since the 1960’s only observations from stations that meet specific criteria and that are calibrated and regularly checked by the Met Office are used for quoting temperatures.
Thermometers must be housed in a Stevenson screen with its door facing north. A Stevenson screen is a white slatted box which houses the thermometer away from direct sunlight and allows air to flow freely through it.
The location of the Stevenson screen is important to ensure consistency across sites. It should be mounted at a height of 1.25m on level grassy ground, away from trees and man made structures. It should also be at least 20m away from any area of concrete or hard-standing, and only half the area within 100m radius of the screen can be covered with buildings or man made surfaces.
The thermometer itself must measure temperatures to the nearest decimal point. This is why observations from some stations – such as METAR stations at airports – aren’t included in official records, as they normally only record the temperature to the nearest whole number.
When are maximum temperatures officially recognised as a new record?
Each day real time data is subject to preliminary quality control before it is released, such as cross checking against nearby stations. A record will not become official until thorough quality control has taken place on the data, which may take several months.
How far do records go back?
Met Office standardised daily temperature records were started in the 1960s, however there are historical paper records for earlier dates held in the National Meteorological Library and Archive. Our National record for the UK goes back to 1910, although there are others that go back further such as the Central England Temperature Record. Individual station statistics go back varying lengths of time dependig on when each station was opened.
Why are some places often the warmest?
Although all thermometers are housed in standard conditions the geographical location of the station does still have an impact on the temperatures recorded. An example is the Gravesend station, which recorded the highest temperature in the UK this Wednesday. The station is surrounded by a built-up industrial area, which could lead to a slightly higher maximum temperature due to factors such as heat being absorbed by buildings.
For full weather extremes listings and historical records go the Met Office Climate section.
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