Maximum temperatures and how they’re recorded

Here at the Met Office we have been collecting weather observations since we were founded, and the earliest daily temperature data we have in our digital climate database is from 1853. However, there are a number of elements to verify temperatures and any associated records which ensure a fair comparison and consistency across our annals.

How are temperatures measured?

Since the 1960s, temperatures quoted by the Met Office have had to come from stations that meet specific criteria and are regularly maintained and inspected by our specialist teams. Only data that is measured to the nearest decimal point are included, meaning that observations from certain stations that measure to whole numbers, such as METAR stations at airports, can’t be used in these standardised results.

Within these stations, thermometers have to be housed in a white slatted box with its door facing north, called a Stevenson screen, which keeps the thermometer away from direct sunlight but air flow constant.

Consistency in the locations of these boxes is also crucial, mounted 1.25m high over level, grassy ground. Man-made materials such as concrete can have a large impact on results through properties such as heat retention, and due to this the Stevenson screens should be located at least 20m away from concrete or hard standing, and only half of the area within a 100m radius should be formed of man made surfaces.

Stevenson Screen. Image: Met Office

When are maximum temperatures officially recognised as a new record?

There is a verification process to the daily real time data which has to go through quality control prior to it’s release, such as cross referencing with nearby stations for any inconsistencies. However, for records to become official more rigorous quality control is carried out over a longer period of time, potentially several months, before they can be recognised as a new record. These include wider cross checking between stations and sites, understanding of the weather on the day and what was expected compared to our forecasts.

Additional to these validations, physical inspections by a team of engineers mean they can check that equipment is working as it should with no anomalies, adding an additional layer of verification to ensure reported records are correct.

What are we learning from the past?

There have been significant impacts from high temperatures over the past century, with many lessons learnt to minimise these effects. In 1911 when temperatures reached 36.7°C, around 4,000 people died in London alone during the heatwave period. Droughts impacted water supplies, lack of grazing for cattle increased milk prices, and certain outdoor employers had to adjust the working hours of their staff in order to avoid the heat.

Graph showing how nine of the top 10 hottest UK days have been since 1990. Image: Met Office

The trend for these high temperatures has also been increasing, with nine of the ten hottest days on record in the UK falling from 1990, three of these since 2019. However, technology has developed, and whilst these temperatures still pose a significant risk to life, the ability to forecast more nearly a week ahead allows people to prepare and adapt their actions to try and minimise the effects that heat can bring.

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