UK records hottest day ever, but when did weather records begin?

Britons are melting on Tuesday after the temperature topped 40C in London. Credit: PA

With temperatures topping 40C in London on Tuesday, Britain is experiencing its hottest day since records began, sizzling in weather hotter than Jamaica, the Maldives and Barbados.

But what does "since records began" actually mean, and how did we predict the weather before that?

When did temperature records begin?

The Met Office is known for keeping a record of temperatures across the UK, but it didn't officially accept responsibility for the custodianship of public weather records until April 1914.

This is when observation stations became more uniform in the way they collected data, explaining why this is often what meteorologists are talking about when they say "since records began."

But actually, various records from weather stations around the country existed before then, though they may have been limited to smaller parts of the UK or not standardised like today's records.

People in the sea in Bournemouth on Tuesday. Credit: Steve Parsons/PA Wire/PA Images

When was the Met Office founded?

The Met Office was founded by Vice-Admiral Robert Fitzroy in 1854.

His seafaring background, as the captain of HMS Beagle, meant he understood how important predicting the weather was for staying safe at sea.

The original purpose of the Met Office was to learn more about the sea, to save the lives of sailors.

How does the Met Office record the temperature?

To measure the temperature accurately, and to ensure readings are standardised between observatories, the Met Office uses Stevenson screens.

These are white boxes used to contain thermometers, with slats that allow air to flow through.

The screens face north, shielding the thermometers from the direct sun and making them a reliable way to measure the air temperature.

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What do we know about the weather before 1914?

The longest-running temperature record in the world is the Central England Temperature record, which keeps a record of the temperature between the Midlands and Lancashire. It dates back to 1659.

The Armagh Observatory Station in Northern Ireland, founded in 1790, began keeping a record of the temperature in 1794.

This has continued uninterrupted ever since, with measurements still taken at 9am every day.

The Met Office holds the Observatory's data from 1853 onwards.

But before we started to measure at weather stations, much of our knowledge of the weather came from seafarers.

Where are the records held and is there an archive?

The UK's meteorological data is stored in a permanent archive, the National Meteorological Archive in Exeter.

The Met Office says this is "vital to climate science," and "helps to inform climate policy."

The archive includes paper observation records for sites in England, Wales and around the world including St Helena and the Falkland Islands.

It also has a collection of private weather diaries from before the Met Office was founded.

The paper records for Scotland are held in Edinburgh, while Northern Ireland's records are held in the Public Records Office in Belfast.