What 77 years of daily readings say about British summers

Everyone says British summers are getting hotter. Almost nobody publishes the count.

So we did. This site stores the daily maximum, minimum and mean temperature for twenty five British towns, going back to 1 January 1950, and computes the statistics itself. That is 27,963 daily readings per town, and a little under 700,000 in total. What follows comes out of that store, not out of anybody’s press release.

The count

Days above 25 degrees, averaged per year, in the 1950s against the 2020s so far. The last column is the trend fitted across every complete year from 1950 to 2025, which is the more honest number:

Cambridge has gone from six hot days a year to twenty. Manchester, which nobody thinks of as a hot city, now averages more in a year than Cambridge managed in the whole of the 1960s. Newcastle has gone from one hot day a decade to three a year.

Every town on the list has risen. Not one has a falling trend.

The 1960s were genuinely cold

It is worth being careful with a baseline. Look down the 1950s column and then picture the 1960s, which were colder still: London drops from 7.4 to 3.3, Bristol from 6.1 to 1.6, Plymouth to none at all. Start a comparison there and the rise looks steeper than it is.

That is why the last column is fitted across every year from 1950 onwards rather than drawn between two decades of somebody’s choosing. The full series is on each town page, year by year, so you can see the shape rather than take the summary on trust.

The nights are a different map

A tropical night is one where the minimum never drops below 20 degrees. They are still rare almost everywhere in Britain, but where they happen is not where you would guess.

Brighton leads at 0.8 a year this decade, then Southampton and Portsmouth at 0.7, then Norwich at 0.5. London manages 0.2. Leeds, Plymouth, Newcastle, Glasgow, Edinburgh and Belfast have effectively none.

So the warmest nights in Britain are not in the biggest city. They are on the south coast, where the sea holds its heat through the evening and gives it back after dark. A Brighton flat has a harder night than a London one, and the housing stock there was built with even less thought for it.

This matters more than it sounds. British houses cool down overnight, and that is the mechanism most of us rely on without thinking about it: open the windows at eleven, sleep, close them at seven. It still works in most of the country most of the time. The question is how long that holds.

The days that broke the records

Fifteen of the twenty five towns set their highest reading on the same day, 19 July 2022. Belfast set its own the day before. That is sixteen out of twenty five inside a single heatwave.

Six more were set in late June 2026, on the 24th, 25th and 26th: Portsmouth, Southampton and Plymouth on the first of those days, Bristol and Cardiff on the second, Norwich on the third.

That is the pattern in a sentence. Not a slow drift, but a small number of extreme days that arrive together and break everything at once.

One caveat on those numbers. They come from a reanalysis grid about eleven kilometres across, which is an average over its whole area, so each figure reads lower than the hottest thermometer in the same town on the same day. The dates are right and the pattern is right. The peaks are conservative.

Why the count matters more than the record

Record days make the news. The count is what you actually live with, and what decides whether a machine is worth buying.

Twenty hot days a year is roughly three weeks when a top floor bedroom is uncomfortable. That is a different purchase decision from six days a year, which is what Cambridge had when most of its housing stock was built.