Updated: 2026-07-31
The point of measurement closest to London is the High Beach station (province of Essex), situated at 110 m altitude, about 20.8 km away. These are its historical records. The real climate of London can differ from this station due to local factors — urban heat island, proximity to the Thames, elevation differences — so treat these as the best available reference, not an exact reading for the city itself.
Here is the direct answer: during the 1980s, the average first summer day (defined as the first day with a maximum temperature of at least 25 °C) occurred on day 172.7 of the year — that is, around late June. In the 2020s, that average has moved to day 140.8 — roughly the third week of May. That is a shift of more than a month in less than four decades, based on data from the Met Office's High Beach station.
The table below shows the average Julian day of the first summer day (tmax ≥ 25 °C) by decade, for the full period of record at High Beach station (1978-2025). A Julian day is simply the day of the year counted from January 1 — day 1 is January 1, day 100 is around April 10, day 200 is around July 19.
| Decade | Average Julian day of first summer day |
|---|---|
| 1970s | 135.0 |
| 1980s | 172.7 |
| 1990s | 149.2 |
| 2000s | 151.8 |
| 2010s | 141.1 |
| 2020s | 140.8 |
The 1980s stand out as a late-summer decade, with an average first summer day at day 172.7 — that is around June 21, almost exactly the astronomical solstice. The 1990s pulled back to day 149.2, and since then the trend has been persistently earlier, with the 2010s at 141.1 and the 2020s at 140.8. The 1970s figure (135.0) is based on a single year (1978), as that is when the station's complete daily records begin.
To put this in perspective: the difference between the 1980s average (172.7) and the 2020s average (140.8) means that, on average, summer-like warmth now arrives about a month earlier than it did in the 1980s. For anyone planning outdoor events, gardening, or seasonal work, this is not a trivial shift — it changes which months are reliably warm.
Beyond the summer-start question, the station's annual mean temperature record shows how the climate has evolved. The table below lists every year from 1978 through 2025 with its annual mean temperature. The period of record runs from 1978-08-01 to 2025-12-31.
The chart shows the complete annual series. The table lists the most notable years; the full series comes from Met Office.
| Year | Annual mean temperature (°C) |
|---|---|
| 1999 | 12.2 |
| 2002 | 11.8 |
| 2006 | 7.7 |
| 2014 | 11.5 |
| 2022 | 11.7 |
| 2023 | 11.5 |
| 2024 | 11.6 |
Looking at the extremes helps frame the whole series. The five warmest years at High Beach since 1978, each with its annual mean temperature, are:
And the five coldest years:
The warmest year on record remains 1999 at 12.2 °C — it has not been beaten since, even though 2022 (11.7 °C), 2024 (11.6 °C), and 2023 (11.5 °C) have come close. The coldest year, 2006 at 7.7 °C, sits well below the average of the surrounding years and has not been repeated. That single cold year aside, the recent run of years from 2014 onward has been consistently warm, with every year from 2014 to 2025 above 10.6 °C except 2015, 2016, 2017, and 2019 which were all at 10.9 °C or above.
Within the full series from 1978 to 2025 at High Beach, the annual mean temperature in 2006 was 7.7 °C — the coldest year in the entire period. That year stands out sharply against the recent run of warm years. It broke the pattern of steady warming and remains the lowest annual mean on record, a reminder that even with a clear trend, individual years can still plunge. The contrast with 1999, the warmest year at 12.2 °C, spans 4.5 °C of annual mean difference between the two extremes in the period.
The shift in the first summer day is consistent with the warming seen in the annual means. As the average temperature rises, the threshold of 25 °C is reached earlier in the year. The effect is not uniform decade by decade — the 2000s saw a slight delay back to day 151.8 — but the overall direction since the 1980s is unambiguous. The 2020s average of 140.8 means that, on typical years, the first genuinely warm summer day now arrives in mid-May rather than late June.
For residents of London, using High Beach as the reference point, this has practical consequences. Planting calendars, the timing of outdoor events, and even energy demand for cooling all respond to when sustained warmth actually arrives. The astronomical definition of summer — the solstice around June 21 — has not changed, but the meteorological reality at this station has moved well ahead of it.
Source: Met Office, High Beach station (Essex). Period 1978-2025. Data from the Met Office MIDAS archive, held at CEDA, Open Government Licence (OGL v3.0).
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© 2026 HistoClima · data: Met Office (OGL v3.0) · not affiliated with Met Office · citation permitted with source.