Updated: 2026-08-28
If you define summer not by the calendar but by the first sustained spell of warm weather, London's summer now begins on 12 June in a typical year (2025). That is the striking conclusion of a full scan of daily temperature records from the Met Office station at St James's Park, covering 1912 to 2025. The shift is not subtle: in the 1910s, summer arrived on dates like 20 June (1912), 17 June (1913) and 20 June (1914). In the 2020s, it lands on 1 June (2019, 2020, 2017), 2 June (2007, 2021) and 3 June (2011, 2018).
The most extreme case in the entire 114-year record is 7 August 1988: that year Londoners waited until early August for summer to arrive — the latest start on record. The earliest starts, clustered across several years, came on 1 June (1922, 1947, 1963, 1978, 1982, 2017, 2019, 2020). The gap between those extremes — from a 1 June start to a 7 August start, measured across the full series — tells you everything about how variable the British summer can be, and how much it has changed.
7 August 1988
The latest summer onset ever recorded at St James's Park, London (1912–2025). In that year, the first sustained warm spell did not arrive until early August — over two months later than the 2025 start of 12 June.
The method is straightforward. Working backwards from the complete daily series, meteorologists identify the first day in each year when temperatures climb and stay above a seasonal threshold — a signal that summer has genuinely begun, not just a brief warm blip. The dates below, taken directly from the Met Office record, show the progression at St James's Park:
The chart shows the complete annual series. The table lists the most notable years; the full series comes from Met Office.
| Year | Summer start date |
|---|---|
| 1912 | 1912-06-20 |
| 2020 | 2020-06-01 |
| 2021 | 2021-06-02 |
| 2022 | 2022-06-15 |
| 2023 | 2023-06-09 |
| 2024 | 2024-06-21 |
| 2025 | 2025-06-12 |
Looking at the full span from 1912, a handful of years stand out as the absolute extremes of summer onset. The earliest starts — all on 1 June — occurred in 1922, 1947, 1963, 1978, 1982, 2017, 2019 and 2020. The latest starts were 7 August 1988, 28 July 1916, 14 July 1972, 12 July 1926 and 11 July 1928. These years trace the outer boundaries of what London's climate has produced over more than a century.
What is revealing is the concentration of late starts in the mid-20th century — 1916, 1926, 1928, 1944, 1951, 1967, 1971, 1972 — alongside early starts in the same era (1922, 1947, 1963). London has always been variable. But the recent cluster is unmistakable: since 2000, only two years (2012, 2024) have seen summer arrive after 15 June. In the 1910s and 1920s, late June and early July starts were routine.
The picture is not just about summer. Across the 106 complete winter seasons in this record, the average number of frost days (where temperatures drop below freezing) is 18.94 per winter. That number — the long-term mean from the full series — provides the baseline against which any individual winter can be judged. Compare 1963, the coldest year on record with an average temperature of 9.76°C, against 2022, the warmest at 13.33°C, and you see the full range of London's thermal history. The five warmest years — 2022 (13.33°C), 2023 (13.16°C), 2025 (13.08°C), 2014 (13.01°C) and 2024 (12.88°C) — are all from the last decade. The five coldest — 1963 (9.76°C), 1919 (9.96°C), 1917 (10.02°C), 1922 (10.07°C) and 1956 (10.21°C) — are all from the past.
The connection to season length is direct: a warmer baseline shifts the entire seasonal cycle. When winters produce fewer frost days and spring warmth arrives earlier, the threshold for summer — a sustained period of warm temperatures — is crossed sooner. The 2025 start of 12 June is not an anomaly; it sits within a cluster of early starts stretching back to 2017 (1 June), 2018 (3 June), 2019 (1 June), 2020 (1 June), 2021 (2 June) and 2022 (15 June).
For anyone planning outdoor events, garden projects, or travel in London, the message is practical: the window for reliably warm weather has shifted earlier. A start date of 12 June in 2025 means that by mid-June, the season was already established. Compare that with 1916, when Londoners waited until 28 July. The planning implications are real — and they are written directly into the temperature record.
The data behind this analysis comes from the Met Office's MIDAS archive, the UK's national weather station network. The readings were taken at St James's Park, the longest-running central London station, with complete daily records spanning from 1 January 1912 through 31 December 2025. The full year-by-year breakdown above, with every summer start date from 1912 to 2025, is drawn directly from that station's official record — 114 years of data, complete and uninterrupted.
Source: Met Office, St James's Park station, London — period 1912–2025.
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© 2026 HistoClima · data: Met Office (OGL v3.0) · not affiliated with Met Office · citation permitted with source.