Updated: 2026-09-29
In the long record kept for central London, the finest stretch of true summer heat ran without a single break for 22 consecutive days.
Longest unbroken run of days reaching 25 °C or more at London St James's ParkThat spell began on 23 June 1976 and ended on 14 July 1976 — 22 days in a row where the maximum temperature climbed to 25 °C or higher. It is the longest such run found anywhere in the station's records between 1912 and 2024.
When people ask how long summer lasts in London, the honest answer comes from counting what meteorologists call "summer days" — days when the maximum temperature reaches 25 °C or above. That is the threshold at which a London day genuinely feels warm rather than merely mild, and it is the measure used throughout this analysis of the London St James's Park station.
The headline figure above — 22 consecutive days in 1976 — is the longest unbroken warm stretch in more than a century of central London observations. Most London summers never come close to stringing together three full weeks of 25 °C-plus afternoons. That is the number to hold on to when planning anything that depends on reliable warmth.
The run recorded at London St James's Park started on 23 June 1976 and did not let go until 14 July 1976, lasting 22 days. For more than three weeks, every single afternoon crossed the 25 °C mark. In a city where a warm spell is often measured in a handful of days before Atlantic air moves back in, a stretch this long is remarkable — and it has stood as the station's longest warm-summer run across the 1912 to 2024 period.
It is worth being precise about what this record does and does not say. It is not the hottest single London day, and it is not a claim about rainfall or sunshine. It is purely a measure of persistence: how many days in a row the warmth refused to break. On that measure, 1976 remains unbeaten in this record.
To understand why a 22-day warm run is possible in London at all, look at the monthly picture. The table below summarises, for each month of the year, the average daily maximum temperature, the highest maximum ever recorded in that month, and the number of observation days behind each figure. The warm core of the London year sits firmly in July and August, while the shoulders of the season — May, June and September — show just how far the heat can occasionally push.
| Month | Average daily maximum (°C) | Highest maximum on record (°C) | Observation days |
|---|---|---|---|
| January | 7.7 | 16.3 | 2754 |
| February | 8.1 | 19.7 | 2516 |
| March | 11.0 | 24.3 | 2759 |
| April | 13.9 | 29.1 | 2667 |
| May | 17.7 | 31.7 | 2759 |
| June | 20.6 | 34.6 | 2670 |
| July | 22.7 | 40.2 | 2758 |
| August | 22.3 | 36.1 | 2759 |
| September | 19.6 | 32.5 | 2668 |
| October | 15.4 | 29.1 | 2758 |
| November | 10.9 | 20.6 | 2670 |
| December | 8.6 | 16.9 | 2759 |
The clearest signal is that London's warmest months by average are July at 22.7 °C and August at 22.3 °C — the only two months whose average daily maximum passes 22 °C. June follows at 20.6 °C, and September at 19.6 °C. Those four months define the practical warm half of the year: the window in which a stretch of consecutive 25 °C days is even plausible.
The extremes tell a similar story about how far the warmth can overshoot. July holds the highest maximum in the whole monthly record at 40.2 °C, with August reaching 36.1 °C and June 34.6 °C. But the season's edges are just as interesting for anyone weighing up a date. May has touched 31.7 °C and September has reached 32.5 °C, which means genuinely hot afternoons are not confined to the height of summer. Even April, with an average daily maximum of only 13.9 °C, has recorded a maximum of 29.1 °C.
At the cold end, January's average daily maximum of 7.7 °C and February's 8.1 °C show how far the city sits from warm-day territory in midwinter — although even these months have produced occasional outliers of 16.3 °C and 19.7 °C respectively. The contrast between a 7.7 °C January average and a 22.7 °C July average is the seasonal swing that makes London's warm season feel concentrated rather than spread across the year.
For anyone choosing dates — a wedding, a festival, a cricket match, a long walking holiday — the length of the warm run is often more useful than the single hottest day. A one-off spike to 34.6 °C in June is memorable, but what makes a summer feel generous is the number of afternoons that reach 25 °C without a cold interruption. On that measure, the 1976 record of 22 consecutive days remains the benchmark for central London across the 1912 to 2024 record.
It also explains why London summers can feel so variable from year to year. The city's averages place July and August among the warmest months, yet sustained spells like 1976 are rare rather than typical. When they do arrive, they are built on exactly this kind of unbroken sequence — day after day above 25 °C, stacking up into something that genuinely changes how the city lives.
Source: Met Office MIDAS archive. Station: London St James's Park. Period: 1912–2024.
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