Updated: 2026-08-06
On the night of 29 July 1948, the temperature in central London never fell below 23.3°C. That remains the warmest night in the city's recorded history, a mark that has stood unchallenged for over seven decades. The reading comes from the Met Office station at London St James's Park, which holds one of the longest continuous climate records in the United Kingdom.
To put it in perspective: the average minimum temperature for July at St James's Park is just 14.0°C. The 1948 night was more than nine degrees above that typical July baseline, a remarkable margin even for a British heatwave.
The Met Office series for London St James's Park extends from January 1912 through December 2025, giving us 114 years of daily minimum temperatures. Scanning that entire record, the single highest minimum temperature — the warmest night — belongs to 29 July 1948, when the mercury bottomed out at 23.3°C. It's a record that has survived every heatwave since, including the intense summers of 1976, 2003, 2018, and 2022.
Tropical nights — defined as nights where the minimum stays above 20°C — are rare in the UK climate. London has seen only a handful of them in over a century of measurements. The top five warmest nights tell a story of two distinct eras: the mid-20th century produced two entries, while the last decade has produced three.
| Date | Minimum temperature (°C) |
|---|---|
| 29 July 1948 | 23.3 |
| 20 July 2016 | 22.1 |
| 12 August 2020 | 22.0 |
| 1 July 2025 | 21.8 |
| 5 September 1949 | 21.7 |
Notice the chronological spread: the 1940s produced two of the five warmest nights, including the all-time record. Then comes a gap of 67 years before the 2016 event. Since then, three dates have joined the top five: 2020, 2025, and the most recent addition on 1 July 2025, which reached 21.8°C. The 2025 reading is the third-warmest July night in the record and demonstrates how recent heatwaves are now routinely producing nights that were once exceptional.
July 1948 was a notably hot month across southern England. The 23.3°C minimum on the 29th came during a sustained heatwave, and the city struggled to cool down even after sunset. Brick buildings and paved streets absorbed heat all day and released it slowly through the night — a phenomenon we now call the urban heat island effect, though it was far less studied then.
By comparison, the normal July minimum at St James's Park is 14.0°C. The 1948 night stayed nearly 10 degrees warmer than that average. What makes this even more striking is that it happened before widespread air conditioning, when Londoners simply had to endure sleepless nights in warm homes.
The annual maximum of the daily minimum temperature — that is, the warmest night of each year — shows how the baseline has shifted. Early decades rarely exceeded 19°C; the 1920s produced a few notable spikes, with 1923 and 1932 both reaching 21.1°C. But the frequency of nights above 20°C has increased markedly since the 1990s.
Here is the full annual series from 1912 to 2025. Each point represents the warmest night of that year, in degrees Celsius.
The chart shows the complete annual series. The table lists the most notable years; the full series comes from Met Office.
| Year | Warmest night (°C) |
|---|---|
| 1948 | 23.3 |
| 1949 | 21.7 |
| 2003 | 10.1 |
| 2016 | 22.1 |
| 2020 | 22.0 |
| 2022 | 21.4 |
| 2025 | 21.8 |
The chart above reveals a clear upward drift. In the first four decades (1912–1949), the warmest night of the year exceeded 20°C only seven times. In the most recent four decades (1990–2025), that threshold was crossed fifteen times. And the magnitude has grown: the three warmest nights of the entire series — 23.3°C in 1948, 22.1°C in 2016, and 22.0°C in 2020 — include two from the last decade.
Unsurprisingly, the warmest nights cluster in mid-summer. July holds the overall record at 23.3°C, with August close behind at 22.0°C and June reaching 20.6°C. But the table below shows that warm nights are not confined to the core summer months. September has produced a 21.7°C night — that's the 5 September 1949 event already mentioned — and October has reached 19.6°C. Even May has managed 18.3°C, a reading that would be a normal daytime maximum for that month.
| Month | Warmest night (°C) | Average minimum (°C) |
|---|---|---|
| January | 12.9 | 2.8 |
| February | 12.5 | 2.8 |
| March | 12.8 | 4.1 |
| April | 14.5 | 6.0 |
| May | 18.3 | 9.1 |
| June | 20.6 | 11.9 |
| July | 23.3 | 14.0 |
| August | 22.0 | 13.8 |
| September | 21.7 | 11.7 |
| October | 19.6 | 8.8 |
| November | 15.3 | 5.3 |
| December | 13.9 | 3.6 |
The winter months are not immune to mild nights either. January's warmest night reached 12.9°C, and December has seen 13.9°C — both far above their respective average minimums of 2.8°C and 3.6°C. These winter warm nights are usually associated with mild Atlantic air masses sweeping across the country, rather than the stagnant heat of summer.
What makes the 23.3°C reading from 29 July 1948 so remarkable is its persistence. It has now stood for 77 years, surviving some of the most intense heatwaves ever recorded in Europe. The summer of 1976 — famously hot and dry — produced a warmest night of 20.7°C at St James's Park, still 2.6°C short of the record. The August 2003 heatwave, which set all-time temperature records across much of Western Europe, produced 21.2°C in London. The July 2022 heatwave, when daytime temperatures exceeded 40°C in parts of the UK for the first time, yielded a warmest night of 21.4°C at this station.
The gap is narrowing. The 2016 and 2020 events came within 1.3°C of the record. The 2025 reading reached 21.8°C, just 1.5°C short. With the clear warming trend visible in the annual series, it would be no surprise if the 1948 record eventually falls. But for now, it remains the benchmark — the warmest night London has ever known, measured at the heart of the city.
A warm night requires a combination of factors: intense daytime heating, high humidity, cloud cover that acts like a blanket trapping heat near the ground, and often a warm wind preventing the boundary layer from cooling. In July 1948, the setup produced a minimum of 23.3°C — a temperature that would feel uncomfortably warm even for a daytime maximum in an average British summer. The city's buildings and roads, which absorb solar energy by day, release it slowly overnight, keeping urban areas warmer than the surrounding countryside. This effect is strongest during calm, clear nights following hot days, and London is one of the most pronounced urban heat islands in Europe.
Source: Met Office, London St James's Park. Period: 1912–2025.
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