The last spring frost, town by town, from 1950 to today

#Is the first warm day a sign that the frosts are over?

In England, mostly yes. At 13 English weather stations with records from 1961, the first spring day of 20 °C now comes around 27 April, and the last frost has usually gone by 14 April. A frost still followed that warm day in 27% of springs in 1996–2025. London’s grid square: 5 of the last 30 springs.

In continental Europe the warm day comes first. At 19 Romanian stations it now comes around 21 March, while the last frost stays until 11 April. Against 1961–1990 the warm day moved 8 days earlier and the last frost only 4 days.

The station analysis also found frost after the warm day: 72% of springs in 1996–2025 had a frost after the first 20 °C day, against 66% in 1961–1990. A 2020 study of up to 1,500 tree species reached a related conclusion for the continent: late-spring frost risk rose in Europe between 1959 and 2017, while it fell in North America.

#When the frosts stop: the map

The colour shows the date after which, over the last 30 springs, it froze in at most three. Yellow: frost leaves in March, on the coasts and in the south-west. Dark blue: it stays into May or June, in the uplands. The second map shows in how many springs it froze after the first 20 °C day.

The largest towns, from the earliest spring to the latest

#The Ice Saints: does it really freeze on 11–15 May?

In the Catholic calendar, 11–15 May are the days of Saints Mamertus, Pancras, Servatius, Boniface and Sophia, known in central Europe as the “Ice Saints”. The saying goes that frost can return until they have passed.

The data say no. On 11–15 May it froze 1.02 times as often as on the surrounding days in 1950–1987 and 0.91 times as often in 1988–2026. Before counting, a real peak was set at 1.2 in both periods; neither period comes close.

What is clear is something else: May frost tends to thin out over the month, with no single date after which it is safe. Romania shows the same in the second period. SRF Meteo in Switzerland, comparing afternoon temperatures, found no significant cooling on those days either.

#Questions

When is the last frost in London?

Over the last 30 springs, the last frost had come by 29 March in half the years and by 14 April in nine years out of ten. The latest since 1950 was 6 May 1959.

When can I plant out tomatoes?

When the frost risk is small enough for you. On the page you choose the day and see in how many of the last 30 springs it froze after it. The RHS also advises waiting for nights consistently above 10 °C for tomatoes.

Where does frost last longest?

In the uplands and in hollows where cold air collects. At Alice Holt, a forest in a frost hollow in Hampshire, the last frost comes 28 days later than in its grid square.

#How it was worked out

Temperatures come from E-OBS v34.0e, a European grid of 0.1 degrees (about 11 by 7 kilometres in Britain) built from weather-station measurements, every day from 1950 to 30 June 2026. For each square and each year: the last day in January–July with the minimum air temperature at or below 0 °C (and at −2 °C or +2 °C), the first frost day in August–December, and the first day in February–June with a maximum of at least 20 °C. A year counts only if every day of March–June (spring) or August–November (autumn) has data.

“In nine springs out of ten, by date X” means: X is the fourth-latest last frost of the last 30 springs (1997–2026), so it froze after X in at most three. “Half the years” uses the sixteenth. The first 20 °C day needs complete February–June data; years without such a day are left out of the count. The page, the map and the cards use the same data and the same rule. Comparisons with the past use 1961–1990, the period the checked stations cover; E-OBS is not homogenised, so a moved or replaced station can show in the grid.

The grid was checked against 14 English and 21 Romanian weather stations (NOAA GHCN-Daily, 1996–2025). At the typical English station the median last-frost date differs from the grid by 2 days; most stations feed the grid, so that check is not independent. The exceptions show what a grid square cannot see: at Alice Holt, a forest in a frost hollow, the last frost comes 28 days later than in its square; at Heathrow airport, 22 days earlier.

The nightly forecast comes from Open-Meteo’s weather models, for the air at 2 metres and the soil surface, hour by hour from 18:00 to 09:00. It is a forecast, not a measurement.

The grid against English weather stations

#Sources

• E-OBS v34.0e, daily minimum and maximum temperature on a 0.1-degree grid, 1950–June 2026 (Copernicus Climate Change Service, ECA&D)
• Cornes et al. (2018), An Ensemble Version of the E-OBS Temperature and Precipitation Datasets, J. Geophys. Res. Atmos. 123
• NOAA GHCN-Daily station temperatures (the grid check against 21 Romanian and 14 English stations)
• Hourly forecast for the air at 2 m and the soil surface: Open-Meteo
• Met Office: how temperature is measured, the grass minimum and ground frost
• RHS: when to plant out tender plants and avoid late frosts
• Zohner et al. (2020), Late-spring frost risk between 1959 and 2017 decreased in North America but increased in Europe and Asia, PNAS 117
• SRF Meteo: the Ice Saints, statistically cool in every second year

E-OBS data: the EU-FP6 project UERRA and the Copernicus Climate Change Service, and the data providers in the ECA&D project (ecad.eu). Used for non-commercial purposes.