The UK has recorded its warmest day of the year so far as temperatures topped 21C on Thursday, coinciding with the spring equinox.

The highest recorded temperature was 21.3C in both Northolt, west London, and Chertsey, Surrey, the Met Office said.

It beat this year’s previous highest figure of 19.7C in Crosby, Merseyside, on 9 March and was just shy of the warmest recorded UK spring equinox temperature of 21.5C in 1972.

Wales also recorded both its hottest day of 2025 so far and its warmest spring equinox as temperatures at Gogerddan reached 20.4C.

The Met Office forecaster Craig Snell said: “It has been a fairly pleasant day, right across the board.

“Sunshine has been the best down across the south, hence, we have kind of seen the highest figures there. Temperatures have been a little bit more hazy further north but everywhere has generally had a dry and fine day today.”

Northern Ireland recorded a top temperature of 17.1C in Derrylin, County Fermanagh, and in Scotland it peaked at 16.1C in Auchincruive, Ayrshire.

Temperatures are expected to dip back to the averages for this time of year of about 11-12C in the coming days.

A frost-free night is expected as cloud and breeze builds up, but sheltered parts of northern England and Scotland could get a touch of frost in the early hours.

A milder start to the day, compared with recent mornings, is expected on Friday when it will be wetter in some parts.

The Met Office meteorologist Aidan McGivern said: “South-western parts of the UK will see showers from the word go, perhaps even a rumble of thunder, though showers will move into west Wales, Northern Ireland, western Scotland, through the day as well.”

The meteorological spring always starts on 1 March while the astronomical spring, or equinox, begins around 20 March each year.

The other equinox is in September and both mark the sun crossing the equator, meaning day and night are about the same length.

The warm weather has brought warnings of pollution and signs that people who are allergic to birch pollen may have tough times ahead.

An air pollution alert has been issued for London, the first this year, after forecasters at Imperial College warned of a rise in particulate pollutants from mainland Europe.

Sadiq Khan, the mayor of London, posted on X: “I urge Londoners to look after themselves and protect those with conditions such as asthma. This includes refraining from engine idling and burning wood or garden waste.”

The Met Office’s UK pollen forecast manager, Yolanda Clewlow, said: “The pollen season has already started and people with a sensitivity to tree pollen may have noticed the early tree species in the air.

“For those who are allergic to birch pollen, the season for this species is likely to be high, largely due to the conditions last spring and summer when pollen was forming on some trees.”