This weekend and into next week temperatures are expected to reach the high twenties Celsius in southern England. This is certainly warmer than we would expect at this time of year – the average maximum temperature for July in England is 20.9 °C – but does it constitute a heat wave?

How hot is a heat wave?

There’s actually no official definition of a heat wave in the UK. In America, where high temperatures are more likely, the official classification is based on the Heat Index. The Heat Index temperature is a ‘feels-like’ temperature calculated by combining the temperature and relative humidity.

Depending on the local climate, an excessive heat warning is issued when the Heat Index is expected to exceed 105 °- 110 °F (40 °C – 43 °C) for at least two consecutive days.

Australia also has variable definitions depending on the state. In Adelaide, a heat wave is defined as five consecutive days at or above 35 °C, or three consecutive days at or over 40 °C.

Heat health watch

Working in association with the Department of Health, the Met Office provides a heat health warning system for England.

The Heat-health watch system comprises four levels of response based upon threshold maximum daytime and minimum night-time temperatures. These thresholds vary by region, but an average threshold temperature is 30 °C by day and 15 °C overnight for at least two consecutive days.

When was the last time we had a prolonged spell of hot weather in the UK?

The last time we saw a long spell of warm weather was in July 2006, where temperatures were above 28 °C in many areas for a fortnight. We have had shorter spells of warm weather since, however, such as the 23 – 26 July last year, when temperatures peaked at 30.7 °C.

How hot will it get this weekend?

You can see the expected maximum temperature range for your area on our website. Temperatures are not currently expected to exceed the heat health watch threshold, but keep up to date with warnings on our website.

Visit our summer pages for activity ideas for hot and sunny weather.