What is a heatwave?

What is a heatwave?

A heatwave is an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity. 

What is the definition for a UK heatwave?

A UK heatwave threshold is met when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by UK county. 

The Met Office heatwave threshold has been updated ahead of summer 2022. The initial heatwave thresholds were calculated based on the 1981-2010 climatology of daily maximum temperature at the mid-point of the meteorological summer (15 July). The revised thresholds will use the 1991-2020 averaging period introduced in January 2022. The geographical differences reflect the differences in climate across the UK.

The central map below, shows the initial heatwave thresholds for the UK, with the right hand map showing the updated thresholds for 2022. As a demonstration of our changing climate, we have also included the left hand map that shows what the thresholds would have been using the 1961-1990 climatology.

heatwave definition

Why do heatwaves happen?

Heatwaves are most common in summer when high pressure develops across an area. High pressure systems are slow moving and can persist over an area for a prolonged period of time, such as days or weeks. They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer. This can allow high pressure to develop over the UK resulting in persistent dry and settled weather.

Are heatwaves linked to climate change?

Summer 2018 was the equal-second warmest summer in a UK series from 1884 for mean daily maximum temperature (shared with 1995) with summer 1976 hottest. 

Heatwaves are extreme weather events, but research shows that climate change is making these events more likely. A scientific study by the Met Office into the Summer 2018 heatwave in the UK showed that the likelihood of the UK experiencing a summer as hot or hotter than 2018 is a little over 1 in 10. It is 30 times more likely to occur now than before the industrial revolution because of the higher concentration of carbon dioxide (a greenhouse gas) in the atmosphere. As greenhouse gas concentrations increase heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year by the 2050s. The Earth’s surface temperature has risen by 1 °C since the pre-industrial period (1850-1900) and UK temperatures have risen by a similar amount.

Heatwave impacts

The UK experiences occasional heatwaves but of a lesser frequency and intensity of those seen elsewhere globally. The summer of 2018 was the equal-warmest summer for mean temperatures in the UK along with 2006, 2003 and 1976. The hottest day of the summer was on 27 July, with 35.6 °C recorded at Felsham, Suffolk.

In August 2003, the UK experienced heatwave conditions lasting 10 days and resulting in 2,000 deaths. During this heatwave, a record maximum temperature of 38.5 °C was recorded at Faversham in Kent. In July 2006, similar conditions occurred breaking records and resulting in the warmest month on record in the UK.

In the summer of 2019, the 2003 maximum temperature record was broken at Cambridge University Botanic Garden on 25 July, with 38.7 °C

To aid in the preparation and awareness before and during a spell of very hot weather, a heatwave plan has been created by UK Health Security Agency in association with the Met Office and other partners. It recommends a series of steps to reduce the risks to health from prolonged exposure to severe heat for:

  • The NHS, local authorities, social care, and other public agencies
  • Professionals working with people at risk
  • Individuals, local communities and voluntary groups

Find out more about staying prepared for summer weather on our WeatherReady advice page, there are also some practical steps you can take to keep homes cool during the day.

What is the difference between the Extreme Heat Warning, Heat Health Alert, and a heatwave?  

  • Extreme Heat Warning – an impact-based warning designed to highlight the potential impacts of extreme heat to protect lives and property, helping people make better decisions to stay safe and thrive.
  • UK Health Security Agency (UKHSA) Heat Health Alert - An England only service considering the impact of prolonged extreme heat on public health, especially those with long-term health conditions. 
  • Heatwave definition - a threshold-based meteorological definition designed to provide the media and public with consistent and reliable messaging.