The Met Office is planning a new heatwave definition for the climate change era

The recent bout of hot weather was deemed to be a “hotspell” not a “heatwave”. 

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When is a hotspell a heatwave? It’s a question the UK’s Met Office say is still not settled.

Speaking in the latest session of the Environmental Audit Committee’s Heatwaves inquiry, the Met Office’s Dr Peter Stott confirmed that there is no “universally agreed definition of a heatwave”.

The present definition used by the World Meteorological Organisation requires the average daily temperature to exceed the normal maximum temperature by five degrees celcius, for more than five consecutive days. It calculates the normal temperature using the period between 1961-1990.  

But this isn’t much help when it comes to establishing how heat feels to the people experiencing it: sun-hardened Texans in air-conditioned homes may be less likely to notice a five degree rise in temperature than sweaty Brits smushed together on the London tube.

So the Met Office is consulting on the creation of a clearer definition, specifically for the UK.

According to press officer Grahame Madge, the new system aims “to help with the broader messaging and communication around spells of hot weather across the whole of the UK”. And Dr Stott told the committee that he hopes it will be ready in time for the summer.

The creation of any such definition is complicated, however, by global climate change. “The impacts of warming become increasingly more significant as we go above two degrees [rise above pre-industrial levels],” Dr Stott told the committee.

The Met Office is working with the EU-funded Helix project to “look at the impacts of different warming levels”, Stott added. Their models for continental European summers predict that large scale temperature anomalies will become a regular occurrence by the middle of the century.

It is not yet known exactly how that trend will relate to the UK, but the country shares the continent’s “land signature” – which sees average temperatures over land rise faster than the global average.

Setting a new definition also requires looking back at the UK’s historical record of heatwave events. “Drawing the boundary to include one and exclude other [periods of hot weather] requires checking with the historical record to ensure the balance is right and that not too few or too many events are captured by the definition,” Madge explains.

At present, the only UK-specific definition in use is limited to England, and targeted primarily at health professionals and emergency planners. Created in conjunction with Public Health England, it uses temperature thresholds that vary by region. In London, qualifying temperatures must be at least 30 degree celcius over two consecutive days, plus an overnight temperature of at least 15 degrees in between.

The recent bout of hot weather did not meet the above criteria and so was deemed to be a “hotspell” not a “heatwave”. Temperatures in St James’s park reached 29.1 degrees celcius – just 0.3 degrees below the hottest ever April on record, Stott explained.

Yet ultimately, whatever new definition is arrived at, it is the increasing variability of UK weather that is likely to cause the biggest challenge of all. As Professor Michael Davies of the UK’s Committee on Climate Change (CCC) pointed out to the inquiry, the heat-related deaths can occur below the temperature set for heatwaves. This means that simple sunny spells, especially when they are unexpected like last week’s, can pose an even greater risk to public health:  “Owing to the relative infrequency of heatwaves, its normal periods of warm weather in summer that contribute to the greatest burden of mortality.”

With this in mind, the CCC has recommended that more needs to be done by the government to assess and reduce the risk of overheating in public buildings and homes. They are also concerned that there has been a drop in the overall percentage of green spaces in urban areas since 2001, which can help manage the heat-island effect. “We didn’t make a recommendation on that in our last report in 2017,” Kathryn Brown, Head of Adaption at the CCC, told the inquiry, “but it’s one we keep looking at.”

According to Defra research, public awareness of increasing hot weather is low: only 10 per cent of people thought average temperatures increased over the last 20 years. When heatwaves hit, "everyone hits the beach" Stott said. But he warned the inquiry, "There’s no narrative about how to look after your old people and babies."

If the UK is to remain safe as the climate continues to change, then public awareness will need to rise even faster than the heat. 

India Bourke is an environment writer and editorial assistant at the New Statesman.