What Temperature Counts as a Heatwave
When the press starts talking about a heatwave, there is a real rule that they have to follow. A heatwave is not just a few hot days in a row, and the number of hot days can vary depending on where you are in the country.
The Official Definition
For a heatwave to be declared, the maximum temperature must reach the threshold for three consecutive days. However, all three days must reach the threshold on each day, not over the course of the three days. In other words, one very hot day is not a heatwave. The temperature must reach the threshold on each of the three days and it must reach the threshold at a weather station near you.
Why There Isn’t One National Number
To respond to the myth that England could have a single heatwave temperature, note that while England is generally a hot country, the climate varies greatly from one end of the country to the other. As such, the temperature for a heatwave can range from 25°C in Scotland and Wales to as high as 28°C in the South East of England, including London. This is not to say that all parts of England have a range of 3 degrees, but rather that there are varying degrees that would be considered to be heatwaves in different parts of the country. For example, a temperature of 27°C would be considered a heatwave in Aberdeenshire but not in Surrey.
Finding Your County’s Threshold
For those wanting to check on the temperature for their own area, the specific temperature for each county isn’t detailed out in a list. Instead, the information is detailed out on a map and you need to check the colour of the county to find out what band your area is in and therefore what temperature would be deemed to trigger a heatwave in your area. It’s worth spending a few minutes looking at this before the summer and saving for future reference rather than trying to look at it whilst the site is being swamped with heatwave related activity.
A Run That Qualifies
I’ve applied this to an example before. Suppose a heatwave is declared for a county with a 27°C threshold. The temperatures for a three day period are as follows: Monday 28°C, Tuesday 29°C and Wednesday 27.5°C. Therefore a heatwave has been confirmed for Wednesday. The fact that Tuesday had been the hottest day did not matter. The fact that none of the three days had fallen below the 27°C threshold had mattered.
A Near-Miss That Doesn’t Qualify
In the example above we have declared a heatwave for the same county as above, but on this occasion the temperatures for the three days were: Monday 29°C, Tuesday 28°C and Wednesday 24°C. This is a near-miss for a heatwave as the temperature only fell on the third day and not on the second, so a heatwave was not declared. The same would be true for a single very hot day sandwiched between two average days.
Who Actually Confirms It
A heatwave is confirmed by the Met Office after they have checked the maximum temperature recorded at one of their climate stations against the relevant county threshold for 3 consecutive days. The term heatwave is often misused by the media to describe a single very hot day or a few hot days interspersed with average temperatures. The threshold temperatures for a heatwave and the associated health advice for working in the heat are based on verified temperature data and not on general impressions of hot weather. This is why employers and businesses need to look at the guidance on working in hot temperatures for verified temperature thresholds rather than just very hot days.
Heat Exhaustion Or Heatstroke
It is also important to recognise signs of heat exhaustion and heatstroke and to know how to treat them. Heat exhaustion typically displays through heavy sweating, dizziness, tiredness, a headache and nausea. Sufferers of heat exhaustion often display pale and clammy skin. People suffering from heat exhaustion can recover from their symptoms within 30 minutes if they are removed from the heat, given something to drink and allowed to rest.
Heatstroke is a more serious condition and you must seek urgent medical help. Patients with heatstroke have a high temperature that does not come down with paracetamol or other remedies. They may be confused, have a fast heart rate and be breathing quickly. They may not sweat despite feeling hot and may become unconscious. If the patient does not improve within 30 minutes of being cooled down then seek further medical help.
Common Questions
Is 30°C for one day a heatwave? No – the single hottest day of the year does not constitute a heatwave. A heatwave is defined as a period of three or more consecutive days of temperatures above the heatwave threshold for a specific area.
Because the definition is based on what counts as unusually hot for each area’s typical climate, not a single UK-wide figure. Scotland and parts of Wales tend to run cooler on average than the south of The reason for this is that the definition of heatwave is based on what is unusually hot for each area’s typical climate, so a lower temperature in Scotland and parts of Wales than in London will be just as significant as a higher temperature in the South East of England.
Who decides a heatwave has happened? Confirmation of a heatwave is provided by the Met Office, using real recorded data from the climate stations across the country. The Met Office check the maximum temperature for each of the climate stations in a county against the defined threshold for that county for each day.
Is it heat exhaustion or heatstroke? Heat exhaustion is characterised by heavy sweating, dizziness, tiredness and a sore, throbbing headache. People with heat exhaustion usually recover within 30 minutes to an hour, after having been given rest, shade and fluids. Heatstroke is more serious and needs urgent medical attention. With heatstroke the sufferer may have a high temperature that doesn’t come down and may appear confused. They may also experience a fast or racing heartbeat and may be breathing very quickly. People with heatstroke do not go red in the face and may even be cool to the touch. In some cases the person may not go red at all and may even be cool to the touch and may not even sweat.
