The definition of a heatwave

[xmetman]

Apparently the WMO definition of a heatwave is five consecutive days with the maximum temperature 5°C above the 1961-1990 long-term average.

This is how the anomalies looked in central England during the summer of 1976:

[Len W]

And what about a tropical heatwave Bruce?

As in the song:

We're having a heatwave

A tropical heatwave

Does the RH have to be above 80% 5 days in a row?

Len

[quaesoveritas]

Bruce,

On the BBC News Channel, on 22/6, Ben Rich gave the definition of a heatwave as 5 days above the average temperature.

From the figures, I think this is the mean temp. not max temp. as you said, which of course would make a heat wave more likely.

Actually his average is given as a range, which I think makes the definition a bit more vague than yours:

[Dick Lovett]

That average of 15-20C for June doesn't make sense, as there is nowhere in the UK that has a mean temperature as high as 20C in any month of the year. Even if you restict it to maximum temperatures, the June means in the London area are at least 21C and probably about 22C in the final week of the month.

Dick Lovett

[xmetman]

The definition is 5°C above the mean MAXIMUM for that day, and not the average or mean temperature for the month.

To do this you need to calculate the 1961-1990 long term average for each day, and that's what I did in the CET chart at the start of this post.

[Brian Wakem]

On Saturday, 23 June 2018 12:31:34 UTC+1, xmetman wrote:

Apparently the WMO definition of a heatwave is five consecutive days with the maximum temperature 5°C above the 1961-1990 long-term average.

There must be other criteria otherwise heatwaves are just as likely in January.

 

[quaesoveritas]

"There must be other criteria otherwise heatwaves are just as likely in January."

I think they probably are.

On the basis of the WMO definition, it is theoretically possible to have a heat wave in the winter.

But judging from this UKMO page, they seem to only happen in summer.Actually, looking at the actual temperature figures, the ones quoted by Ben Rich seem to be average maximum temperatures, although I don't entirely understand how there can be a range for the UK.

https://www.metoffice.gov.uk/learning/temperature/heatwave

[Dan Grey]

Great graph. Would love to see how this summer is shaping up.

It's interesting how the 61-90 average is used. There must be a mildly political element to that, in order to emphasise 'heatwaves' becoming more frequent across the decades.