Keep your cool, it’s not an official heat wave yet
YOU may have been seeing the words “heat wave” thrown around a bit this week. Like desperately finding an exotic location that we are officially warmer than (this week, it’s Ibiza), declaring a heat wave is something that seemingly must be done whenever the mercury nudges past 77F (25C).
A heat wave, though, is a strictly defined thing, not that everybody agrees on quite what that definition should be. The World Meteorological Organisation has it as five or more consecutive days during which the daily maximum temperature surpasses the average maximum temperature by 41F (5C) or more. Seems reasonable.
Particular countries have their own versions. In Denmark there is a sensible two-tier system, in which a heat wave is declared when more than half the country exceeds 82.4F (28C) for three days, and a “warmth wave”, when it’s over 77F (25C).
And in the Netherlands, Belgium and Luxembourg, it is a heat wave if five consecutive days see more than 77F (25C), but only if at least three days in this period are over 30C (86F).
Just thinking about it is enough to warrant a lie down and an ice lolly. But what about here? The Met Office has settled on “a period of at least three consecutive days with maximum temperatures meeting or exceeding the heat wave temperature threshold”.
That threshold, confusingly, is different depending on the area. London’s is 82.4F (28C), the rest of the South East and Midlands 80.6F (27C), the next band of counties 78.8F (26C), and the rest – Cornwall, Devon, most of Wales, the north of England and all of Scotland and Northern Ireland – at 77F (25C).
So yesterday may have been the hottest day of the year, but just as one swallow doesn’t make a summer, one scorcher doesn’t make a heat wave. We are just experiencing a “hot spell” – which doesn’t sound quite so exciting.