How hot does a ‘heatwave’ have to be?

Some like it hot

Are heatwaves becoming a devalued currency? Last year the Met Office defined a heatwave as three consecutive days when maximum temperatures exceed the 90th percentile maximum temperature for mid-July. In London that means when the maximum exceeds 28˚C. For the rest of the south-east, as far west as Hampshire and as far north as Nottinghamshire, the threshold is 27˚C.