Watching TV last night, the weather forecaster commented that although today the temperatures would reach the 90's for a second day in a row, we can't talk about a heatwave because there is unlikely to be a third day of high temperatures. A heatwave in New England is defined (by one of the local TV station weather forecasters) as a string of three 90+ degree Fahrenheit (over 32° Celsius) days in a row. Two days of those temperatures would definitely count as a heatwave in the UK.
Some people laugh when people in the UK complain about heatwaves when the temperatures are only in the 70's, (mid 20's Celsius) and you see headlines like "Britain swelters in the seventies." It makes complete sense though that the official definition of a heatwave will vary from place to place. In the Netherlands it's classified a heatwave when there are at least 5 days with a maximum temperature above 25°C (77°F) of which at least 3 days have a maximum temperature above 30°C (86° F). (Source: World Health Organization)
Looking for information on the topic this afternoon, I discovered that depending on where you are in the UK, let alone where you are in the world, the temperature threshold is different. In a large country such as the USA, I would have expected that. 'Hot' weather in New England is not the same as 'hot' in the southern USA. I was surprised to find regional differences in the definition of a heatwave for the UK. The temperatures must be reached on at least two consecutive days and the intervening night.
Scroll down - for some reason the table below has decided it needs lots of personal space and nothing I do will persuade it to move back up the page :-(
| Region | Threshold temperature | |
| Day max | Night min | |
| North East England | 28°C / 82.4°F | 15°C / 59°F |
| North West England | 30°C / 86°F | 15°C / 59°F |
| Yorkshire and the Humber | 29°C / 84°F | 15°C / 59°F |
| East Midlands | 30°C / 86°F | 15°C / 59°F |
| West Midlands | 30°C / 86°F | 15°C / 59°F |
| East of England | 30°C / 86°F | 15°C / 59°F |
| South East England | 31°C / 87.8°F | 16°C / 60.8°F |
| London | 32°C / 89.6°F | 18°C / 64.4°F |
| South West England | 30°C / 86°F | 15°C / 59°F |
| Wales | 30°C / 86°F | 15°C / 59°F |
In the United States, the National Weather Service suggest that a heat advisory be issued when the daytime heat index reaches 40.6°C / 105°F and a night time minimum temperature of 26.7°C / 80°F persists for at least 48 hours. Local definitions are used: in Dallas the medical examiners office define a heatwave as three consecutive days of temperatures over 37.8°C / 100.4°F - that's a good 10 degrees warmer than a New England heatwave.
I wonder how the definitions will change over the next few years as global warming takes effect and heatwaves become more common? Will we simply redefine what a heatwave is?
