When is a Heatwave Not Actually a Heatwave? 

In the U.S., record-breaking heatwaves have wreaked havoc across the country, with droughts, wildfires, and schools switching to remote learning. Portland, Ore., and Seattle, Wash. hit all-time highs of 116 and 108°F on Monday

The black X’s on this map indicate all-time record high temperatures. Source

While there is no doubt that the current weather conditions here can be described as a heatwave, it’s not always so clear-cut. Sometimes, we might colloquially refer to a period of hot weather as a heatwave, but meteorologists would disagree. 

Heatwave Thresholds

The UK is better known for its rain than its sun, especially during outdoor events that depend on good weather, such as Wimbledon. But this isn’t always the case. 

After a particularly rainy May, large parts of the UK, including London, basked in glorious sunny and hot weather in early June with temperatures reaching highs of 86°F (if you’re a resident of Texas or Arizona, you might question whether that counts as a high or if that’s just typical weather). 

Yet, despite this heat spell that saw a spike in interest in fans and air conditioning units, meteorologists did not class it as a heatwave. This is because forecasters in the UK must record daily maximum temperatures that meet or exceed the heatwave temperature threshold for three days in a row before classifying a weather system as a heatwave. This threshold varies according to the region. 

In South West England, the threshold for a heatwave is 25°C (77°F), while in London, it is 28°C (82°F). However, due to thunderstorms and a resultant drop in temperatures, the weather system did not meet the threshold for a heatwave. 

Meteorologists also pointed out that the temperature might have felt hotter than normal due to the unseasonably cold weather in the UK in May. 

Heatwaves By Location

There is no standard definition of a heatwave, and it varies across the world. However, the World Meteorological Organization says that daily maximum temperatures must exceed the average maximum temperature by 9 °F for more than five consecutive days to be considered a heatwave. 

In the U.S., definitions vary by region, but a heatwave is generally defined as a period of at least two or more days of sweltering weather. In California, they even have a term for an extended heatwave –– a heat storm. This occurs when the temperature reaches 100 °F for three or more consecutive days. 

The Greek National Meteorological Service defines a heatwave as three consecutive days with daily maximum temperatures at or above 102 °F and minimum temperatures at or above 79 °F. 

In Denmark, they have both heatwaves and warmth waves. A heatwave (hedebølge) is defined as a period of at least three consecutive days when the average maximum temperature across more than 50% of the country exceeds 82.4 °F. A warmth wave (varmebølge) must meet the same conditions but for a temperature of 77.0 °F. 

Read more here on how to stay safe and cool during the warmer months.

Featured Photo by Becky Phan on Unsplash 

 

 

 

 

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