The northern hemisphere has been hit by extreme heat over the past two weeks, with record-breaking temperatures recorded in the US, Canada, Europe and the Middle East. An unprecedented week-long heatwave has killed at least 18 people in the Canadian province of Québec, with the temperature in Montreal reaching an all-time record 36.6C on Monday.

The western US has been sweltering too, with 40.5C in the “mile high city” of Denver, which is 5,280 feet above sea level. The British Isles and other temperate parts of north-west Europe have been unseasonably hot for several weeks. Scotland’s hottest day on record saw a temperature of 33.2C in Motherwell. The Middle East and south-west Asia experienced extraordinary extremes even for a region famous for summer heat. Most remarkable was the hottest night ever recorded on Earth: in Quriyat, Oman, the temperature over a 24-hour period fell to a minimum of 42.6C. These meteorological events are linked by the behaviour of the jet stream, the band of fast winds blowing west to east in the upper atmosphere.

Movements in the wavy path that the jet stream follows around the globe play a key role in determining weather conditions. In places affected by heatwaves over recent weeks, the jet stream has been further north than usual, allowing large anticyclonic domes of hot air to build up to the south — which resist the encroachment by cooler air masses.