This is a series of articles examining heatwaves from every angle, exploring their causes, consequences, and the science behind them. It includes eight parts:
- Introduction
- Heatwave patterns
- Impacts on human health
- Impacts on urban areas
- Extreme heat in 2024
- Are heatwaves linked to climate change?
- Adapting to more frequent periods of extreme heat
- Temperature, apparent temperature, and heat indices
Synoptic patterns typical of heatwaves
Shorter heatwave episodes, lasting one to three days, are often the result of the advection of very warm air from lower latitudes, particularly from the Sahara in the case of Europe. Under sunny conditions, this warm air heats up further as it passes over the hot land surface, leading to a continued rise in temperatures.
The advection of very warm air was the cause of the heatwave that struck parts of Western Europe in mid-July 2022. In the United Kingdom, a new temperature record of 40.3 °C was set, surpassing the previous all-time high by 1.5 °C. Temperatures exceeded 40 °C in France, 45 °C in Spain, and 46 °C in Portugal. During the exceptionally hot summer of 2022, extreme heat claimed the lives of more than 60,000 people across Europe.
Prolonged heatwaves are often the result of a persistent and extensive area of high pressure, known as a heat dome. Subsiding air in the centre of the anticyclone suppresses upward movement of warm air from the surface. Additionally, the descending air warms adiabatically, further increasing near-surface temperatures. Due to minimal vertical air movement, cloud formation is inhibited, allowing more solar radiation to reach the ground. This further heats both the soil and the air above it, potentially leading to long-lasting heatwave conditions.
A heat dome often forms in situations where upper-level jet streams curve toward the polar regions. Its development can also be influenced by air descending from mountainous areas, which warms adiabatically as it sinks.
A striking example of a heatwave caused by a heat dome is the extreme nine-day heatwave that affected western Canada and the U.S. Pacific Northwest in late June and early July 2021. In many locations, temperatures exceeded 40 °C, with an extraordinary 49.6 °C recorded on June 29 in the Canadian town of Lytton, British Columbia, which set a new national record. The following day, the town was completely destroyed by a large wildfire, largely fuelled by parched vegetation resulting from the extreme heat. The heatwave had devastating consequences. It claimed more than 1,400 lives, triggered widespread wildfires, and caused severe air pollution from smoke across the region.
Occurrence of Heatwaves
An analysis of extreme heatwave data over the past 65 years shows that their occurrence has increased significantly, particularly in the last five years (2024 study). The most affected regions include Europe, densely populated central China, Japan, Korea, the Arabian Peninsula, eastern and southern Australia, and scattered parts of Africa. Other impacted areas include northern Canada and its Arctic islands, northern Greenland, the southern edge of South America, parts of Siberia, and to some extent, Texas and New Mexico. Regions where heatwaves are intensifying more rapidly than elsewhere are referred to as so-called heatwave hotspots.
One possible explanation for why heatwaves repeatedly occur in certain regions is a hypothesis based on the behaviour of the jet stream and Rossby waves within the global atmospheric circulation.
A detailed analysis of climate data from 1979 to 2018 showed that Rossby waves known as wave-5 and wave-7 can form stable and spatially organized meanders in the jet stream that repeatedly appear over specific regions. These waves can persist over the same areas for weeks, creating blocking anticyclonic conditions at the surface. Other types of waves do not exhibit this regularity.
Wave-5 patterns often bring stable, long-lasting weather over North America, eastern Asia, and eastern Europe, while wave-7 patterns tend to focus over western and central North America, western Europe, and western Asia. In both cases, the effect is similar: warm air is drawn northward into the crests of these waves, leading to extreme and prolonged temperature increases, often associated with heatwaves and drought.
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