For the second time this year, many parts of western and central Europe are experiencing a prolonged spell of extreme heat, with record-breaking June temperatures expected this week.
The heat wave is being driven by a strong area of high pressure parked over continental Europe. This phenomenon is often referred to as a “heat dome.”
What is a heat dome?
A heat dome is a large, persistent area of high pressure high up in the atmosphere that allows heat to build over a region for several days or longer. They occur all over the world.
The Royal Meteorological Society in Reading, England, describes the high-pressure system as being a lid on a pot. It limits rising air, meaning warm air near the Earth’s surface cannot rise and form clouds. With fewer clouds, there is more sunshine, which allows the ground to heat up day after day.
France’s weather agency, Météo-France, says these stubborn high-pressure systems can also block or divert passing weather fronts, leading to conditions with few clouds and little rainfall.
The “lid” effect also helps trap and recycle heat. As warm air sinks within the system, it becomes compressed and warms even more, much like how air heats up inside a bicycle pump when it is squeezed, making the trapped air hotter and hotter, steadily increasing temperatures.
The combination of prolonged sunshine and sinking, warming air allows heat to steadily accumulate, increasing the likelihood of extreme temperatures during heat waves.
This week’s temperatures are likely to break records.
According to Britain’s weather service, the Met Office, this week’s European heat wave is likely to push temperatures to around 10 degrees Celsius, or 18 degrees Fahrenheit, above normal for this time of year for many parts of western and central Europe.
Parts of southern Britain have been issued a rare red warning for extreme heat, the highest level, for Wednesday morning through Thursday night. Forecasters at the Met Office said temperatures could reach 40 degrees Celsius (104 degrees Fahrenheit) for parts of Wales and central and southern England, including London. This would make it the hottest day ever for this early in the year and only the second time on record the 40-degree mark has been met or exceeded.
In France, forecasters said temperatures in Paris could hit 40 degrees Celsius this week, which would be unprecedented for June. More than half of the country is under a red alert for heat wave conditions, the highest level. Météo-France said “exceptionally high temperatures, both day and night,” were expected.
Spain, Portugal, Switzerland, Luxembourg and Germany are all under high-level heat warnings, with temperatures forecast in the high 30s to mid-40s Celsius (the upper 90s to lower 100s in Fahrenheit).
The episode of extreme heat comes not long after a record-breaking heat wave in May, when temperatures in both Britain and France reached their highest ever recorded for the month.
Heat domes are happening more frequently.
While high-pressure systems or heat domes are part of natural weather variability, scientists say climate change is increasing the frequency and intensity of these extreme heat events.
Hannah Cloke, Regius Professor of Meteorology and Climate Science at the University of Reading, said a warmer atmosphere is effectively acting as a “warming springboard,” meaning that when heat domes develop, they are starting from a hotter base line and can more easily set off heat wave conditions.
“At the same time, there is some evidence that more persistent high-pressure blocking patterns can develop,” she said, “allowing the famously changeable British weather to become stuck in one position for longer, enabling heat to build and linger.”
This combination of a warmer climate and more persistent weather patterns means that once a heat wave develops, it is more likely to intensify and linger.
The EU’s Copernicus Climate Change Service adds that these prolonged heat waves also have wider impacts on the environment. Hot conditions during heat waves, combined with a lack of rainfall, can dry out vegetation, which can create “ideal conditions for large wildfires,” it said.
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