Factors Contributing to the Heatwave's Severity
This week, England and Wales are bracing for temperatures in the mid-to-high 30s Celsius, potentially setting new June records. For many, the perceived heat will be even more intense than the actual thermometer readings, a phenomenon attributed to an unyielding heat dome, high humidity, and a succession of warm nights.
The Role of the Heat Dome
A significant and stationary high-pressure system, often referred to as a heat dome, is the primary cause of the intense heatwave affecting the UK and Europe. Dr. Akshay Deoras, a senior research scientist at the University of Reading, describes this as the “driving force” behind the extreme temperatures. This weather pattern causes air to descend, warming and drying as it does, which in turn leads to more direct sunlight heating the ground.
Unlike the May heatwave, where the heat dome was centered directly over the UK, this week's dome originated in the sub-tropics and has been positioned more over France, where temperatures have surpassed 40C. Its slight shift north and east has channeled a south-easterly airflow, bringing hotter and more humid air into the UK. This increased moisture significantly amplifies the sensation of heat stress. Dr. Deoras characterized the situation as “not just a heatwave, it is a heat-dome driven furnace.”
The Impact of High Humidity
The hottest regions of the UK, particularly southern England and parts of Wales, are expected to reach 38 or 39C. The discomfort is greatly exacerbated by high humidity, which measures the amount of moisture in the air. The human body cools itself by sweating, with the evaporation of sweat from the skin dissipating heat. However, in humid conditions, sweat cannot evaporate as effectively, hindering the body’s natural cooling process.
Meteorologists use a “feels like” temperature to account for the combined effect of air temperature and humidity. For instance, an air temperature of 35C might actually “feel” like 41C. While the July 2022 heatwave saw higher headline temperatures, with the UK recording 40C for the first time, the air was considerably drier. Consequently, this June’s heatwave, despite potentially lower absolute peak temperatures, is expected to feel more oppressive and could pose greater health risks due to the humidity.
Unrelenting Tropical Nights
Overnight temperatures are predicted to remain high throughout the week, with some areas not dropping below 20C, a phenomenon known as “tropical nights.” It is anticipated that Thursday night could be the warmest June night on record, with minimum temperatures across Wales, the Midlands, and the South East potentially staying above 21 to 23C. Some locations might even exceed the current UK and England record of 22.7C from 1979, and the recent Wales record of 20.3C.
The “feels like” temperature during the night may be particularly notable, potentially reaching 27C even at the coolest point. This contrasts sharply with previous heatwaves; for example, during the July 2022 heatwave, while many areas experienced tropical nights, few had more than two consecutively. This heatwave could see some locations endure three or four consecutive tropical nights, which is far more impactful. This prolonged warmth prevents the body from cooling down adequately between daytime heat spikes, posing risks not only to vulnerable individuals but also to healthy people, leading to disrupted sleep, cardiovascular strain, and an increased risk of heat exhaustion.
As global temperatures continue to rise, such heatwaves are expected to become more frequent. Dr. Deoras emphasized that without significant reductions in greenhouse gas emissions, global warming will accelerate. He noted, “A warmer atmosphere holds more moisture, so the combination of increased temperatures and humidity is what is likely to make humid heatwaves like this become more frequent and more intense.”