June temperature records are set to be broken this week as the second heatwave of the year grips England and Wales. Temperatures are expected to reach the mid- to high 30s Celsius – similar to May – but for many, this heatwave will feel far more intense. Where the forecast is a maximum of 35C, it may feel like 41C.
That discomfort comes down to a combination of factors: an unrelenting heat dome, high humidity, and back-to-back tropical nights. The intense heat has been caused by a static and large area of high pressure – a heat dome – which Dr Akshay Deoras, senior research scientist at the University of Reading, called the “driving force” behind the heatwave. This weather pattern forces air to sink through the atmosphere, compressing and heating as it hits the ground. The sinking air also dries out, preventing clouds from forming and allowing strong sunshine to heat the ground further.
“June records set to break as heat dome and humidity make 35C feel like 41C in UK.”
During the May heatwave, the centre of the heat dome was over the UK, so heat built underneath it rather than being pulled in from more humid places. This week, the heat dome originated in the hot and humid sub-tropics and has been centred more over France, where temperatures have soared into the 40s Celsius. As it shifted slightly north and east, a south-easterly airflow has brought hotter, moister air into the UK.
“This is not just a heatwave, it is a heat-dome driven furnace that will grip most of the southern UK and push temperatures into truly exceptional territory,” said Dr Deoras.
The moist air results in much higher humidity, adding significantly to heat stress. Humidity is a measure of how much moisture is in the air. During hot weather, the body cools itself by sweating; as sweat evaporates from the skin, it cools the air next to us. But when the air is humid, sweat cannot evaporate effectively, reducing our ability to cool down. Meteorologists calculate a “feels like” temperature that combines air temperature and humidity. So while the thermometer may read 35C, high humidity can make it feel like 41C.
The hottest areas – southern England and parts of Wales – will see temperatures of 38 or 39C, and this becomes particularly uncomfortable when humidity is factored in. Some have compared this week to the historic heatwave of July 2022, when the UK recorded 40C for the first time. But during that heatwave, the air was very dry. This time, the humidity is making the heat far more oppressive – a furnace rather than a dry oven.