Britain is bracing itself for another sweltering week as the second significant heatwave of the year grips England and Wales. Temperatures are forecast to soar into the mid-to-high 30s Celsius, prompting many to wonder why this hot spell feels so much more oppressive than previous ones. According to experts, several key meteorological factors contribute to the heightened discomfort, including a relentless 'heat dome', elevated humidity levels, and an unprecedented series of tropical nights.
The primary driver behind this intense heat is a static and expansive area of high pressure, often referred to as a 'heat dome'. Dr Akshay Deoras, a senior research scientist at the University of Reading, explained to BBC Weather that this phenomenon causes air to sink, compress, and heat up as it approaches the ground. Unlike the May heatwave, where the heat dome was centred directly over the UK, this week's dome originated in the hot and humid sub-tropics and has been primarily situated over France, where temperatures have already surpassed 40C. Its slight shift north and east has drawn hotter, moister air into the UK via a south-easterly airflow, leading to significantly higher humidity.
This increased humidity is a crucial factor in why the current heatwave feels so much worse. Humidity measures the amount of moisture in the air, and it directly impacts the body's ability to cool itself. When it's hot, our bodies sweat, and the evaporation of this sweat from our skin helps to cool us down. However, in highly humid conditions, sweat struggles to evaporate effectively, diminishing our natural cooling mechanism. Meteorologists can calculate a 'feels like' temperature, which accounts for both air temperature and humidity. While the actual air temperature might be 35C, the high humidity could make it feel closer to 41C, according to these calculations.
Adding to the discomfort are the unusually high overnight temperatures. Many locations are not expected to see temperatures drop below 20C, a phenomenon known as a 'tropical night'. It is anticipated that Thursday night could become the warmest June night on record, with minimum temperatures in parts of Wales, the Midlands, and the South East potentially remaining above 21C, or even 23C in some areas. This far exceeds the current UK and England record of 22.7C from 1976 and the recent Welsh record of 20.3C. Unlike previous heatwaves, such as July 2022 where few places experienced more than two consecutive tropical nights, some areas this week could endure three or four, offering little respite from the heat.
The combination of a moisture-laden heat dome, high humidity, and prolonged warm nights distinguishes this heatwave from others, even those with higher peak temperatures. For instance, the July 2022 heatwave, which saw the UK record 40C for the first time, was characterised by very dry air. Consequently, despite potentially lower headline temperatures, the current heatwave feels more oppressive due to these additional factors.