What are ‘tropical nights’ as phenomenon to hit UK amid third heatwave

The once extremely-rare weather event is becoming increasingly common

Tropical nights are set to hit the UK later this week as a third heatwave of the year was officially declared. Temperatures expected to climb as high as 35C in some areas later this week, the Met Office has said.

South east England has now officially met the heatwave criteria, after three days of temperatures above 28C, according to the forecaster.

And temperatures are set to continue to rise across the UK throughout the week, and more regions are likely to reach their local heatwave thresholds it is predicted with the mercury peaking into the 30s in multiple areas.

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Parts of southern England could see temperature highs of about 32C on Tuesday, rising to 33C on Wednesday and 34C on Thursday, with the highest readings of 35C forecast for isolated areas on Friday and Saturday.

By the weekend, the warm air is forecast to reach northern parts of the UK, and conditions are set to become increasingly humid, with a growing risk of showers or isolated thunderstorms. Amber heat health alerts have also been issued by the UK Health Security Agency (UKHSA) for the Midlands and southern England, from 9am on Wednesday until 9pm on Sunday.

Meanwhile, yellow heat health alerts have been issued by the agency for Yorkshire and northern England, also between 9am on Wednesday to 9pm on Sunday.

The Met Office said UV levels are expected to be high or very high across many parts of the UK throughout the week, and advised people to ensure they use sun protection.

The UK is also expected to experience tropical nights by the end of the week. These are a weather event which are becoming more increasingly common as temperatures continue to rise.

What are tropical nights?

As the UK experiences increasingly warmer summers, the phenomenon of tropical nights is becoming more relevant to both weather forecasting and public health, the Met Office website states. They are identified by consistently warmer nights but also pose health risks.

Tropical nights are defined by a night when the temperature does not fall below 20C. Recent studies show that these are not just confined to warmer climates, and that the UK is now seeing more frequent occurrences of these events and particularly during heatwaves.

The likelihood of experiencing three consecutive tropical nights during July has increased significantly due to climate change. In a pre-industrial climate the probability was less than one per cent per year. However, in today’s climate, that probability has risen to approximately 20 per cent per year.

“In July 2022, the UK experienced three consecutive tropical nights, marking a significant milestone in the country’s climate history,” The Met Office website reads. “This event was characterised by minimum temperatures remaining above 20C for three successive 24-hour periods. Such conditions pose serious health risks, especially for vulnerable populations.”

Tropical nights can also affect human health, the Met Office explained. This is because the body relies on cooler night time temperatures to regulate core temperature and recover from daytime heat.

When overnight temperatures remain high, the recovery process can not be carried out and can lead to increased cardiovascular stress and sleep disturbances. High night time temperatures are also linked to increased mortality, particularly among older adults and those with pre-existing health conditions.