How Saharan sand in France is making nights even hotter

The third major heatwave of the summer continues to see France experience extreme temperatures, leading to uncomfortable days and nights for millions.

One particular feature of the weather is the high night-time temperatures recorded across several areas of France, including the south.

France’s overnight temperature record was broken earlier in the heatwave when temperatures in Vivès (Pyrénées-Orientales) did not drop below 30.6C across the night. 

Overnight temperatures as high as 25C across most of the south are forecast in the coming days by state forecaster Météo France, rising to potentially 27C in Paris where dense urban conditions make it difficult for heat to escape.

Saharan sand exacerbates conditions

A factor which may cause temperatures to remain high overnight in the coming days is Saharan sand blowing in from North Africa.

Better known for causing dust particles to fall across cities leaving vehicles and streets streaked with dust, the sand also prevents radiation from leaving the atmosphere. 

“At night, the Earth emits radiation to cool down. However, if this radiation encounters something in the atmosphere, part of it is reflected back to its source,” said Météo France meteorologist Tristan Amm, quoted in La Dépêche. 

This in turn heats up the ground even more, bringing the warm radiation back. A similar pattern is seen during periods of the ‘heat dome’ effect, common during heatwave periods.

“Sand particles make clouds more opaque, meaning they reflect more radiation back toward the Earth,” Mr Amm said. 

A new bout of Saharan sand is set to be blown across France this week, arriving over the Pyrénées. 

The sand, often limited to the south and south-west, is expected to reach as far north as the Grand Est, Bourgogne-Franche-Comté, and Auvergne-Rhône-Alpes regions according to Mr Amm. 

This burst of hot air will also reinforce intense temperatures across France, blocking out a mass of cold Atlantic air. 

“The cold-core low (or weather depression) located near Portugal is acting like a heat pump. It draws up the hot air accumulating over the Iberian Peninsula and North Africa and pushes it toward France,” Mr Amm said. 

Current estimates see the heatwave lasting until the weekend. 

More tropical nights forecast 

High overnight temperatures are one of the principal triggers for heatwave warnings initially being raised. 

Generally, day- and night-time temperatures have to reach above a certain departmental threshold for three consecutive days for canicule warnings to be implemented. 

Nights where temperatures do not drop below 20C are usually classified as ‘tropical nights', nuits tropicales. 

This threshold is notable because above this human bodies and buildings alike struggle to cool down during the evening hours, with extreme day-time heat making this all but an impossibility during daylight hours.

In areas of France facing cooler winters – including many currently facing red-level heatwave alerts – difficulties are even more pronounced as buildings are built with the purpose of retaining heat overnight.

A lack of suitable ventilation can make properties feel suffocating in the intense night heat, with residents unable to escape high temperatures. 

Note that if you leave your windows open and you are burgled, there is a risk you will not be covered by your insurance.