The Singapore Grand Prix has been declared a heat-hazard race, the second weekend in a row that the designation has been used in Formula 1 and the fourth time this year.
The definition allows teams to fit cooling devices to their cars that pump liquid through a vest worn by the driver to manage temperatures in the cockpit.
A heat hazard is an official status for an event where it is forecast that temperatures will exceed 31C at some point during the race. It was first introduced in 2025.
Singapore is close to the equator and has long been known as one of the most demanding for drivers - in 2024, George Russell and Lewis Hamilton were treated for heat exhaustion after the race.
Temperatures in the city are always in the 31-33C region across the race weekend, with high humidity and a chance of rain every day.
The race is held at night, but temperatures remain in the region of 30C even after dark.
A cooling kit is not mandatory, but any driver who chooses not to use it has to carry 5kg of ballast in their car to ensure he does not have a competitive advantage.
The system uses a cooling tank and pump, usually situated in the car's nose, that delivers a cooled liquid, such as glycol, through a system of pipes in a fireproof top worn under their overalls.
Some teams and drivers prefer not to use the system because of two key compromises.
It is difficult to keep the liquid cool enough for long enough so that it lasts for an entire grand prix - a problem exacerbated in Singapore because it is the longest race of the year.
And some drivers find the cooling top uncomfortable in the tight confines of the cockpit.
If the liquid cannot be kept cool, it reverts to car temperature, which is considerably higher than ambient. It is not unusual for temperatures in an F1 cockpit to rise above 40C.
Mercedes are one of the teams who have decided against running the system because they preferred to devote the resource that would have been involved in making it robust enough to last a race into car performance.
Governing body the FIA has granted teams an allowance under the cost cap of $50,000 (£38,000) if they can make a system that keeps the liquid cool for an entire grand prix, and a further $75,000 (£57,000) if they could make the top comfortable.
Mercedes deputy team principal Bradley Lord described the allowance as "small", adding: "We didn't believe that allowance was sufficient for us to be able to develop a solution that would be robust enough to last for the entire race."
Singapore follows the Austrian, Italian and last weekend's Bahrain Grand Prix in Malaysia in being declared a heat-hazard race this year.