Why do you see this warning light so often in winter?

If you own a car, you may have noticed that a small warning light depicting a flat tire with an exclamation mark in the middle comes on much more frequently during the cold winter months.

First of all, it is a warning light that should not be ignored. This means that one or more of your tires have lost pressure, which may indicate a tire failure. But it’s not just punctures that can cause tire pressure to drop – the temperature can also cause tire pressure to drop.

When you pump air into a tire, it inflates. So far, so simple. You pump until the tire is so full of air in a small space that it pushes against the tire walls enough to support the weight of your vehicle. Since the air pressure inside the tires is higher than the pressure of the surrounding air and the car on them, it prevents your car from scraping and screeching along the ground while driving.

However, the air molecules in your tires are not static clumps, but are constantly bouncing back and forth, exerting this pressure outwards. And as you probably know from school, or at least from the “thermometers are just speedometers for atoms” meme, when the temperature drops in the winter, the air in your tires stops moving as quickly, and so the outside pressure drops.

“A good estimate is that vehicle tire pressure adjusts by approximately 1 psi for every 10°F (5.6°C) change in air temperature,” explains Tire Buyer. “If the outside air temperature drops by 30°C (16.7°C) compared to the last tire pressure setting, expect the tire pressure to drop by approximately 3 psi.”

That’s about 6.9 Pascals per 5.6°C of temperature drop for all metric fans in the house, although this depends on your tires and is just a rough rule.

You may be wondering how the outside temperature affects the air trapped in your tires. When the temperature in the material of your tires drops, it means that the individual molecules within them move less. As this happens, the air inside collides with the material and transfers energy to it in the form of heat, eventually causing the air inside to slow down (thus lowering the temperature).

On hot days or after a long journey where your tires heat up due to friction, tire pressure increases. You should take this into account when refilling with air.

“Michelin recommends waiting two hours after a journey before checking tire pressure, unless the journey was only a few miles at low speed,” explains the AA. “If you check the tires on the forecourt of a gas station while driving, then as a rule of thumb add 4 or 5 psi to the (cold) pressure value specified in the manual.”

But regardless of whether the weather is cold or hot, whenever you see this warning light, please don’t ignore it completely.

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