Every fall, tire shops get flooded with people whose tire-pressure-monitoring (TPMS) light just came on. The car is fine. The temperature dropped 30 degrees overnight, and the pressure inside the tires dropped with it. Here's the math and how to handle it.
The 1-PSI-per-10°F rule
Air pressure follows temperature roughly linearly. For every 10 degrees Fahrenheit that the ambient temperature drops, your tire pressure drops about 1 PSI. The reverse is also true — pressure goes up 1 PSI per 10°F warming.
So a tire set to 35 PSI on a 75-degree October afternoon will read 30 PSI on a 25-degree January morning. That triggers the TPMS warning at about 25% below the recommended pressure.
Why this matters in winter, specifically
Underinflated tires have a larger contact patch (sounds good for snow, but it isn't — see below), increased rolling resistance (worse fuel economy in winter, which is already worse than summer), and faster shoulder wear.
And contrary to a long-running myth, you do NOT want to underinflate tires for winter traction. Modern winter tires are designed to grip best at their door-jamb-recommended pressure. Underinflation hurts braking and cornering grip more than it helps anything.
What to do
Check pressure cold once a month in fall and winter — more often than the rest of the year because the temperature swings are larger. Set the pressure to the door-jamb spec at whatever the current cold-soak temperature is. As winter deepens, you may need to top off another 2-3 PSI mid-season.
If the TPMS light comes on
Don't ignore it — but don't panic either. Drive carefully to a gas station with a working air pump, and top off all four tires to the door-jamb recommendation. The light may need a few miles of driving above 20 mph to reset itself once pressures are correct.
Edge case — going to the mountains
If you drive from a warm low-elevation area to a cold high-elevation one (say, St. Louis to Colorado), both temperature drop and altitude rise will lower your pressure. Plan to top off at altitude after the temperature has stabilized.
Cold weather doesn't damage your tires. Ignoring the pressure drop does.