How to keep your car cool when it is parked

Why a parked car gets so hot

Sunlight passes through glass as visible and near-infrared light, hits the dark surfaces inside, and is re-radiated as longer-wave infrared. Glass is much less transparent to that longer wavelength, so the energy is trapped: it comes in easily and leaves slowly.

Which tells you exactly where the leverage is. Stop the energy coming in — that is the windshield shade and the parking angle — or give the trapped heat a way out, which is the cracked window. Everything on this page is one of those two things.

The seven tactics, ranked by effort

The free one people skip

The windshield is the largest and most steeply angled window on the car, which makes it the dominant path for solar energy. Point it away from where the sun will be when you come back and you have made the biggest available change for nothing.

The practical version: if you park at nine in the morning and return at five, the sun will be in the west. Park facing east. In a lot with a choice of orientation, that is a decision you make in three seconds.

Cracking the windows: how much, and the trade

Hot air rises and accumulates against the headliner. A gap of a few millimeters at the top of two windows lets it escape and lets cooler air in low, which is a convection loop rather than a seal.

The trade is security and rain, and it is real. A few millimeters is not an entry point; two inches is. Use your judgment about where the car is parked, and do not do it if rain is possible — a wet interior is a worse problem than a hot one, and it lasts longer.

The shade, which is the thing to actually buy

Under $25, blocks the dominant heat path, and helps everything in the car: the dashboard plastic, the steering wheel, the adhesive holding a phone mount on, and the dash cam living in the hottest strip of glass in the car.

Manufacturer claims in this category range from a certified 99.87% UVB blocking figure to a much rounder “up to 40 F cooler” with no stated conditions. The shade page sorts out which claims are testable and which are marketing, and covers sizing, which is the part that decides whether it works at all.

Cooling it down once you are back

The instinct is to get in, close the doors and put the air conditioning on maximum. That asks the system to chill a cabin full of very hot air, which takes a while.

Faster: open the windows for the first minute of driving with the fan on and the air conditioning off, so the hot air is physically pushed out and replaced with outside air. Then close up and switch the air conditioning on. You have removed the heat instead of cooling it.

And use fresh air rather than recirculate for that first minute — recirculating is recirculating the hot air you are trying to get rid of. Switch to recirculate once the cabin is close to outside temperature, which is when it becomes the more efficient mode.

What does not do much

• Solar-powered vent fans. Small, in a window gap, moving a trivial volume of air. The gap they need is doing most of the work on its own.
• Leaving the air conditioning running while parked. Fuel, and a running engine, for a car nobody is in.
• Dashboard covers alone. They protect the dashboard from UV, which is worth something, and they do not stop the energy entering the cabin. A windshield shade does both.

What not to leave in a hot car

Anything with a lithium battery, which is most portable electronics. That specifically includes a jump starter, which is a lithium pack that lives in your trunk precisely so it is there when you need it — heat shortens its life and it is worth knowing that a jump starter which has spent three summers in a trunk may not have the charge it claims.

Also: aerosol cans, sunscreen, anything in a plastic bottle you intend to drink, and anything electronic you care about. A dash cam is designed for the environment; the ones with published heat ceilings are on this page, and it is the only category on this site where makers publish the number at all.