A parked car doesn't care what the season is. Even highs in the 50s can turn your car into an oven. Sunlight passes straight through glass and heats the inside, and that heat has nowhere to go. 

Most of us think of dangerous heat inside a car as just a summer problem. But temperatures we might consider "cool" can still be dangerous, and even deadly.

If the outside air is a cool 57 degrees, it can still push a parked car's interior past 110 within about an hour, according to research cited by the National Highway Traffic Safety Administration.

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In the first 10 minutes, interior temperatures can climb by roughly 19 degrees. Most of that rise happens fast.

Cracking a window doesn't help as much as you might expect. Research on the subject has found a slightly open window makes almost no difference to either how fast a car heats up or how hot it ultimately gets inside.

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Hot car deaths get the most attention in July and August, because that's when about two-thirds of them happen. But that also means close to a third of these events take place during cooler months.

Cases have been recorded in every month of the year except January. The heat inside a car can get to dangerous levels at any season, and at outside temperatures most people wouldn't think twice about.

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How quickly a mild day can heat up

As a general rule, 80% of the rise happens within the first half-hour. And even on a mild day, the inside of a car can turn dangerous quickly.

• 60 degrees outside → 94 degrees after 30 minutes
• 65 degrees outside → 99 degrees inside after 30 minutes 
• 70 degrees outside → 104 degrees inside after 30 minutes
• 75 degrees outside → 109 degrees inside after 30 minutes

The science behind why this happens is called the greenhouse effect.

What is the greenhouse effect?

The greenhouse effect is the natural process where gases in Earth’s atmosphere trap the sun’s heat and keep the surface of our planet warm enough to support life. Without the greenhouse effect, our planet would be uninhabitable. 

However, when it comes to vehicles, the windows trap heat in a similar way, with no way for it to escape. 

The result is the car gets hotter and hotter, reaching dangerous temperatures in a matter of minutes.