Why doesn't cracking the car windows keep a dog safe in the heat?

Sunlight passes through the glass and heats the dashboard, seats, and air inside, and that trapped heat radiates back out far more slowly than it came in, similar to how a greenhouse stays warm long after the sun has moved. On a mild 22°C day, a car's interior can pass 35°C in about 20 minutes and keep climbing from there, well past what most people expect from checking the outside temperature alone.

Testing consistently shows that cracking a window a few centimeters only reduces the peak interior temperature by a small margin, often just a couple of degrees, nowhere near enough to offset the greenhouse effect from the rest of the glass. The gap also does little to add real airflow, since there is no breeze moving through a stationary, mostly sealed car.

The only reliable approach is never leaving a dog alone in a parked car, even for a quick errand, regardless of the outside temperature or how long you plan to be gone. If a dog must be in the car during a stop, one person stays with it and the air conditioning runs, or the dog comes along on the errand instead.

This applies even on days that do not feel especially hot, since a mild 18 to 20°C day outside can still push a closed car's interior into the mid 30s given enough direct sun and time. Planning errands around the dog, rather than planning to leave the dog in the car for a few minutes, avoids the risk entirely.