How fast does a parked car heat up?
Set today's temperature and where you parked. Then watch an hour pass in a few seconds, and see what happens to everything you left inside.
What happens, and when
Two hours parked: sun or shade
About 80% of the rise happens in the first 30 minutes. Shade helps a lot. Cracking a window barely helps at all.
What to do
Not even for a minute, not even with the windows open. If you see a child alone in a car, call 999.
Phones, laptops and power banks come with you. Never charge them in a hot car. If one swells, smells or is hot, let it cool away from the car.
Insulin and many medicines must stay below about 25–30°C. Check the leaflet.
Do the supermarket at the end of your trip. Keep lighters, aerosols and perfume out of the car in summer.
Sources: McLaren, Null and Quinn, "Heat stress from enclosed vehicles" (Pediatrics, 2005): after an hour in the sun the cabin was on average about 23°C (41°F) hotter than outside, 80% of the rise came in the first 30 minutes, and cracking the windows made almost no difference. Vanos et al. (Temperature, 2018), Arizona: on days above about 38°C, cabins reached about 47°C after an hour in the sun and about 38°C in the shade. NHTSA heatstroke prevention: a child's body temperature rises three to five times faster than an adult's. MedlinePlus (US National Library of Medicine): insulin in use should be kept at 15–30°C and never on a car dashboard. EU Aerosol Dispensers Directive (75/324/EEC): aerosol cans carry the warning "do not expose to temperatures exceeding 50°C". The other item limits are rough estimates. Not medical or safety advice. Live outside temperature from Open-Meteo.