Have you ever noticed how hot it can get inside a car on a summer day — far hotter than it is outside? That's because a car acts like a greenhouse, trapping the sun's heat.

A study by the Animal Protection Institute showed that even moderately warm temperatures outside can quickly lead to deadly temperatures inside a closed car.

The study, conducted during a local heat wave, compared an outside temperature of a shaded area with the inside of an automobile in three states: fully closed, with four windows cracked, and with two windows cracked. Inside temperatures were measured with an indoor/outdoor thermometer and an oven thermometer (both readings are given). All temperatures use the Fahrenheit scale.

Day 1

Outside Temperature

Inside Closed Automobile

  Indoor/Outdoor Oven Thermometer
9:00 am 82° 109° ----
9:30 am 87° 115° ----
10:00 am 91° 115° ----
10:30 am 94° 114° 115°
11:00 am 98° 114° 119°
11:30 am 100° 117° 124°
12:00 pm 101° 119° 127°
1:30 pm 112° 124° 130°
2:30 pm 125° 130+° 159°
4:00 pm 98° 110° 110°

Day 2

Outside Temperature

Inside Auto - 4 Windows Cracked

  Indoor/Outdoor Oven Thermometer
9:15 am 84° 98° 98°
10:00 am 88° 103° 105°
10:30 am 90° 108° 108°
11:00 am 92° 109° 109°
12:00 pm 95° 113° 113°
1:00 pm 101° 114° 115°
2:00 pm 110° 123° 120°
3:40 pm 112° 129° 128°
4:00 pm 115° 132° 130°

Day 3

Outside Temperature

Inside Auto - 2 Windows Cracked

  Indoor/Outdoor Oven Thermometer
8:30 am 72° 72° 72°
9:30 am 80° 95° 95°
12:00 pm 88° 105° 105°
1:50 pm 99° 109° 109°
2:30 pm 104° 120° 120°
(both thermometers showed identical readings)   

Other studies show similar results: