On a warm, sunny day windows collect light, trapping heat inside the vehicle, and pushing the temperature inside to dangerous levels.
On an 85-degree Fahrenheit day, for example, the temperature inside a car with the windows opened slightly can reach 102 degrees within ten minutes. After
30 minutes, the temperature will reach
120 degrees. At 110 degrees, pets are in danger of heatstroke. On hot and humid days,
the temperature in a car parked in direct sunlight can rise more than 30 degrees per minute, and quickly become lethal.
Link...
http://www.cabq.gov/pets/weather.html