That's because heat can cause problems for pets, even on days that we would consider merely warm.
Dogs are most vulnerable.
We get comfortable with taking our dogs on errands in the cooler months, leaving them in the car while we run into the grocery, dry cleaner or pharmacy. While no intelligent pet owner would leave a dog in the car on a day in the 90s or above, it's easy not to realize the risk when the temperature is in the 70s and 80s. The trouble is, a temperature that's pleasant outside of the car could end up being very hot inside, even with the windows cracked. Cars are like greenhouses, collecting heat through the glass of their windows and allowing it to build up inside. Even on days in the 70s or 80s, a car in the sunshine can become uncomfortably warm - and even deadly if left long enough.