This is an excerpt from Dr Becker's blog. You can sign up for her newsletters here

On an 85-degree day it takes only 10 minutes for the interior of your parked car to climb to 102 degrees. In a half hour, it can reach 120 degrees. Leaving windows partially open doesn't help to cool things down inside the vehicle. To make matters worse, dogs have a higher body temp than people and they don't cool down as efficiently as we do. Your canine buddy is designed more for insulation from the cold than for cooling down. You have sweat glands all over your body, but your dog's are confined to her nose and the pads of her feet. A dog that is heating up can only normalize her body temperature through panting, which just doesn't get the job done under extreme conditions. In a very short period of time, an overheated dog can suffer critical damage to her brain, heart, liver and nervous system.

Symptoms of overheating include:
* Heavy panting * Excessive thirst * Glazed eyes * Vomiting and bloody diarrhea * Bright or dark red tongue, gums * Staggering * Elevated body temperature (104ºF and up) * Weakness, collapse * Increased pulse and heartbeat * Seizures * Excessive drooling * Unconsciousness

If your dog's body temperature gets to 109ºF or higher, heatstroke is the result. The cells of the body rapidly start to die. The brain swells, causing seizures. Lack of blood supply to the GI tract creates ulcers. Dehydration leads to irreversible kidney damage. All these catastrophic events take place within a matter of minutes. In the early stages of a heat-related illness it can be difficult to assess your dog's condition, since it's normal for him to pant when he's warm or while exerting himself.

I recommend you learn from your dog's vet how to take his temperature (rectally – I'm sorry), and invest in a digital thermometer that you designate for doggie use only. It could come in handy if you're ever concerned your dog is overheated and need to know his body temperature.
I can't stress enough how important it is for dog owners to take every precaution to keep their pets from getting overheated. By the time a dog is exhibiting symptoms of heatstroke, it's often too late to save him.

Read the whole article here