A parked car on a warm day behaves like a greenhouse: sunlight pours through the glass, the interior temperature climbs well above the air outside within minutes, and a dog — who cools herself mainly by panting — cannot keep up. Cracking the windows makes little difference, and shade is unreliable because the sun moves. That is why the honest answer to this question starts with a rule rather than a technique.
The rule that prevents almost every case
Never leave your dog alone in a parked car when the day is warm, and treat even mild, bright days with suspicion, because the cabin heats on sunshine, not just on hot air. Most tragedies involve an owner who meant to be gone for two minutes and was held up. So plan the trip backwards: if any stop on your route will not let her come inside with you, she stays at home for that trip, or the errand waits until someone can sit with her in the car with the engine and air conditioning running. A dog left at home in a cool room is bored; a dog left in a heating car is in danger.
Travelling with her on hot days
When she does travel, choose the cooler ends of the day for longer drives and keep the cabin comfortable while moving — air conditioning on, or windows set so air flows without letting her put her head out into traffic fumes and flying grit. Secure her on a harness or in a crate so a sudden stop does not throw her, and schedule water breaks on long journeys, offering small drinks often rather than one huge bowl she may bring back up. Never let her ride in an enclosed boot space with no airflow, and on arrival at a hot destination, walk her on grass rather than scorching tarmac, which burns pads quickly.
Reading the early signs of overheating
Learn what struggling looks like so you can act before it becomes an emergency: panting that grows frantic and noisy, thick ropy drool, bright red gums, restlessness followed by wobbliness, and in serious cases vomiting, collapse or confusion. A flat-faced dog, an overweight dog, a puppy and an older dog all overheat faster than a fit adult, so their margin is thinner still. If your dog shows these signs after heat exposure, move her into shade or a cool room, offer small sips of cool — not ice-cold — water, wet her coat with cool water, and contact a veterinarian straight away rather than waiting to see whether she perks up; overheating can worsen after the dog looks better.
Making the safe choice the easy choice
Build hot-weather habits before the next trip: a note on the dashboard, a rule that whoever drives checks the back seat aloud, and a standing plan for which stops she joins and which trips she skips. If you ever find another person's dog shut in a hot car and clearly distressed, note the time and the car's details, try to locate the owner through nearby shops, and contact the local police or animal authorities if the dog is deteriorating. For anything you still want to check before the next trip, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.