"I'll just be a minute" is one of the most common last thoughts before a pet suffers heatstroke in a parked car — and it's almost never actually true, both because errands take longer than we expect and because a car doesn't need long to become dangerous in the first place. The greenhouse effect inside a parked vehicle is dramatic and well documented, and it catches even careful, loving owners off guard because the outside temperature alone doesn't tell the whole story.
How Fast a Parked Car Actually Heats Up
On a 70°F (21°C) day — a temperature that doesn't even feel particularly hot — the inside of a parked car can climb about 20°F within just 10 minutes, and nearly 30-40°F within an hour, even with the windows cracked. Cracking a window barely changes the outcome, since it does very little to release trapped heat. That means on a day that feels comfortable outside, a car's interior can reach the mid-90s to low 100s Fahrenheit surprisingly quickly, and on an actually hot summer day, interior temperatures can exceed 120°F (49°C) within a short window.
Dogs, being far less efficient at regulating their temperature than humans, can develop heatstroke in this kind of environment quickly — sometimes within 15 to 20 minutes in a hot, poorly ventilated car. Because the underlying mechanism is the same, everything in our guide to recognizing and responding to heatstroke applies here too, except a hot car often has no shade, no airflow, and no way for the dog to escape the heat source — which is what makes it so much more dangerous than heat exposure outdoors.
It's Not Just Summer, Either
Most warnings about hot cars focus on summer, and for good reason, but the greenhouse effect that heats a parked car starts mattering at much milder outdoor temperatures than people expect. Spring and fall days that feel pleasant — the kind where you might not think twice about leaving a window cracked for a few minutes — can still push a car's interior into a genuinely risky range if the sun is out. Dark-colored cars and vehicles parked in direct sun heat up fastest, but even a lighter-colored car in partial shade can climb well beyond what feels comfortable for a person, let alone a dog with far less capacity to shed heat.
Why "Just a Few Minutes" Never Actually Holds
Even a quick errand rarely stays quick — a short line becomes a long one, a parking spot is harder to find than expected, or a conversation runs long. None of that matters to a dog sitting in an increasingly hot car; the danger accumulates regardless of your intentions. And because heat builds cumulatively and the dog can't escape it or actively cool themselves the way they could outdoors with shade and airflow, there really isn't a "safe" amount of time on a warm day, even with windows cracked or parked in what looks like shade (shade shifts as the sun moves, often faster than expected).
What actually helps
- Leave your pet at home on errands where they'd have to wait in the car, even briefly.
- If you're traveling with a pet, plan stops around pet-friendly options rather than leaving them behind in the vehicle.
- Use a sunshade and good airflow while actually driving, but don't treat that as license to leave a pet in a stopped, unattended car.
- If you see a distressed animal alone in a hot car, act — note the location, vehicle description, and time, and contact local authorities or store staff for help right away.
The Bottom Line
Parked cars heat up faster and more severely than most people expect, even on mild days, and cracked windows don't meaningfully change that. The only truly safe rule is not leaving a pet alone in a parked car at all — no errand is quick enough to make it worth the risk, and the cost of being wrong is far too high. If a trip involves stops where your pet can't come with you, it's worth reworking the plan itself — running that errand separately, or leaving your pet safely at home — rather than gambling on how long "just a few minutes" will actually take.