Why does a car parked in the sun get 20+ degrees hotter than outside?
― Cracking a window isn't enough
It's 30°C outside, but the moment you open the car door, a wall of hot air pushes back at you. That's not your imagination. A car's interior is a room where the sun's energy comes in easily but struggles to leave. And the cause isn't that cars are special — it's a property of the material itself: glass.
You parked in the sun for just 15 minutes while shopping. You come back, open the door, and the air inside is so hot it's hard to breathe. The steering wheel is too hot to hold with a bare hand.
What's strange is that you, standing right outside, feel fine. The same sun is shining on you both, yet the inside of the car has become a different world.
People often say "just crack a window and it'll be fine." But once you understand the mechanism, you can also see why that trick barely works.
There are really just two reasons
Sunlight passes straight through the glass and gets absorbed by the seats and steering wheel. Once warmed, they give off their own light — heat-light — but that light can't pass back out through the glass. There's a door in, but no door out.
You don't overheat outdoors because the wind carries the hot air around your body away. Inside a closed car, the air barely moves. The warmed air just sits there, and heat keeps piling up.
Both effects push the same way: heat gets in easily, but has a hard time getting out. So the gap just keeps widening. Let's look at each in turn.
Glass behaves differently coming and going
Sunlight and the heat-light given off by warm objects are, in fact, close relatives. Both are waves called electromagnetic radiation — the only difference is wavelength.
The sun's surface is thought to be around 6000°C. The light it sends out has short wavelengths — mostly visible light plus the near-infrared just beyond it. Glass readily lets this short-wavelength light through. That's why you can see the scenery outside, and why sunlight reaches into the car.
Meanwhile, the seats and dashboard, having absorbed that light and warmed to 40°C or 60°C, also give off light of their own. But because they're at a much lower temperature, it's far-longer-wavelength infrared. Glass barely lets this long-wavelength infrared through — it absorbs it instead. Look at Figure 1: the trip in is a free pass; the trip back is blocked. The very same sheet of glass behaves differently depending on the wavelength of the light.
Why "crack a window" helps less than you'd think
This is where the second reason kicks in. Getting rid of heat doesn't only mean giving off light. You can also carry heat away by swapping out the warmed air itself. The reason people outside feel fine is that wind does exactly that job.
But inside a car, the air is trapped. A few centimetres of open window only lets a tiny amount of air exchange happen. Worse, warmed air becomes lighter and clings to the ceiling, so the heat lower down barely moves at all. A test by an automobile association also found that cracking a window only slightly lowered the interior temperature.
On a clear summer day, a car parked in the sun can reach an interior temperature above 50°C, with the dashboard surface nearing 70°C. Even on a day that isn't quite peak summer — with outside temperatures around 25–30°C — the interior topping 40°C is nothing unusual. The feeling that "it's not that hot today" simply doesn't apply inside a car.
The reason glass or greenhouse plastic "lets short waves in but blocks long waves" is exactly why greenhouses stay warm. The carbon dioxide and water vapour wrapped around the Earth play a similar role. What happens inside a car is a tiny, fast-forward version of what happens across the whole planet.
Black paint absorbs light well, so the body itself gets hotter. But the heat created when light passes through the windows and converts to heat inside happens regardless of colour. Colour matters, but changing it is no guarantee of safety.
So what should you actually do?
The most important lesson follows directly from the mechanism. It's a room that keeps gaining heat and can't let it out — so never leave a person or animal inside it.
- Never wait alone in the carEven "just a few minutes, I'll be right back" is enough for the inside temperature to keep climbing. Get out together, even for a short stop.
- Choose shade, and use a sunshadeIf light can't get in, it can't turn into heat inside. A windshield sunshade is a tool for blocking the doorway.
- Before getting in, open all the windows fully to air it outNot just a crack — open them all the way, to push out the built-up hot air before you get in.
Limp, not sweating, slow to respond when spoken to — these can be signs of advanced heatstroke. Call emergency services immediately, and move the person to a shaded or air-conditioned area. Cooling the neck, armpits, and groin helps cool the blood efficiently, since large blood vessels run close to the skin there. If the person is unconscious or can't drink on their own, don't force them to drink. Young children and elderly people may not report feeling unwell, so check on them frequently. Follow the instructions of emergency responders or local authorities for care until help arrives.
Summary
A car's interior gets hot not so much because the sun is strong, but because glass "lets light in, but won't let it back out." Add to that the fact that the air never gets exchanged, and you get a room that only ever accumulates heat. Both why cracking a window barely helps, and why a sunshade works, come down to this one mechanism.
A car's interior isn't hot because a lot of heat gets in.
It's hot because the heat that gets in can't leave.
For what happens inside the body, see Why is heatstroke life-threatening?. The idea of "moving" heat rather than "erasing" it also connects to Why does a refrigerator warm up the room?.
- On a sunny day, place a sealed clear container and an identical open one side by side in the sun. After 20 minutes, compare the warmth of the air inside each with your hand. The sealed one will be noticeably hotter.
- Next, cover the sealed container with white paper to shade it, and wait another 20 minutes. You'll find that blocking the light itself matters more than whether the lid is on or off.
- Before getting into a car parked in the sun, open all the windows fully and wait one minute, then feel the air inside again. Just exchanging the air changes how it feels quite a lot.
To avoid burns, don't touch hot metal or glass with a bare hand for long. Keep experiments brief, and do them in the shade on hot days.
For readers who want more ― terms, formulas, and textbook linksClearly labelled by level, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics / Physics"
- HS+Advanced high-school content, or textbook sidebar material
- Univ.Not covered in high school — university specialist courses (heat transfer engineering, radiative heat transfer)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Greenhouse effect: when an enclosure lets short-wavelength light in but blocks long-wavelength light from leaving, the temperature inside rises. The same name applies to a car, a greenhouse, and the whole planet.
- Radiation (thermal radiation): objects give off light according to their own temperature, carrying heat with it. This is why heat transfers even without contact.
- Convection: heat carried by the movement of warmed air or water itself. It works well outdoors where there's wind, but barely at all inside a closed car.
- Specific heat: the amount of heat needed to raise something's temperature by 1 degree. Air is light, so a small amount of heat raises its temperature a great deal.
MSHSChecking with a formula: how fast does the air inside a car heat up?
Let's estimate how long it would take to raise the temperature of just the car's air by 1 degree, assuming all the sun's energy went into heating the air alone. In reality some heat goes into warming the seats and body panels, and some escapes outside — so this is an "upper-bound speed."
| Sunlight intensity reaching the ground on a clear day | about 1000 watts per square metre |
| Area of windows/glass letting light in (typical small car) | about 3 square metres |
| Volume of air inside the car (typical small car) | about 3 cubic metres |
| Weight of 1 cubic metre of air | about 1.2 kilograms |
| Heat needed to raise 1 kilogram of air by 1 degree | about 1000 joules |
| Rate of light energy the car receives (watts) | 1000 × 3 = 3000 |
| Weight of the air inside the car (kilograms) | 3 × 1.2 = 3.6 |
| Heat needed to raise all the air by 1 degree (joules) | 3.6 × 1000 = 3600 |
| Time to rise by 1 degree (seconds) | 3600 ÷ 3000 = 1.2 |
It's easier to follow by units rather than symbols. A watt means "joules per second." So dividing joules by watts gives you seconds. For the air alone, that works out to just over a second per degree. In reality it's much slower, because heat gets siphoned off into the seats and body panels, and gradually leaks outside. But the direction doesn't change: the temperature keeps climbing without stopping.
HSHS+Why does temperature change the wavelength of light given off?
HSEvery object gives off light according to its own temperature, and the hotter it is, the shorter the wavelength of that light. The sun's surface is around 6000°C, and the peak of the light it gives off falls around visible light. The peak of the light given off by a 60°C seat, by contrast, sits much further toward longer-wavelength infrared.
HS+The intensity of the light an object gives off is known to be proportional to the fourth power of its absolute temperature. In other words, doubling the absolute temperature makes the light given off 16 times as strong. So as the car's interior gets hotter, its drive to shed heat outward also grows stronger — meaning the temperature doesn't rise forever, but levels off once the incoming and outgoing amounts balance. This article is about that plateau temperature being 20 or even 30 degrees above the outside air.
Univ.A glass window opens and closes depending on wavelength
Heat transfer engineering uses a quantity describing how much light a material transmits at each wavelength to handle this phenomenon. Ordinary sheet glass is said to transmit visible light through near-infrared well, but transmits almost nothing once wavelength exceeds roughly 4 micrometres. Most sunlight falls into the former category, while most of the light given off by objects near room temperature falls into the latter. This mismatch at the boundary is the true identity of the greenhouse effect. Only by solving not just radiation but also conduction through the car body and convection of the interior air together can you actually predict the real interior temperature.
ResearchWhat's still not fully understood
- Temperature distribution inside the car Interior temperature varies greatly between top and bottom, and between the sunlit and shaded sides, so a simple average can't capture the risk. Which spot best represents the burden on a person is still being studied.
- Effects on small bodies Small children's body temperatures are thought to rise faster than adults'. But it's not possible to run experiments on humans to find out how long it takes to become dangerous, or how dangerous. Estimates rely on mannequins and computational models, and confirming their accuracy remains a challenge.
- Heat-reflecting windows Technology to apply films to glass that reflect only near-infrared is advancing. But the trade-off between how much visible light still gets through and how much heat gets blocked, and the cost-effectiveness of it, is still being refined.
In other words, this article too is "an explanation based on what's currently understood." The figures vary greatly with conditions, so treat them as rough guides.
Links to textbooks (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science: properties of light / how heat travels | Glass letting light through, and heat carried by convection |
| HS | Basic Physics: heat and temperature / Physics: electromagnetic waves | Estimating temperature rise using specific heat, and the difference in wavelength |
| HS+ | Physics: thermal radiation (advanced/sidebar) | Shorter wavelength at higher temperature, and the fourth-power relationship |
| Univ. | Heat transfer engineering / radiative heat transfer / building environmental engineering | Wavelength-dependent transmittance, and combining radiation, conduction, and convection |
| Research | Environmental physiology / heatstroke prevention research | Temperature distribution inside cars, and estimating effects on small bodies |
| ― | Everyday connections | Using a sunshade, airing out fully before getting in, never leaving people or animals in the car |
- Ministry of the Environment Heatstroke Prevention Information Site (環境省 熱中症予防情報サイト)
- Ministry of the Environment, "Heatstroke Environmental Health Manual" (環境省『熱中症環境保健マニュアル』, section on symptoms and first aid)
- Japan Automobile Federation user test, "Summer Car Interior Temperatures" (日本自動車連盟 ユーザーテスト「真夏の車内温度」)
- Japan Meteorological Agency, "Observation of Solar and Terrestrial Radiation" and related meteorological observation guidance (気象庁「日射・放射の観測」)
- Japan Society of Mechanical Engineers, "Heat Transfer Engineering Data" (日本機械学会『伝熱工学資料』, section on radiative heat transfer and spectral properties of materials)
※This article is a general-audience science explainer. The figures given are approximations meant to help explain the mechanism. Actual car interior temperatures vary greatly by vehicle, colour, weather, and time of day. If you notice any signs of illness, follow the instructions of emergency services, local authorities, or medical professionals.