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Name: Zach
Status: Student
Grade: Other
Location: NV
Country: United States
Date: May 2007


Question:
Is it possible for a car to achieve a terminal heat or a heat level above which the air inside the car cannot rise, no matter how long it is in direct sunlight?



Replies:
I would say yes, for a given set of conditions, there is a 'terminal temperature' (a scientist might call this a "steady state"). That steady state can be very different depending on the conditions, though, and it is a very complicated situation to try to predict. So there is not a single 'terminal temperature' for all cars or all days.

As sunlight hits the windshield, enters the car, and heats up the interior, the air inside warms up, and the windows warm up. The windows, because they are now hotter than the air outside, transfer heat back to the air around the car (heat moves from hotter bodies to colder ones, and the bigger the difference, the faster the heat moves). So the 'steady state' is reached when the amount of heat going into the car equals the amount of heat going out of the car.

The hotter the inside temperature, the more heat will be transferred to the surroundings. Thus, when the car is cool, the sun will heat it up more quickly, but eventually it will heat up so much that the heat gained from the sun is matched by the heat lost to the air. Other factors that matter are the presence of wind (just like wind cools you faster, it will cool your car faster too).

Now, based on my experience having lived in Texas, it only takes a couple of hours for the car to get really hot, but it depends a lot on the time of day and the amount of breeze. I did not find that metallic sun screens cooled down the car after a full day parked, but it slowed down the heating and did make the steering wheel a little more comfortable to grab. So I would guess that it takes several hours or more to reach a 'steady state'.

Hope this helps,
Burr Zimmerman


Dear Zach,

Yes. When the heat carried away from the car by radiation, convection, and other processes is equal to the heat given to the car by the sunlight, the car temperature will reach equilibrium.

This is similar to the earth in that the earth's temperature seems to be rising because the heat given to the earth by the sun is greater than the heat radiated out into space because the increasing amount of CO2 in the atmosphere acts like a blanket in reducing the amount of heat radiated.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Since the car loses heat from conduction, and some minor air exchange. As the conduction losses increase as the temperature of the car increases, it will eventually reach a terminal temperature at some point. Otherwise we'd see an awful lot of melted dashboards and seats.

R. Avakian


Zach-

The short answer is, uh, no. Convection can stop, but not because of that. But your question could use a better understanding of relative vs. absolute temperature, and of temperature-level (level of heat at a place) vs. heat-energy (what travels between places).

No matter how hot the average temperature of the car is, the air inside can still get hotter, so it can have small temperature differences. If the air in the bottom is 1 degree warmer than the air in the top (both are trapped inside the car), then convection will occur. The warm air in the bottom will pick some path to rise towards the top, and the cooler air on top will pick a complementary path to the bottom, to replace the air which rose. It is a cycle, a circulation of air. And the air either circulates like that, or it sits stagnant, depending on the sign of the temperature difference. If the air in the top of the car is already hottest, and the windows are closed, then yes, it cannot rise any more. Air inside such a car can be very stagnant.

What determines these small temperature differences is _where_ the heat-energy is injected into the car. Sunlight beating on the roof makes air in the top hot first, so it actually tries to stop convection. But if more sunlight goes through a clear window to the floor where the light is absorbed by dark carpeting, then it makes the bottom air hotter, and tries to start convection.

You might also ask yourself where the heat energy is removed from the car. 'Cause cold air sinks, just like hot air rises. It is a purely relative thing, just like "big" vs. "small". There is no absolute temperature at which air is "certifiably hot" so it rises. It is merely a little warmer than the other bunch of air above it, so they trade places.

Heat energy does come out. Heat only going in would make the car hotter and hotter forever, way past melting, and we know that does not happen. As the car gets hotter and hotter, heat energy starts coming back out faster and faster, because the car's temperature is higher than the outside air's temperature. Eventually the car is so hot that the heat energy going out matches the amount of heat energy going in as sunlight.

That is when the car's temperature stops rising. Then there is still sunlight going in, and the temperature inside is no longer rising, so we know an equal amount of heat energy is coming back out somewhere. I guess heat energy usually comes out evenly all over the car's outer surface, by causing convection outside, or by warming the breezes drifting by. This would be a car that's hot to touch, and maybe you can see hot air swirlies rising from it. But there are other possible cooling paths too. Cooling at the top, such as through a partly-open window, would tend to start convection inside. (cold air sinks...) Cooling on the bottom, such as a hot car suddenly landing on snow, makes cold air already sunk as far as it can, so it would tend to stop the air-convection inside. I cannot think of many realistic things that cool a car on the bottom more than on the top. So the typical cooling paths tend to allow convection inside.

see if that turns your motor...
Jim Swenson


Well, sure, but what the particular temperature is depends on the conditions. The greater the difference in temperature between the car and its surroundings, the faster it will transfer heat to its surroundings. As the car's temperature rises upon absorbing energy from sunlight, it loses energy by conduction, convection, and radiation. There will be some temperature at which its rate of energy loss equals its rate of energy gain, and thus it will hold at that temperature until conditions change. Its temperature will change only when its rates of energy loss and energy gain are unequal.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming


This question requires a bit more data....... if we say the car is in a surrounding area that or where the temperature is increasing of course the ext and internal temp goes up. As to internal temperature consider the car,size of car, ....on a typical basis we can say empirically the temp will be 10-20 degrees hotter, but please consider all the other factors length of time in sun etc...... questions requires a bit more refined parameters

Prof. Harry Przekop, Physicist



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