cars arent airtite: why so hot under sun

ok this has been buggin me for a L O N G time: cars are nowhere near airtight, heck drive on open roads with fan off, windows/sunroof closed, and the combination of low pressure at the rear (either C pillar vents or under bumper) and high pressure where the hood meets windshield is enough to get a nice breeze thru the vents.

so, why do they get so darn hot when parked in teh sun? can't the hot air escape naturally??

I think heat rises ... if that is the case, then the cab of a car with the windows rolled up and sunroof closed would likely contain the heat coming in from outside.

A car in motion would have air naturally flowing through the grill and vents. A car at rest would not have that type of air movement. A dog sticking its head out of the window of a moving car gets its fur blown around; no such fur blowing with a parked car (and this being summer, don't forget to NEVER LEAVE YOUR PET UNATTENDED IN AN ENCLOSED VEHICLE DURING THE DAY! The heat inside of an enclosed vehicle can be fatal!).

Me not a physicist, so the above information is certainly open for debate (except for the pet in the enclosed vehicle part - don't do it) ... /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/icon3.gif

right, the air would get hot, but wouldn't there be some sort of diffusion as well? very rarely is the air dead still--always a slight breeze. also i would venture to guess that air pressure in the cabin would be higher than ambient: open the door after the car sits for a while baking, you will be greeted with a blast of hot air rushing out. all that, at least to me, suggest that there should be some natural convective current, but there isn't apparently, given all the hot car cabins i get into /ubbthreads/images/graemlins/smile.gif

at the bottom of my logic (or illogic?) is the idea that cracking the window a bit or using the vent on the sunroof will make a difference as to how hot the cabin gets: but why? the aircon plumbing also connects outside air to the cabin. why doen't that play in, when a cracked down window does.

and no worries about pets, i have none. thankfully the aircon is a goodly powerful unit, and car has HSEA glass all around, so any passengers, should they wish to stay in the car for any reason when i step out, run the a/c and crank the cd /ubbthreads/images/graemlins/smile.gif

Well, I can quickly think of the metal of the car being an excellent heat conductor (or is that insulator)... If you ever noticed, if you get into your car when it's a very cold day outside, inside your car would actually feel a bit colder. It's probably due to the metal inside your car retaining the temperature out outside climates...

I'm guessing it's a combination of pressure buildup inside your car that's making it so hot, and with direct sun rays, worsens it. And given that your car's metal, it's like having a solar cooker ready to go...

The car may not be airtight, but the rate of osmosis is quite small; too small with the windows rolled up, to make much of a difference...

...conductor, which is why it either saps heat from you or transfers heat to you (unlike, say Ti watch bracelets).

everything said basically amounted to: incoming heat flux is larger than any heat carried out. still nothing had been said to address why a cracked window/sunroof will make a dramatic difference when the aircon plumbing should theoretically work the same way

An open window or sunroof should release that pressure, as well as increase rate of osmosis to outside air... In a hot day, it's not going to be a huge differencemaker, but it will mean the difference between 180 degrees inside your car and, say 120 degrees if the window's rolled down...

Something like just a cracked windshield shouldn't make much of a difference...

mea culpa...cracked meaning window cracked open a small amount to allow venting but not enough to allow anyone attempting break in!

closed windows/sunroof can easily top 120 fer shure! that's the critical temp when things go kabooom. friends/family have had the misfortune of such things as cosmetics and fragrances blow up in the car :/

ahh...
/ubbthreads/images/graemlins/thumbsup.gif

One word: Glass.

Magnifies the sun's rays, and the colored seats and dash absorb this energy and heat up, and everything inside gets much hotter than the outside ambient air. More heat gets generated than air flow alone can dissipate, until a HOT equilibrium is reached. Notice that a car in the shade stays the same temp as ambient (outside air).

So to me, it's not so much whether heat is carried out of a parked car via a cracked window or the ventilation ducts, its that the sun going thru glass just turns the interior into a hothouse.

Just my theory. I'm not a thermodynamics engineer, I don't play one on TV, and I've not recently stayed at Holiday Inn Express.

Dave

infra-red radiation? it goes in the car but is prevented from escaping by the glass windows (greenhouse effect).

Cars are very efficient at heating up in the sun due to the greenhouse effect allowed by all the glass. The amount of ventilation that might seep in through the vents is insignificant compared to the incoming heat. You would need a very large flow of air through the vents to keep a parked car from heating up in the sun, and that would require forcing it through the vents with a fan.

The natural convection forces of the hot air in the car might increase ventilation a little bit, but primarily in an upward direction. The ventilation system's low openings and convoluted passages aren't oriented to efficiently allow air to flow upward. If you open a window slightly, the opening is near the top of the cabin and is more likely to allow air to flow upward out of the cabin. It is also more likely to catch any breezes outside the car, especially if you have a window open on each side of the car allowing cross-ventilation.

Have you ever looked at a cars ventilation system? There's a reason that little to no air comes in [without the help of a fan] when the car is not moving. It's the same reason that going through a carwash doesn't result in water comigng out the vents.

The air ducts of a car are highly engineered complex passages with lots of twists and turns.


Not all cars are the same so this isn't globally true, but if you are running the air in recirculate mode and turn the car off, the inside ducts are essentially cutoff from the outside world.

I work in a green house...in the summer with the fans on (big HUGE fans) blowing the hot air outside it still gets to about 125 inside when it is 85 outside.

Ah, yes.. Max Planck and his Magical Quantum Physics stuff.
Something about the relationship of temperature and wavelength being inverse. Add glass being transparent at visible and near IR wavelengths (400-7000nm) and absorbent/reflective at longer wave lengths. Result is a black (Planck) body of approx 330K (55C/135F).

Here's what happens;

Sun (6000K) peak energy is at 500nm> Glass very transparent at 500nm. Interior surfaces at 20C. Energy from Sun heats interior surfaces of car. Peak energy at 10,000nm (more or less), glass reflects and absorbs and radiates this energy. Temp goes up a little, say to 25C. More energy from sun, interior surfaces get hotter, now radiating as a black body of 9990K (MOL). Round and round we go till interior temp goes toward 50-60C where the interior energy wavelength reaches the point where the glass is more transparent and equilibrium is reached.

The roof and other metal parts increase the rate at which the temperature rises but has little effect on the final temp. Opening the door windows a little bit slows down the rate but unless there is a good cross wind only moderates the temp a little. Tinting and metalizing the wind shield slows the process but has little effect on the equilibrium point.

What helps? A cloudy day where the temp of the cloudy sky is around 290K or about the same as the starting point of the interior of the car..

wow. thanks for all the info!

It all has to do with sunlight.
Heat is generated inside the car by sunlight hitting the cars seats, dash etc. Becuase of the type of glass used in cars, and the angles,,, sunlight is able to pass into the car,,, but bounces around inside the car, and doesnt excape.
Cars with dark window tint actually stay COOLER in the sun, despite them being darker. The tent filters the sunlight, not allowing it to heat the inside of the cars surfaces as much.

This process doesnt really have anything to do wiht air in side the car, venting would help the air cool alittle, but the main cuase is the sun heating the surfaces inside the car.

I witnessed a Maxima pulled indoors here at work on a very hot humid day. It had been detailed about 2 hours prior and the carpet was still wet. It was pulled under an covering because a strong storm was approaching. As the storm hit and the temp. dropped about 20 degrees in 10 minutes I jumped as I heard the loud boom behind me. The back window of the car imploded. This leads me to believe the cars are at least somewhat airtight. I believe a combination of the temp and pressure change could not be equalized fast enough and the resulting interior under pressure caused the implosion. It was a black car, which made the heat worse than it would have been in a lighter colored car. The other cars did not suffer this fate because they had the advantage of the cooling rain during the temp. drop.

Wow, now that is crazy.
I have seem solar powered fans that you fit onto your side windows and roll up on it, so you can vent your vehicle without risking theft.

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avusblue said:
Just my theory. I'm not a thermodynamics engineer, I don't play one on TV, and I've not recently stayed at Holiday Inn Express.

Dave

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LMAO! That's my kind of humor! Good one, guy!

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DieselDave said:
I witnessed a Maxima pulled indoors here at work on a very hot humid day. It had been detailed about 2 hours prior and the carpet was still wet. It was pulled under an covering because a strong storm was approaching. As the storm hit and the temp. dropped about 20 degrees in 10 minutes I jumped as I heard the loud boom behind me. The back window of the car imploded. This leads me to believe the cars are at least somewhat airtight. I believe a combination of the temp and pressure change could not be equalized fast enough and the resulting interior under pressure caused the implosion. It was a black car, which made the heat worse than it would have been in a lighter colored car. The other cars did not suffer this fate because they had the advantage of the cooling rain during the temp. drop.

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Actually Dave, I've seen this before. It happens when the steel of the car body cools and contracts faster than the rear window glass. This phenomena is almost always cause by an anomaly in the opening of the car body, such as a high seam in the "C" pillar or hatch or by a protruding piece of MIG wire left over from welding said seam or a dimensionally incorrect window opening caused by a previous body repair. When the rear window opening contracts faster than the glass, the high points in the opening exert high pressure on a very small area on the edge of the glass, causing it to shatter. This also occurs in cold climates when the body is cold and the rear window defogger is turned on, causing the glass to expand faster than the window opening.

Dan