Night Cooling Below Ambiant Temperature

[DreamTransit1]

I've been full time in my Transit for over six weeks now but still stuck in MI tieing up loose ends. Now that it's been cooling down at night I'm still trying to figure out why my un insulated van is always 5 degrees cooler than the outside temp from sunset to sunrise. It's 47 degrees inside right now!
What are the physics behind this?
I had noticed it with my other vans too but never could figure it out.

 

[BobBski]

Hey, we have 2 things in common , own Transits and presently in Michigan. I'm heading to Wisconsin via the UP. Regarding physics..can't help you there, I just know the inside of my insulate van is considerably warmer inside, at night, just from body heat. Haven't used the heater yet and keep a window cracked and vent open. Stove in the a.m making coffee knocks of any chill real quick.

 

[DreamTransit1]

It's got something to do with heat loss from the steel van to the atmosphere but I would assume after a daytime temp warmer inside than out it would reach a steady state when it is even with the outside. But for some reason it's consistantly five degrees cooler.
Wish I knew how to REVERSE the effect. Or harness it for a cooling system when it's hot out at night!

 

[Richard]

This Q&A may shed some light on the matter:

http://mechanics.stackexchange.com/questions/4215/how-is-outside-temperature-measured-in-a-car

 

[Mobilesport]

Mines always about 5 degrees hotter , I wish mine went cooler. .

 

[29chico]

It is possible for objects with high emissivity  to be cooler than the ambient temp as the temp is dropping.  A painted metal panel has very high emissivity so it can cool down quicker than the surrounding environment after the sun goes down.

Emissivity is an object's ability to emit thermal radiation.

 

[highdesertranger]

how a you measuring the temp? with the same thermometer? with an indoor outdoor thermometer? have you every checked it for accuracy? highdesertranger

 

[Optimistic Paranoid]

I frankly don't see how this is possible.  Heat flows from the hotter item to the cooler one.  When they reach the same temperature, heat stops flowing.  Insulation and/or Low emissivity or high emissivity controls the SPEED at which this happens.  But heat can't keep flowing out from your car into a hotter ambient area.

Like HDR and others have hinted, I suspect thermometer errors and false readings.

 

[TrainChaser]

One other possibility: especially if the sun has been shining, you may be parked on top of a heat sink called Mother Earth. The ground (dirt or pavement) absorbs a lot of heat from the sun, and then gradually releases it at night. I don't know where your thermometer sensor is, but I would imagine it's under the shell. The heat rises from the ground, and is trapped for a bit by the shell and that is what the sensor is reading.

It would be interesting if you recorded both at various times: 9 p.m., midnight, 3 a.m., 6 a.m. If the difference between them decreases, it's probably ground temp, and if it stays exactly the same, it's probably inaccuracy of one of the thermometers.

 

[highdesertranger]

hey a real penny. nice thermometer Guy. highdesertranger

 

[AngryVanMan]

29chico got it! On clear evenings with no cloud cover, emissivity is a very large factor - it's why you can get frost on your car in above 32 degree weather. The steel structure and glass radiates very effectively, which is why it can freeze water (frost) in exterior ambient temperatures as high as almost 40 degrees. The night sky is very close to absolute zero, so things with low mass and good radiation properties literally lose heat to outer space! In turn, through the process of convection the closed and sealed interior air begins cooling to match the surface temperature of the skin of the vehicle.

If you have cloud cover, wind, or even park under something like an awning or a tree this effect will be counter acted. Rolling down windows or running a vent fan will also prevent this from happening.

It could be measurement error, but it also could be a real effect in an uninsulated vehicle. Science be crazy man!

 

[AngryVanMan]

I think this would be most noticeable in a vehicle with little or no insulation or headliner - anything that blocks the convection surface area of skin to interior air would decrease the effect until eventually it was not significant any longer.

Daytime "sky" on a clear day is technically the same temperature, but solar radiation is orders of magnitude larger than what is lost through radiative cooling, imparting more photons of infrared energy per unit of time than what is lost in space ♪ queue Space Odyssey music ♪

 

[DreamTransit1]

Forget thermometer accuracy. When I get in the van from outside it is noticeably cooler than outside. And in all other conditions the readings are as expected/predicted.
I knew there was definitely some radiation cooling thing going on but wasn't sure how it worked. Now I do.
Thanks guys!

 

[DreamTransit1]

BTW, I did notice that it was always with a rapidly cooling air mass due to a clear sky.