MadSci Network: Physics

Re: What causes the temperature in a car to soar?(physics facts & details)

Date: Wed Sep 20 01:21:42 2000
Posted By: Steven Miller, Undergraduate, Mechanical Engineering, San Diego State University
Area of science: Physics
ID: 968805843.Ph

Hello Matthew,

    Myrtle Beach in the Summer! Is it hot there? I'm going to assume that 
you mean "why is it so hot inside the car after it's been parked outside 
in the sun".  The explanation can be found in a branch of thermodynamics 
called "heat transfer".
    There are three basic methods of heat transfer; conduction, convection,
and radiation.  This phenomenon deals with all three.

Radiation:  The sun is the source of the energy responsible for increasing
the temperature in the parked car - the sun radiates this energy.  Thermal 
radiation is electromagnetic in nature; that is to say that thermal 
radiation encompasses the visible light spectrum, as well as some 
ultrviolet and infrared frequencies (wavelengths from 10E-1 to 10E2 
micrometers*).  The car acts as the receiver or absorber of this 
radiation.  A darker color vehicle generally absorbs more heat than a 
lighter one.  If sunlight is hitting the car, it is receiving thermal 

*Page 598, Figure 12-3; Incropera, Frank and David Dewitt. "Introduction 
to Heat Transfer", 3rd Ed. 

Conduction:  This is one of the mechanisms which transfers heat from the 
outer surfaces to the inner surfaces of the car.  Although the finish of 
the car reflects some of the energy away, the balance of the energy 
is "conducted" or transmitted to the inner surfaces of the passenger 
compartment.  This accounts for most likely only a small amount of the 
heat entering the passenger compartment, and it is transferred to the air 
inside by "convection". 

Convection:  Most likely the majority of the thermal energy is transmitted 
through the glass (windshield, etc.) in the passenger compartment where it 
is absorbed by the seats,dash etc.  Again, dark colors absorb more energy 
that lighter colors, so be careful on those black vinyl seats - you could 
get burned!!  The process becomes somewhat more complicated here.  
Initially, the energy would be transferred from the seats/dash/etc to the 
closest layer of air by conduction, but as the layer of air warms up it 
becomes less dense (lighter) and rises.  This begins the natural 
convection process as air begins to move in the passenger compartment due 
to the buoyancy forces. As air is drawn over the hot surfaces it is warmed 
and becomes lighter and continues the process.  Theoretically this can 
continue as long as the air can absorb heat, reheating over and over. 

This is probably a somewhat oversimplified explanation, but the three heat 
transfer mechanisms are appropriate descriptions of the basic problem.

How do we reduce the heat transfer from the sun to the inside of the car?  
By parking in the shade we can minimize the effects of the sun's thermal 
radiation by letting a tree or building absorb the majority of the energy 
instead of the car.  If we roll down the windows just a crack at the top 
the lighter hot air can escape the confines of the passenger compartment.  
Another means of blocking or reflecting the sun's energy is to use the 
cardboard shields that you may have seen people put behind the windshield 
to stop the solar radiation from hitting the dash and seats.

Well, that's about it in a nutshell!  I hope this answered your question.  
If you require more details such as governing equations for convection, 
conduction and radiation consult any heat transfer text, such as the one I 
referenced above.  It is probably worth noting here the dangers involved 
in leaving a pet or small child in a vehicle with the windows rolled up 
for a period of time.  These processes can take less time than one might 
think and the sun's energy is impressive!

Steven Miller
Undergrad - Mechanical Engineering
San Diego State University

P.S.  If you are interested in solar energy please see the National Solar 
Thermal Test Facility (NSTTF) website at:  http:

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