Why Does the Inside of Cars Get so Hot?

We’ve all heard the warnings: don’t leave your children or pets in your car (or truck) during this time of year.  The temperatures inside the passenger compartment can reach over 100 degrees easily and quickly causing heat stroke and death.  This sounds like common sense but, every year, during the summer, reports of a death of a child or animal left in a hot car, are not uncommon.  The reason that passenger compartments get so hot is because of the way heat is transferred into and out of the compartment.  First, in order for heat to flow, there has to be a temperature difference.  As long as the inside is cooler than the outside ambient temperature, heat will flow from outside to inside.  More specifically, heat transfer will occur by convection from the ambient air to the outer surfaces of the passenger compartment, including glass.  Heat will then be transferred by conduction through the roof, insulation and headliner.  Heat transfer by conduction will also occur through windows.  There is also a radiation component that serves to heat the interior.  The sun’s rays will heat the solid parts of the interior such as dashboards and steering wheels.  The heat absorbed by these objects is then radiated to the air inside the passenger compartment.  The real clincher to this process is that the heat entering the vehicle enters at a rate faster than it is dissipated.  As a result, the temperature inside the passenger compartment can only increase.  It is not until the rate of heat transfer entering the vehicle is reduced below the rate of heat transfer out of the vehicle that the temperature in the passenger compartment is reduced.  This point usually starts to occur at dusk. 

Remember, we still have approximately one and a half months of summer left.  Please be mindful of your young passengers and pets and don’t leave them locked in a death trap!

The photos below illustrate how hot a passenger compartment can get. The photographs were taken inside a Chevrolet extended cab pick up truck.

Temperature in cab after 7 minutes with windshield shaded
Temperature after 30 minutes, windshield uncovered

Temperature after 1 hour
Temperature after 2 hours

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About R.J. Hill, P.E.
R. J. Hill is the author of two blogs: R.J. Hill Consulting and the Descendants of James Alexander Hill. Mr. Hill is a registered professional (mechanical) engineer with over 39 years of experience, 34 years in private practice. Please visit www.rjhill.com to see the kinds of forensic investigations that Mr. Hill performs.

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