Car Temperature in Morning; Equilibrium?
Country: United States
Date: Winter 2009-2010
Hi, I have a very simple question. If you leave your car
outside all night, with the windows and doors shut, and the
temperature drops to zero degrees, when you get in the car in the
morning will the temperature inside the car be the same as the
temperature outside the car? I am assuming that there is no
additional heat source for the car (it has not been running) and
that you are getting in the car right before the sun comes up.
The answer is "yes.... if the night is long enough"! With no
of heat, the inside of the car will eventually reach thermal
equilibrium with the
outside, and both will have the same temperature. If the night is too short,
and other sources of heat under the hood will not have fully cooled off, they
may still cause a small amount of warming in the passenger compartment.
This is only true if night time temperature is stable and does not change
quickly. It is even possible for the interior of the car to be colder than
outside. Why?... There is a time delay where the interior temperature lags
slightly behind the changing outside temperature. What can happen is that
the outside temperature may have remained at a steady low temperature for
most of the night, and the interior will have cooled to this same
But if the outside temperature started to rise before dawn, the interior
temperature will also rise, but lag behind the rising exterior
As a result, the interior temperature will remains slightly cooler than the
rising exterior temperature, as both are rising.
It depends on how well insulated the car is, and how the temperature
varies with time during the might. Generally, the temperature inside
the car will lag behind the outside temperature, because it takes time
for heat to be conducted -- the rate is proportional to the difference
in temperatures, and to the thermal conductivity of all the paths by
which heat can be conducted. As the temperature inside the car
approaches the outside temperature, the rate at which heat is conducted
You did not specify whether you meant zero degrees F. or zero degrees C.
That makes a big difference.
The temperature in the car's interior will probably be similar to the
exterior. That is different than what is going on under the hood of the car
where you have that huge bunch of metal called the engine. It warms up more
slowly just because of its large mass. On the long scale, the interior and
exterior temperatures will be pretty much the same.
A car is not perfectly insulated, so there is heat exchange between the car and
its surroundings. A big factor is radiative heating due to sunlight. Conduction
due to air temp is also a factor. Look up "conduction" and "radiative heat
transfer" if you need background on these terms.
Heat exchange does not happen instantaneously, so the internal temperature of
the car will lag behind air temperature a bit as the heat moves in or out. In your
example you are talking about morning, so I am assuming the air temperature is
rising at the time you are talking about. In the case of a cold morning (but a rising
air temperature) and a cold car, the car can actually be colder than the air because
the air is heating up faster than the more-massive car interior. If the sun is shining,
the car interior could be warmer than the air as conduction and radiative heating
both warm it. If the air temperature is not changing, and if there is no radiative
heating, then your car could exactly equal air temp (this is called thermal
equilibrium), but in reality, things are never steady enough to truly reach
Hope this helps,
When you get in your car in the morning, the temperature inside the car
should be the same as the temperature of the air outside the car because the
heat of your engine, and any that may be left in the passenger area will
dissipate into the surrounding air.
So in an isolated system, If the car is parked in a garage, the heat of the
engine will dissipate into the air of the garage. The garage air will get
warmer, and the car will get cooler.
This process was summarized by Isaac Newton in his statement of the second
Law of Thermodynamics. One way to state it is that heat travels from hot to
cold bodies and not the other way around until they reach a common
equilibrium temperature. More technically: heat is a measure of disorder,
or entropy, and entropy always increases for an isolated system (in this
case the air in the garage).
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Update: June 2012