
Why is the temperature inside of a car hotter
than
the temperature outside on a sunny day?

Question Date: 20130320   Answer 1:
Energy is flowing into the car when the
sunlight flows in through the windows. The
Sun's energy flows as electromagnetic radiation,
most of which is visible light and infrared
light. When the light hits the solid objects in
the car like the seats it heats up the surface.
The air itself doesn't absorb much heat from the
sunlight, but it's touching the hot surfaces in
the car so it gets heated up from that.
The air outside the car is also heating up in
the same way, but there's a huge amount of air
outside for miles above the surface of the earth
which is constantly moving around, so the same
energy is heating up the air outside, but the
energy is spread over a much larger volume of
air so it doesn't get as hot as the small amount
of air in the car.
Sometimes the air outside can get very hot,
for instance in the middle of a large asphalt
parking lot. In the parking lot, the black
surface doesn't reflect much light so it absorbs
more of the sunlight as heat and it heats up a
lot. Because it is a large area, it takes
longer for the hot air to get blown around by
the wind so the air can get pretty warm. If you
just had a tiny patch of asphalt, the asphalt
would heat up but the wind would easily move the
air around to spread that heat over a larger
amount of air, so the air wouldn't get very hot.
  Answer 2:
The reason that the air inside a car on a
sunny day is generally hotter than the air
outside the car has to do with the confinement
of the air. Light enters the car through the
window and heats the interior of the car. Yet,
since the car is a closed system, the air inside
the car does not escape and mix with the outside
air to transfer heat. The only way heat can be
dissipated from the car is by thermal contact
between the car and the outside environment (as
well as black body radiative transfer by light
which we will neglect in this explanation). Such
a heat transfer mechanism can be modeled by
Newton's law of cooling: dQ/dt=h*A*ΔT(t)
where
dQ/dt is the rate of heat transfer, h is the
heat transfer coefficient, A is the area over
which thermal contact is made, and ΔT(t)
is the
difference between the temperature of the object
and the environment as a function of time. At
steady state (if you leave the car out for a
long time and it heats up), the rate that heat
energy enters the car as light has to exactly
cancel the rate at which heat energy exits the
car due to thermal contact with the environment.
If the sun gives us about P W/m^{2}
(light energy per second per meter squared), the
car absorbs a fraction f has heat, and we assume
that the total area of the windows in a car is
about 1 m^{2}, then we can say that the
rate of heat absorbed by the car on a sunny day
is roughly P*f. If the temperature in the car is
at steady state, then P*fh*A* ΔT=0 (heat
I
write ΔT instead of ΔT(t) because we
are
assuming ΔT is a constant with time under
our
steady state assumption) . From this we see that
the difference between the temperatures inside
the car and outside the car is then given by
ΔT=
(P*f)/(h*A). If we just assign some very
approximate values for these variables (P~1300
W/m^{2}, f~0.5, h~10 W/(m^{2}
K), A~5 m^{2}) we get ΔT=13 K
which is
about 23 degrees Fahrenheit hotter inside the
car than outside.
In summary, because the air is confined in
the car and the only heat transport out of the
car (neglecting black body radiation) is due to
the thermal contact between the car and the air
outside, the car will heat up until the rate at
which energy is absorbed by the sunlight is
canceled by the rate at which heat can be
dissipated from the car.
  Answer 3:
This is a great question! This is phenomenon
called the "greenhouse effect." When the sun's
light hits the glass windows of the car,
electrons in the glass are "excited" to higher
energy levels. They eventually "return" to lower
energy levels, meaning they release energy by
emitting radiation. This in turn, heats up the
car!
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