|Where does hot air rise and cold air fall? Because
I have looked, and looked but I can't find an answer.|
|Question Date: 2018-05-30|
Think of a hot air balloon. When the air gets
hot, the air molecules move around more and push
each other farther apart. Some of the air gets
pushed out of the hot air balloon, so it doesn't
have as much air as it did before it got hot, and
it rises in the cooler air around it.
When air gets cold, the air molecules don't move
around as fast, and they don't push each other
apart as much. Because of that, there are more
molecules of cold air, or fewer molecules of hot
air, in the same amount of space. The air with
more molecules [cold air] is heavier than the air
with fewer molecules [hot air], so the hot air
rises up and the cold air sinks down.
You can also think about the density of
the hot air and the cold air. With more molecules
in the same amount of space, the cold air is more
dense - the cold air has a higher density.
Density is the weight for some volume of
material. [Water has a density of 1 gram in
1 cubic centimeter of water.] The hot air has
fewer molecules in the same amount of space, so it
is less dense, and less dense fluids rise up
above more dense fluids.
You could do an experiment with hot and cold
water, and with food color in it. Put hot water
in a glass and then gently add cold water.
What happens to the colored water? Then
put cold water in a glass and gently add hot
water. What happens to the colored water?
I should try that. It sounds fun.
Hot air exerts more pressure per unit volume, and
as a result expands to become less dense than cold
air. Anything of high density in a fluid will
sink and anything of low density will rise.
Thus, hot air, being less dense, rises, while cold
air, being more dense, sinks.
You can read answers related to this question
on our database, on the links below.
You are right that hot air rises, and cold air
descends. This is a physical law that we can
observe everywhere, which scientists call
"convection". Try experimenting in your
house, or another two-story building. In summer,
when it is very hot outside, go into a lower level
of a building (that does not have air
conditioning) and measure the temperature. Then go
into the upper floor and measure the temperature.
Hot air rises, so the upper floor will be
That experiment is a great way to test the
physical law that heat rises. So how does this
work on a global scale?
The earth is heated by the sun. Based on the
spherical shape (like a ball) of Earth, the
equator is much warmer than the poles (North and
South poles). Large-scale convection on Earth
moves hot air at the equator to the North/South
When air gets hot it expands, which
causes it to weigh less than the air around it. So
on Earth, hot air will usually rise if the sun
heats it up. For example, the equator gets lots of
direct sunlight, but when there is less heat then
the air can become heavier and fall. Air that
rises from the tropics around the equator then
rises so high that it becomes cold, but it can't
return to where it was before because hot air is
rising there! So air will travel away from the
equator, lose a lot of its water, and then
fall. The falling dry and cold air is often
where the largest deserts form, like the Sahara or
the Sonoran desert in Mexico.
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