I think that I can start answering this
question by first asking another question: why
does a helium balloon rise?
Maybe you’ve thought about this before; maybe
you already know the answer. It has to do with
density, which is how much stuff something
has (mass) divided by the amount of space it
occupies (volume). Oil is less dense than water,
and that’s why it floats on top. Ice is also less
dense than water, which is why the tips of
icebergs poke out above the ocean. A helium
balloon floats and escapes if it slips from a
child’s grip because it is less dense than the
So to summarize, something rises above
another thing when the density is lower.
That must mean that hot air is less dense than
cold air! So is hot air less dense than
cold air? You might know that even though you
can’t see it, air is not nothing. It contains
tiny, tiny stuff that we call molecules. These
molecules zip around and bounce off of things back
and forth all of the time. If you increase the
temperature, they gain energy and zip around
faster, just like if you ate an energy bar or
drank a caffeinated beverage. When the molecules
move faster, they move further away from each
other. That means that hot, fast-moving
molecules are taking up more space – they have
more volume! Since the density goes down when
you increase the volume, hot air is less dense.
That means it will rise!
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