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 surrounding air.

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!