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.

link 1

link 2

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 significantly hotter.

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 poles.

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.