Why does hot air rise? Hot air rises because it is less dense than cold air.
When something heats up it is having thermal energy applied to it in some form. If you hold your hand out on a sunny day, infrared radiation is supplying energy to the molecules in the skin of your hand, making your hand warm. A microwave is the same. The microwaves make the water molecules in the food vibrate, supplying them with energy.
In cold conditions, molecules don’t move much, which is why ice is a solid. When molecules stop moving completely, it is absolute zero, which is -273℃. It is not possible to get down to this level because there will always be movement on the quantum level. As something warms up, the molecules move faster and faster.
The more energy molecules have, the more they move. This is why water becomes steam. When molecules warm, the extra energy makes the molecules move around more and they spread out, making them take up more space and become less dense.
Density is the amount of matter in a certain space. If you have a cubic meter of lead and a cubic meter of feathers, the lead is going to weigh more because there are more lead molecules in the space. Density is how tightly or loosely packed the molecules of a substance are within a given space. For example, the core of a neutron star has the densest material in the universe. 1 cubic meter of gold would weigh 19.3 tons. A cubic meter of neutronium would weigh 10,000,000,000,000,000 tons.
When air is warmed up, it has more energy, the molecules move more and it spreads out, becoming less dense. When air cools down, it loses that energy, the molecules move less, it contracts, and it becomes denser.
Now, two forces act upon the air. Gravity and buoyancy. Gravity acts upon all of the air molecules and pulls them down towards the Earth. Buoyancy is where the pressure from the surrounding air pushes the less dense air molecules up.
Objects will always move from an area of high pressure to an area of low pressure. Think about a cork released at the bottom of the sea. It will keep moving up until the pressure within it is equal to the pressure outside of it.
The cold air is more dense than the warm air, and thus has a higher pressure. The warm air is less dense and thus has a lower pressure. The pressure from the cold air pushes the warm air up. The warm air will keep rising until it reaches so high that the air pressure drops. When this happens there is nothing constraining the air molecules and they can spread out more. However, when they spread out more, they lose the energy that they were carrying and stop moving as much, which causes them to contract and the warm air stops rising. As it becomes cool, it becomes denser and eventually it starts to sink again. Once it reaches the bottom, the ground warms it up again and the cycle continues. This cycling of hot and cold air is what drives our atmospheric system.
This is the reason why a hot air balloon can float. A cubic meter of air at sea level weighs 1.2 kg. If you heat the air by 40˚C, the cubic meter of air weighs about 35 grams. That means, the pressure of the cooler air around it pushes the balloon up. The balloon can rise for as long as the air is heated. Once it starts to cool, the balloon will begin to sink. A cubic meter of hot air can lift about 1 kg of weight, which is why hot air balloons have to be so massive. A hot air balloon canopy can hold about 2,500 cubic meters of air, which means it can lift about 2.5 tons. The basket and canopy usually weigh about half a ton. Adding passengers makes it heavier, but it can still float.
So, hot air rises because its molecules move faster and spread apart. This makes it less dense. It is pushed up by the denser cold air around it, which is, in turn, pulled down more by gravity. When the hot air reaches the high atmosphere, the surrounding pressure drops, the hot air molecules slow down, and it sinks again. And that is what I learned today.
https://www.grc.nasa.gov/www/k-12/Summer_Training/FranktonES/Convection_main_page.html
https://wonderopolis.org/wonder/does-hot-air-always-rise
https://scienceline.ucsb.edu/getkey.php?key=3901
https://teachearthscience.org/Resources/HotAir.pdf
https://theconversation.com/curious-kids-why-does-cold-air-go-down-and-hot-air-go-up-171408