Why does a hot-air balloon rise?

Early in the morning in Putrajaya, a huge balloon slowly leaves the ground. It has no engine and no wings. The only thing that lifts it is hot air. How can air carry a basket full of people?

Everything around us, air too, is made of very small particles. In air, the particles are far apart, and they move all the time in every direction. When the burner under the balloon heats the air, the particles get more energy and move faster. They hit each other harder and push each other further apart. The air expands: the same amount of air now takes up more space.

This changes the density of the air. Density tells us how much mass there is in a space. Inside the balloon, the warm particles are spread out, so one box of hot air has less mass than the same box of cold air outside. We say that hot air is less dense than cold air.

Here is the main idea. Air and water are both fluids: they can flow. Anything that is less dense than the fluid around it rises up through it. A piece of wood rises to the top of water because wood is less dense than water. In the same way, the hot air in the balloon rises through the cold air around it, and it pulls the balloon and the basket up with it.

The pilot controls the flight with the burner. To go higher, she heats the air more. To come down, she lets the air cool. The particles lose energy and move more slowly, so they move closer together. The air contracts, it becomes denser again, and the balloon slowly falls.

The same idea explains many things in daily life. Warm air rises above a hot road, and hot water rises to the top of a pan on the cooker. When a warm fluid rises and a cooler fluid falls to take its place, this movement is called convection.