The faster molecules move, the hotter the air. … So air, like most other substances, expands when heated and contracts when cooled. Because there is more space between the molecules, the air is less dense than the surrounding matter and the hot air floats upward. This is the concept used in the hot air balloons.

Why does cold air go up?

When a mass of air takes up more space, it has a lower density. When you have a lower density fluid immersed in a higher density fluid, the lower density fluid rises and the higher density fluid falls. … Warm air rises when it’s surrounded by cold air because of its lower density.

Which is heavier hot air or cold air?

Air is made of molecules, and therefore has mass. Barometric pressure is a measure of how much mass of air, i.e. how many air molecules, exist above the point of measurement, all the way up to the top of the atmosphere. … Cold air is denser than warm air.