Heating the gas (air) in a hot air balloon makes it rise because the hot air becomes less dense (lighter) than the cooler air around it, so the balloon experiences an upward buoyant force that can overcome its weight and lift it into the sky.

Why would heating the gas in a hot air balloon make the balloon rise?

The core idea in simple terms

When you heat the air inside the balloon, the air molecules move faster and spread out, so the same “amount” of air now takes up more space. This means the air inside the balloon becomes less dense than the cooler air outside. Because of that density difference, the surrounding cooler air pushes up on the balloon harder than gravity pulls it down, so the balloon rises like a wooden block floating on water.

Step-by-step: what actually happens

  1. Burner heats the air inside
    • The burner at the top of the basket shoots a flame into the balloon’s envelope, raising the temperature of the air inside.
 * Higher temperature means air molecules inside move faster and collide more energetically.
  1. Hot air expands and density drops
    • Faster-moving molecules spread out; the air inside the envelope expands and may even push some air out of the opening at the bottom.
 * Density is “mass per volume.” Because the volume increases more than the mass, the density of the air inside goes down.
  1. Buoyant force becomes larger than weight
    • The cooler outside air is now denser than the hot air inside, so it exerts a greater upward buoyant force on the whole balloon (envelope + air inside + basket + passengers).
 * If this upward buoyant force is greater than the total weight, the balloon accelerates upward and rises.
  1. Controlling going up and down
    • To rise: the pilot adds more heat, making the inside air hotter and less dense, increasing lift.
 * To descend: they let the air cool or vent air from the top; the inside air becomes denser again, reducing lift so gravity “wins” and the balloon comes down.

Mini-analogy: floating on “air water”

Think of air as an invisible “sea” of fluid, just like water in a pool.

  • A boat floats when it is overall less dense than water.
  • A hot air balloon “floats” when it is overall less dense than the surrounding air.

When you heat the gas in the balloon, you’re changing how much “air water” it displaces and how heavy the balloon is compared to that displaced air, which is exactly the principle described by Archimedes’ law applied to gases.

Key facts at a glance

  • Hot air has lower density than cold air at the same pressure.
  • Lower-density air inside the balloon means greater buoyant force from the surrounding cooler air.
  • If buoyant force > weight of balloon system → the balloon rises.
  • Pilots steer altitude mainly by adjusting the temperature difference between the inside and outside air.

Bottom note: Information gathered from public forums or data available on the internet and portrayed here.