The molecules in air, like all gases, expand to fill their container. Place air in a balloon, whether a large hot air balloon or a birthday size balloon, and the molecules spread out. Besides changing the size of the balloon, one way to affect the molecules' movement is to change the temperature.
How close air molecules are to each other refers to the density of air. Temperature affects density as the molecules move further apart when heated. Warm temperatures mean a lower density of air. This explains why low pressure on weather maps means warm temperatures. Colder temperatures have the opposite effect. As air cools the molecules move closer together and the air becomes denser. High pressure means colder temperatures. The air in balloons follow these same basic principles.
Hot air rises. Raise the temperature and the molecules in air expand outward, causing the air temperature to rise. You can experience this effect in a two-story house. The upstairs is warmer, both in summer and winter, as heat from the downstairs rises. It also explains why rooms with high ceilings are expensive to heat; all the hot air rises to the top. The more you heat the air the greater the effect.
Cool air falls to the ground. As air cools the molecules become compact. This makes for dense air. That nice cool air from the air conditioner stays on the first floor and does not rise to the second floor. As with rising hot air, the more you cool the air the greater the effect. The air just keeps getting denser. For the air in a balloon the effect is to make the balloon fall.
Operation of a Hot Air Balloon
An operator of a hot air balloon utilizes the effect of temperature on air molecules to make the balloon go up and down. Lifting the balloon off the ground requires enough heat to expand the molecules inside the balloon so the balloon has enough lift. The balloon continues to rise as long as heat is applied. When the desired altitude is reached the heat source is extinguished. Over time the air in the balloon cools and the balloon gradually descends. Periodic heat is necessary to maintain altitude. To lower the balloon quickly the operator pulls a cord which opens the top of the balloon, allowing warm air to escape. The balloon descends until the top is closed.
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