Air is drawn by the force of gravity at any given time. Because of this, the air pressure creates a force with the opposite direction; an upward force. At this point, gravity is not as strong to pull down a greater number of air particles, so the air density builds to whatever level balances the force of gravity.

At the surface of the Earth, the air pressure is higher because here, the air is supporting the weight of all the air at any given height. As you move up in atmosphere, you would notice that the air has less mass. The balancing pressure decreases and that's why pressure drops the higher we go.

To push something up in the air, it has to be lighter than the volume of air around it. But it seems like nothing is lighter than air.

Vacuum can have volume, but it doesn't have mass. So a balloon with vacuum inside should be lifted by the buoyancy of the air around it.

Another option would be to fill the balloon with air that is less dense than the surrounding air.

But again, fewer air particles per volume means lower air pressure, so the surrounding air pressure would squeeze the balloon until the air density inside was equal to the air density outside.

We can increase overall pressure in two ways:

• Increase the number of air particles so there is a greater number of particle impacts over a given surface area.
• Increase the speed of the particles so that the particles hit an area more often and each particle collides with greater force.

By heating the air, you can lower air density without losing air pressure. The air particles become more excited as they absorb the heat energy, and move faster.

Thus, hot air exerts greater air pressure per particle than cold air, so you don't need as many air particles to build to the same pressure level. So a hot air balloon rises because it is filled with hot, less dense air and is surrounded by colder, more dense air.

For more information, the formula and other conversions you can use our free Pressure Converter.