There are two basic types of balloons that fly or float: hot air balloons and helium-filled balloons, both of which depend on buoyancy.
Particles of air in Earth’s atmosphere are subject to the downward force of gravity. However, air pressure produces an upward force working against the pull of gravity. The density of the air around us increases to the level that balances the force of gravity, because at this point, the force of gravity isn’t strong enough to pull down a greater number of air molecules.
This pressure level is highest right at the surface of the Earth because the air at this level is supporting the weight of all the air above it – more weight above means a greater downward gravitational force. As you move up through levels of the atmosphere, the air has less mass above it, and so the balancing pressure decreases. This is why pressure drops as you rise in altitude.
Atmospheric pressure is greatest at the surface of the Earth because at this level, the air is supporting the weight of the air above it. The more air weighs, the greater the gravitational pushing downward. The higher you go in the atmosphere the less mass of air above it, hence, air pressure decreases.
The air pressure at sea level is 14.7 pounds per square inch, and while this is enough to overturn a chair, or even crush, the air pressure is applied equally from the right, left, top, bottom and all other angles. Therefore, the force created by atmospheric pressure on the chair is balanced by an equal force in the opposite direction. That is why we don’t notice the air pressure around us.
This differential air pressure creates an upward buoyant force. This means that the air pressure is greater below an object than above it, which means that air has a greater upward push. However, this force is relatively weak in comparison to the force of gravity, its strength is determined by weight of the air displaced by an object. A solid object is typically heavier than the air it displaces, so buoyant force does not exert any appreciable force. Buoyant force can only move objects that are lighter than the surrounding air.
In other words, something has to be lighter than an equal volume of the air around it for it to “float” in the air.
One way to lift a balloon into the air is to fill it with air that is less dense than the surrounding air. Since the air inside the balloon will have less mass per square inch than the air in the atmosphere, it will be lighter than the air it is displacing, causing it to rise. The problem with that method is that fewer molecules of air per volume translates into lower air pressure. The result is that surrounding air pressure would squeeze the balloon until there was equal air density inside and outside of the balloon.
There are two ways to increase the air pressure inside a balloon:
Increase the number of the molecules of air so there is a greater number of molecular collisions on a specified surface area, or increase the speed of the molecules so that they strike an area with more frequency, and each molecule collides with increased force.
To lower the atmospheric density in a balloon without losing air pressure, all you need to do is increase the speed of the molecules. Heating the air inside the balloon will achieve this goal.
The molecules absorb the thermal energy which makes them move faster, which in turn increases the frequency of collisions and the force of the collisions.
This is why hot air produces increased air pressure per molecule than cold air. This means a hot air balloon rises because it is filled with hot, less dense air and is surrounded by colder, more dense air.
The amount of lifting power is determined by the temperature of the air inside the balloon. A cubic foot of air weighs approximately 35 grams at 32 degrees Fahrenheit. The same volume of air at 132 degrees Fahrenheit will weigh 25 percent less, or roughly 26.5 grams. Since the difference is somewhat small a hot air balloon must be large to support equal weight
Helium balloons rise in the air because helium is lighter than air, the difference is 1.075 grams per liter. This is not a huge difference and this is why balloons and blimps have to be so large in order to carry any appreciable payload. Carrying a 33,000-pound payload a balloon must be 100 ft in diameter. This explains why the Chinese spy balloon was so immense.
At one time, hydrogen was used instead of helium for passenger airships, the problem is it is quite flammable, and the Hindenburg disaster was the result of hydrogen being ignited.
A helium weather balloon can rise up to 22 miles above the earth but the higher it goes the more the balloon expands and at some point the air outside air pressure will be so weak that the balloon will burst.
