Hydrogen and helium are both lighter than air so a balloon filled with either of them will tend to rise. We can refer to a balloon filled with hydrogen or helium as a gas balloon, to distinguish it from a hot air balloon, although today as hydrogen is very seldom used we usually refer to gas balloons as helium balloons.
There is no way of controlling the horizontal movement of a helium balloon at a constant height: it moves at the speed of the wind in the direction of the wind. This is what makes ballooning so fascinating: there is no wind blowing on you and no wind noise.
But we can control its vertical movement. The wind speed and direction may be different at different heights and the balloon pilot can take advantage of this.
Air is compressible, which means that if we take a syringe full of air and push in the plunger the volume of the air gets smaller - the higher the pressure the smaller the volume. And of course if we change the volume without changing the mass we change the density.
As a balloon goes higher into the air the air pressure (and also air temperature) gets less, and because air is compressible this means that the density of the air gets less.
At sea level, if the air temperature is about 15oC and the pressure of the atmosphere is about 1013 mb the density of air is about 1.22 kg/m3, and the density of helium at this temperature and pressure is about 0.18 kg/m3, a difference of 1.04 kg/m3. But at an altitude of 3000 m atmospheric pressure is only about 701 mb, the density of air is about 0.91 kg/m3 and the density of helium is about 0.13 kg/m3, a difference of 0.78 kg/m3. This means that at an altitude of 3000 m we shall need 1282 m3 of helium to support 1000 kg. At any other altitude we shall need more, or less, helium than this, so the balloon is neutrally buoyant and will remain at this altitude.
In this respect a balloon is totally different from a submarine. Water is incompressible and therefore the density of the water does not change with depth, so if the buoyancy of a submarine is less than its weight at the surface it will sink to the bottom of the sea regardless of the depth of the water. Submarines are discussed in another Page on this web site - to link to it click here 
At sea level we shall need only about 960 m3 of helium to lift 1000 kg, but the balloon will be neutrally buoyant and so will not gain height.
An ordinary party balloon is made of quite stretchy rubber. As we blow it up it stretches, the higher the pressure the more it stretches - if we inflate it too much it will burst. If we fill an ordinary party balloon with helium its buoyancy will be greater than its weight and it will start to climb. As it gets higher the atmospheric pressure gets less so the difference in pressure between the helium inside the balloon and the air outside it will get bigger and the balloon will stretch more. At some height it will be neutrally buoyant, unless it bursts first. The rubber which party balloons are made from is slightly porous so the helium will gradually leak away and the balloon will eventually come down.
Scientific and weather balloons carry instruments to study the atmosphere at different heights, for example wind speed and direction, dust from an erupting volcano, levels of radiation from sunspots etc. These balloons are often designed to burst when they reach a certain height, and the instruments then fall back to the Earth on a parachute. Man-carrying balloons are not designed to burst, in fact they must be designed not to burst!
Man-carrying balloons, in fact all balloons designed to be re-used, must be made of something much stronger than the sort of rubber used to make party balloons. But the materials used are not anything like as stretchy. So if we fill a balloon completely with helium at sea level it will not stretch as the pressure gets less, so its volume will not increase as it gets higher and it cannot climb. If we want a balloon weighing 1000 kg to rise to 3000 m the envelope needs to be big enough to hold 1282 m3 of helium when fully inflated, but at sea level we need to fill it with only 960 m3. Then as it rises and the pressure gets less the helium expands, and by the time the balloon reaches 3000 m the envelope will be fully inflated.
A balloon may also receive heat and other radiation from the Sun and this will warm it up. We must allow for this when we fill the balloon, otherwise if the helium warms up and there is no room for it to expand its pressure will rise and the balloon may burst. But this can be a major problem if the balloon then passes from sunlight into darkness, when there will be a very sudden cooling and loss of buoyancy - this is discussed later, under Composite balloons - to link to this Page please click here .
Controlling the height of an airship is discussed on the Page on airships; in a helium balloon you have only two ways of controlling your height: you can change the buoyancy of the balloon, or you can change its weight. You can change the buoyancy only by releasing some helium; you can change the weight only by releasing some ballast - this consists of bags of sand carried in the basket.
Once you have landed safely you must release all the rest of the helium so the balloon envelope can be folded up for transport and storage - all the helium is lost after every flight. This makes pleasure flying in a helium balloon a very expensive hobby - hot air ballooning is much more affordable.
© Barry Gray January 2006
