When air is heated, it gradually starts ascending in the atmosphere, and is eventually replaced by cold air from the surroundings. This ascent of hot air can be attributed to the fact that its density is lower than that of cold air around it. Simply put, the density of air falls as it is heated, and this in turn causes it to rise. The same principle is used in hot air balloons, wherein air is heated using a source of heat placed in the suspended wicker basket, and trapped in the 'envelope' above. The buoyant force produced by the presence of hot air within the envelope and cold air outside it, makes the hot air balloon ascend gradually.

The buoyant force that a hot air balloon is subjected to is similar to that described in the Archimedes' Principle - which states that:

"Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object".

If the said object is heavier than the displaced fluid, it tends to sink and settle at the bottom. But if it is lighter than the fluid displaced, it tends to float (liquid) or rise (gas). In case of hot air balloon, it tends to rise because the air within is hot and hence less dense than the air outside. Like hot air, even helium is lighter than air at ambient temperature, and that explains why helium balloons rise when they are let loose. More importantly, the use of helium balloons is not just restricted to amusement parks or family celebrations today, but goes well beyond that. In fact, these helium-filled balloons are widely used in the field of advertising (tethered helium balloons) and meteorological studies (NASA's Scientific Ballooning Program).

How Helium Balloons Work?

Helium balloons are basically gas balloons that are filled with helium gas, which is considered to be the best option among the various lighter-than-air gases that can be used in gas balloons. In fact, helium is the second lightest element in the observable universe - with only hydrogen being lighter than it. As helium is lighter than air, using it in balloons helps them rise in the sky with atmospheric air acting as the fluid. Basically, density of matter plays a crucial role in determining whether it will float/rise or sink/fall. As the density of air is less than the density of water, a plastic bottle filled with air tends to float on water. If you tie this air-filled bottle and stone to either ends of the rope and throw it in the water, you will see the bottle floating in the water like a gas balloon in the air. This floating of the bottle can be attributed to 'buoyancy', which is defined as the upward force exerted on an object which is immersed in the fluid partly or fully.

While the law of buoyancy does apply to air (gas), the buoyant force of air is much less than that of water (liquid) as its density is relatively low. In case of helium, its density is less than the density of atmospheric air. Specifically speaking, the density of air is 1.25 g/L, while the density of helium is just 0.1785 g/L. The considerably high density of atmospheric air can be attributed to the fact that nitrogen (which has a density of 1.2506 g/L) constitutes nearly 80 percent of it. Similarly, the molecular weight of air is 28.9, which is seven times more than that of helium with molecular weight of 4.0. As helium, which is used to fill a helium gas balloon, is lighter than the air outside it, this balloon is subjected to buoyancy and begins a gradual ascend. Owing to the ability of unmanned helium balloons to reach considerably high altitudes, they are frequently used in the field of atmospheric research to study the weather conditions prevailing in the upper atmosphere.

As in case of helium, even hydrogen and neon are categorized as 'lighter-than-air' gases, as their density is lower than that of air. It is this low density of these gases that adds to their buoyancy. While hydrogen, with a density of 0.08988 g/L, is lighter than helium, it is not the first choice for gas balloons owing to its highly inflammable nature. Similarly, even neon - with its density of 0.9002 g/L, is lighter than air, and thus can be used to fill gas balloons. However, the fact that it is rare on the Earth, heavier as compared to helium and expensive to produce industrially, gives it a thumbs-down when it comes to its use in gas balloons.

How Much Weight Can Helium Balloons Lift?

As far as helium balloon lift is concerned, it is directly proportional to the volume of helium within the balloon, which, in turn, is directly proportional to the size of the balloon. Basically, 1 cubic foot of helium is known to lift 28.2 grams of payload. In order to find out how much weight a particular helium balloon can lift, you need to multiply the volume of helium in it into 28.2.

Step #1: Find the volume of helium in the balloon using the volume of sphere formula.

Volume of Sphere = 4/3 π.r3 where r = radius and π = 3.14 (approx)
If the diameter of balloon is 10 ft, its radius will be 5 ft.
Therefore, the volume of helium will be 4/3 × 3.14 × 5 x 5 × 5 = 523.33 cu ft.

Step #2: Find the payload capacity of the helium balloon.

Payload Capacity (gm) = Volume of Helium (cu ft) × 28.2 gm (approx)
If the volume of helium is 523.33 cu ft, its payload capacity will be 523.33 × 28.2 = 14757.90 gm (32.53 lbs).

Interestingly, the large balloons used by NASA for scientific studies are designed to carry a payload of 8,000 lbs. The state-of-the-art Columbia Scientific Balloon Facility (CSBF) operated by NASA can facilitate the launch of 400 ft diameter high-altitude unmanned balloons for the purpose of scientific research.

As the helium balloon continues to rise, the density of helium within it continues to come down. But there is a limit to how high it can go. Basically, a helium-filled balloon can continue to ascend as long as the air in the surrounding is denser than helium within it. As the density of air falls with altitude, there comes a point when it equals the density of helium within the balloon. Theoretically, the maximum altitude a helium balloon can reach is believed to be somewhere around 110,000 ft. However, many things come into play when determining how high a helium balloon can reach, and that makes the theoretical maximum of 110,000 disputable. At the same time, that also explains the spectacular ability of the unmanned, high-altitude balloons developed by NASA for scientific studies, to reach an altitude of 120,000 ft.