<P> A range of envelope sizes is available . The smallest, one - person, basket-less balloons (called "Hoppers" or "Cloudhoppers") have as little as 600 m (21,000 cu ft) of envelope volume; for a perfect sphere the radius would be around 5 m (16 ft). At the other end of the scale, balloons used by commercial sightseeing operations may be able to carry well over two dozen people, with envelope volumes of up to 17,000 m (600,000 cu ft). The most - used size is about 2,800 m (99,000 cu ft), and can carry 3 to 5 people . </P> <P> The top of the balloon usually has a vent of some sort, enabling the pilot to release hot air to slow an ascent, start a descent, or increase the rate of descent, usually for landing . Some hot air balloons have turning vents, which are side vents that, when opened, cause the balloon to rotate . Such vents are particularly useful for balloons with rectangular baskets, to facilitate aligning the wider side of the basket for landing . </P> <P> The most common type of top vent is a disk - shaped flap of fabric called a parachute vent, invented by Tracy Barnes . The fabric is connected around its edge to a set of "vent lines" that converge in the center . (The arrangement of fabric and lines roughly resembles a parachute--thus the name .) These "vent lines" are themselves connected to a control line that runs to the basket . A parachute vent is opened by pulling on the control line . Once the control line is released, the pressure of the remaining hot air pushes the vent fabric back into place . A parachute vent can be opened briefly while in flight to initiate a rapid descent . (Slower descents are initiated by allowing the air in the balloon to cool naturally .) The vent is pulled open completely to collapse the balloon after landing . </P> <P> An older, and presently less commonly used, style of top vent is called a "Velcro - style" vent . This too is a disk of fabric at the top of the balloon . However, rather than having a set of "vent lines" that can repeatedly open and close the vent, the vent is secured by "hook and loop" fasteners (such as Velcro) and is only opened at the end of the flight . Balloons equipped with a Velcro - style vent typically have a second "maneuvering vent" built into the side (as opposed to the top) of the balloon . Another common type of top design is the "Smart Vent," which, rather than lowering a fabric disc into the envelope as in the "parachute" type, gathers the fabric together in the center of the opening . This system can theoretically be used for in - flight maneuvering, but is more commonly used only as a rapid - deflation device for use after landing, of particular value in high winds . Other designs, such as the "pop top" and "MultiVent" systems, have also attempted to address the need for rapid deflation on landing, but the parachute top remains popular as an all - around maneuvering and deflation system . </P>

How many can fit in a hot air balloon