REPORTS of Richard Branson's attempt to circumnavigate the world have been confusing. Sometimes his craft has been referred to as a hot air balloon, which in its conventional form works on a principle that is simple and well known - that warm air tends to rise.
The balloon itself is like an inverted bag, and the bubble of air trapped inside is heated by a jet of flame, fuelled normally by propane gas. You increase the flame whenever you want to climb, and turn it off altogether for descent in the meantime, balloon and gondola drift quietly with the wind.
The problem for prolonged uninterrupted flights is that great amounts of fuel are required.
The confusion arises because of references to helium. Helium is an inert gas that is lighter than air, and which therefore affords a buoyancy to a balloon containing it, more or less regardless of its temperature.
The problem with helium for manned balloons, however, is that in order to descend the gas must be expelled further ascent depends on additional supplies of helium, and this causes problems for a flight as long as that envisaged for the Global Challenger.
It is likely that the best solution for the circumglobal trip was found to be a compromise helium could provide the necessary buoyancy to keep the aircraft in the air, and height could be controlled by heating the helium or allowing it to cool, thereby varying its volume and the amount of lift it would provide.
As a general rule, the winds aloft blow in an undulating stream from west to east around the world. Once aloft, therefore, a balloon should complete its circumglobal trip if it can stay airborne for sufficient time to do it, although its path may not be as direct as the passengers might hope.
Sometimes the upper level winds proceed on their eastward journey in a steady stream, but often they meander sharply, the flow bending north, then south, then north again in tortuous convolutions. The advantage of the lower latitudes, and a launching spot like Marrakech, is that the worst of these undulations are likely to be avoided they are more pronounced and common in the middle latitudes, much further north.
The surface winds, for take off, are also gentler in North Africa than in the often stormy environment of a European January. While strong winds when airborne pose no major problems for balloonists, sooner or later, as we have seen, the craft must come to ground, and a soft landing is hard to achieve in a laundry basket at 40 mph.