A hot air balloon consists of a bag called the that is capable of containing heated air. Suspended beneath is a or (in some long-distance or high-altitude balloons, a ), which carries passengers and (usually) a source of heat, in most cases an open flame. The heated air inside the envelope makes it since it has a than the relatively cold air outside the envelope. As with all , hot air balloons cannot fly beyond the . Unlike , the envelope does not have to be sealed at the bottom since the air near the bottom of the envelope is at the same pressure as the surrounding air. In today's sport balloons the envelope is generally made from fabric and the mouth of the balloon (closest to the burner flame) is made from fire resistant material such as . Beginning in the mid-1970s, balloon envelopes have been made in all kinds of shapes, such as hot dogs, rocket ships, and the shapes of commercial products, though the traditional shape remains popular for most non-commercial, and many commercial, applications.
Copyright Vernon L. Underwood