The internal temperature of the air in the envelope is raised  or lowered to change altitude. To climb, the temperature in  the envelope is raised by heating the air which creates more  lift. To descend, the air in the envelope is allowed to cool.  Cooling of the envelope is also possible by allowing hot air  to escape through a vent. This temporary opening closes  and seals automatically, due to the upward pressure, when  it is not in use.  A balloon’s weight when in flight is not only the figure as  stated in the flight manual, but also includes the weight of  the air within the inflated envelope, the balloon components  and equipment, as well as the pilot and passengers. The  average 77,000 cubic foot hot air balloon contains an air mass  that weighs over 3,000 pounds. By adding all these factors  together, it is easy to understand how weight influences the  balloon’s response to pilot actions during flight maneuvers.  The weight and sheer momentum of a balloon in flight make  it difficult for a pilot to effect rapid changes.  of the object. The net upward buoyancy force is equal to the  magnitude of the weight of fluid displaced by the object. This  force enables the object to float or at least to appear lighter.  An object must make room for its own volume by pushing  aside, or displacing, an equal volume of liquid. For example,  an aircraft carrier exerts downward force on the water and  the water exerts upward force on the aircraft carrier. A solid  object floats when it has displaced just enough water, or air  in the case of a balloon, to equal its own original weight. $_{Figure 2-2.}$  The air inside the balloon envelope is heated to create  buoyancy. Balloon Components A hot air balloon consists of three main components:  envelope, heater system, and basket. In addition, flight  instruments, fuel tanks, and other support equipment are  needed for a safe balloon flight.  [Figure 2-3]  The most  common ballooning terms are used in the following text, in  To create the necessary buoyancy for flight, the air inside the  balloon envelope is heated which causes the air to expand,  making it less dense.  [Figure 2-2]  Once the interior air weighs  less than the non-heated ambient air (air that surrounds an  object), the balloon becomes lighter in weight and rises in an  effort to find a level where the interior air density matches  that of the exterior air density. The envelope is carried along  “for the ride,” as it does little more than contain the heated  air mass. The balloon rises to a point where the lift created  by the action of heating the air is equal or greater than that of  the balloon itself. The balloon rises because it has reached a  state of “positive buoyancy” and the amount of lift is greater  than the weight of the balloon. The greater the heat differential between the air inside the  envelope and the ambient air, the faster the balloon rises. Hot  air is constantly being lost from the top of the envelope by  leaking through the fabric, seams, and deflation port. Heat is  also lost by radiation. Only the best and newest fabrics are  nearly airtight. Some fabrics become increasingly porous  with age and some colors radiate heat faster than others  do. Under certain conditions, some dark colored envelopes  may gain heat from the sun. To compensate for heat loss,  prolonged flight is possible only if fuel is carried on board  to make heat.  Figure 2-1. Basic balloon terms. Figure 2-3.  Basic balloon terms. 2-3 Deflation port Envelope Horizontal load tapes Vertical load tapes Basket Crown line Heating system Mouth/Throat Skirt or scoop