When calculating the effect of changing pressure and  temperature, it is necessary to multiply the sea level lift by  the ratio of pressures and temperatures. For a nonstandard  ambient pressure, multiply the lift at the ISA level either by  29.92 "Hg or 1,013.25 millibars (mb).  Temperatures at altitude must also be calculated and  compensated for. The factor for temperature is a ratio of  absolute temperatures expressed in either degrees Kelvin  or Rankine. To get temperature in degrees Rankine, simply  add 459 to the normal Fahrenheit temperature. For a new  temperature, multiply the lift calculated at the ISA by the  factor: (59 °F+ 459)/(new temperature + 459). When using  temperature in degrees Centigrade (°C), add 273 to convert  to absolute temperature (i.e., Kelvin). This is: (15 °C +  273)/(new temperature + 273). Various lift factors at differing  altitudes, comparing helium and hydrogen, are illustrated in  Appendix F. For further discussion of gas balloon calculations, the book  A Short Course on the Theory and Operation of the Free  Balloon , by C. H. Roth, Goodyear Tire and Rubber Company,  is recommended reading. This manual provides a good  overview of the physics and operation of gas ballooning as  of 1917. It is long out of print, but photocopies are readily  available. The pressure ceiling is the altitude at which the lifting gas  inside the envelope would expand to just completely fill the  envelope, assuming the balloon rose to that altitude. Rising  above the pressure ceiling causes lifting gas to be expelled  from the appendix and establishes a new, higher pressure  ceiling. Exceeding the current pressure ceiling causes loss of  lifting gas, reduces gross lift, and typically causes the balloon  to eventually begin to descend. Ballast must be expended to  maintain the new higher altitude. However, maneuvers that  result in altitude changes below the pressure ceiling, do not  result in loss of lifting gas or gross lift. Very little ballast is  required to ascend while below the pressure ceiling. For these  reasons, the gas pilot should always be aware of what the  approximate current pressure ceiling is and should consider  the consequences of penetrating that ceiling. When studying weather for gas ballooning, one must look  for trends both further into the future and higher above  the ground. The best weather for any flight is determined  by the flight’s objectives. A flight to set a duration record  (maximum time aloft) benefits from light winds and clear  skies while a distance competition requires high winds aloft  with lighter winds in the landing zone. A competitor in a  long competition is likely to encounter several different  weather patterns during flight simply due to the length of  the flight. Examples of these include precipitation, snow,  icing, thunderstorms, lightning, high winds, mountain winds,  unstable air, or convective currents. This discussion again focuses on the most common type of  competitive flight, a Gordon Bennett type, with the objective  to maximize great circle distance covered. Since winning  distances can be well over a thousand miles at altitudes of  up to 18,000 feet MSL with times aloft possibly exceeding  seventy hours, a much larger area of the weather map must  be studied than for a typical hot air flight. The following approximations generally apply to a balloon  below 18,000 mean seal level (MSL). In contrast to hot air flights, landing conditions are most  likely to be different from those at launch and several weather  patterns may be encountered during the flight. Freezing  levels and the moisture content of the air should be checked  to predict the possibility of icing. Any icing that occurs has  multiple negative impacts on the flight. It adds weight to  11-6 Additional Factors That Affect Lift Meteorological Differences From Hot Air  Ballooning Weather Considerations for Gas  Ballooning Pressure Ceiling 2.  When the lifting gas inside the balloon is warmer (i.e.,  super heating) than the ambient air, additional lift is  generated. The reverse happens when the lifting gas  is colder than the ambient air. 3.  Nonstandard atmospheric conditions, such as  inversions, affect a balloon’s stability. 4.  The atmospheric humidity has a small effect on lift  with more humidity resulting in slightly less lift. 5.  The purity of the lifting gas directly affects lift. Most  commercially produced gas is assumed to be greater  than 99 percent pure, but purity can be reduced as a  result of improper filling technique. 1.  For a 1,000 cubic meter balloon at its pressure ceiling,  an ambient pressure decrease of 1 "Hg causes a  decrease in gross lift of about 80 pounds. 2.  For a 1,000 cubic meter balloon at its pressure ceiling,  an ambient and gas temperature decrease of 3.3 ºF  causes a lift increase of about 16 pounds. 3. For a 1,000 cubic meter balloon at its pressure ceiling,  a discharge of about 64 pounds of ballast results in  approximately a 1,000 foot increase in altitude. 1.  A balloon flying below its pressure ceiling (i.e., a  flaccid balloon) responds differently from one flying  at its pressure ceiling. 