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The following approximations generally apply to a balloon below 18,000 mean seal level (MSL).
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.
Additional Factors That Affect Lift
1. A balloon flying below its pressure ceiling (i.e., a flaccid balloon) responds differently from one flying at its 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.
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.Weather Considerations for Gas Ballooning
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.Meteorological Differences From Hot Air Ballooning
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
11-7
Figure 11-7. Standard temperature lapse chart.
14,000
12,00010,0008,0006,0004,0002,0000Temperature (°F)Altitude (feet)–20.00 0.00 20.00 40.00 60.00 80.00Standard Lapse Rate
Figure 11-8. Atmosphere exhibiting an inversion.
14,000
12,00010,0008,0006,0004,0002,0000Temperature (°F)Altitude (feet)–20.00 0.00 20.00 40.00 60.00 80.00Inversion Aloft
the balloon and can interfere with the functioning of the valve by preventing it from either opening properly or from sealing tightly after activation. The weight of the ice should initiate a natural descent to a lower altitude and warmer temperatures. If this does not happen naturally, the pilot may initiate a descent by valving. Be aware that the melting ice at lower altitudes lightens the system and additional valving is necessary to prevent a second ascend back above the freezing level. Falling ice shards have also been blamed for equipment damage on occasional flights. A second strategy against icing is to seek drier air above the saturated air but the higher altitude is unlikely to provide the warmer temperatures needed to melt the accumulated ice.
Encountering thunderstorm activity is much more common in gas versus hot air flights. This is especially dangerous when flying an explosive gas such as hydrogen. Isolated thunderstorms can develop due to solar heating of moist air in the afternoon over the great plains of the United States with very little warning. Prediction and avoidance are the best tools against thunderstorms. Real time contact between the pilots and a ground-based meteorologist with access to forecasting tools to predict the formation of thunderstorms can help to avoid these storms. During the different stages of a thunderstorm, rapid accelerations due to inflows or outflows, as well as rapid ascents or descents, are very likely. The presence of any of these effects, in conjunction with any thunder or lightning are indications that an immediate landing is prudent.Meteorological Flight Planning
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Balloon Flying Handbook(135)
