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Flight planning for a gas balloon flight starts several days prior to the planned flight. The gas balloon pilot examines numerous meteorological tools, looking at frontal movement to get the big picture, wind speed and direction forecasts at several altitudes, and future times to predict a flight path. Then, they will study forecast precipitation probabilities and freezing levels over the flight period. Modern trajectory predictors, such as the HYSPLIT (www.arl.noaa.gov/ready.html) program, maintained by National Oceanic and Atmospheric Administration (NOAA) can be a great help. However, the HYSPLIT model gives no forecast for precipitation along the route, so the possibility of rain, snow, or icing must be assessed using other weather models. Often, ideal weather is found just after a frontal passage, after any moisture has cleared but while the air mass is still moving with the departing front.
For serious competitions, a professional meteorologist is an invaluable team member and is consulted before and during the flight. A good starting point for weather investigations is NOAA’s aviation weather web site www.aviationweather.gov. Flying in Inversions
Proper utilization of atmospheric temperature inversions during gas balloon flights can result in increased flight stability and ballast conservation. Figure 11-7 shows a simplified atmospheric temperature lapse chart when no inversion is present. Figure 11-8 is similar but with an inversion present. In both charts, altitude increases up the vertical scale and temperature increases going to the right on the horizontal scale. Figure 11-7 shows temperature decreasing consistently with altitude, while Figure 11-8 has an inversion zone (inside red circle) from 2,000 to 4,000 feet
11-8
in which the temperature increases with increasing altitude. A gas balloon flying in this inversion has the advantage of increased stability as compared to the altitudes above or below the inversion which exhibit normal lapse rates.
To explain this, it is important to understand the term “stability” with respect to a gas balloon. Stability can be imagined as an invisible hand that gently pulls the balloon down whenever it starts to rise or alternately pushes the balloon back up as it starts to fall. A balloon flying in stable weather tends to fly level with very little intervention from the pilot. However, stability is a weak condition and can be overcome by many factors, such as gain or loss of solar heating, orographic winds, and ballasting or valving.
To understand why an inversion creates stable flying conditions, think of a gas balloon flying at 3,000 feet in the middle of the inversion. If, for some reason, the balloon starts to ascend, two things happen. First, the balloon enters warmer ambient air. Second, the lifting gas inside the balloon expands and cools adiabatically as it reaches the slightly lower pressure atmosphere of the higher altitude. Both of these effects cause the balloon to lose lift and to descend. It is helpful to remember that, as a hot air balloon either cools or enters hotter air, the temperature differential between outside and inside air decreases and lift is lost. The same principle applies here, as applied to gas balloons. Subsequently, if the gas balloon descends from its starting point, it encounters cooler outside ambient air and its interior gas compresses and warms adiabatically, therefore gaining and ascending back to its original altitude. Remember, in an inversion, either motion (ascending or descending) tends to cause an opposing force to passively initiate a return to the original altitude.
With proper planning, the gas balloon pilot can take advantage of this scenario. Weather theory teaches that inversions often set up at night either right at the surface (sometimes referred to as “surface inversion”) or at some altitude above the ground (generally referred to as an “inversion aloft”) as shown in Figure 11-8. Visible signs of an inversion may be pollution trapped below the inversion causing reduced visibility and dirty looking air. Invisible signs of an inversion may be significantly more stable flying conditions in the inversion area. If a slow, steady initial ascent is initiated (best done by launching with a flaccid balloon), the balloon may find an inversion with no help from the pilot by leveling off as it enters the inversion zone. More likely the pilot has to hunt for the inversion; inversion levels can be determined from the use of the Skew-T charts as previously discussed in Chapter 4, Weather Theory and Reports.
As an example, during flight at night very near the ground, a pilot may feel he or she is continually ballasting to fly level. If a pilot suspects an inversion aloft may exist above, he or she can initiate a slow ascent. If the envelope is flaccid and the ambient air at this altitude is not inverted, then the balloon tends to continue rising until it either encounters an inversion or becomes full as it reaches its pressure ceiling. If the balloon levels out while it is still flaccid, this may indicate that it has entered an inversion zone. If the balloon continues to fly level passively, it is flying in an inversion.
　
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