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. 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. For a pilot with a hot air rating, certification to fly gas  balloons requires the removal of the “with airborne heater”  limitation on his or her certificate. For a private pilot gas  rating, aeronautical experience requirements in Title 14 of  the Code of Federal Regulations (14 CFR) section 61.109(h)  (1) require “…at least two flights of at least 2 hours each  that consists of 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.  The regulation for removal of the airborne heater restriction  from an existing hot air pilot’s certificate (14 CFR Section 61.115 Balloon ratings: Limitations) states: 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. NOTE: Only a logbook entry is required to complete the  process (no check ride), since the gas authorization is a  removal of a restriction from an existing rating rather than  an issuance of a new rating. The regulations for the aeronautical experience for a  commercial rating are virtually identical according to 14  CFR section 61.129(h)(4)(i) except that the student must act  as pilot in command on both flights and the controlled ascent  must be to “…5,000 feet above the launch site.” As an example, during flight at night very near the ground,  a pilot may feel he or she is continually ballasting to fly  11-8 (2) The limitation may be removed when the person  obtains the required aeronautical experience in a  gas balloon and receives a logbook endorsement  from an authorized instructor who attests to the  person’s accomplishment of the required aeronautical  experience and ability to satisfactorily operate a gas  balloon.  (iii) At least one flight involving a controlled ascent to  3,000 feet above the launch site. ” (ii) At least one flight performing the duties of pilot  in command in a gas balloon with an authorized  instructor; and (i)  At least one training flight with an authorized  instructor within 60 days prior to application for the  rating on the areas of operation for a gas balloon; Gas Balloon Regulations The Practice of Gas Ballooning