$_{Figure 4-21.}$  Surface obstructions cause eddies and other irregular  air movements. may extend beyond this boundary. Varying types of surfaces,  and the resultant thermal conditions, can affect a balloon to  a considerable extent.  The balloon pilot caught in a thermal will recognize the  condition by an increase in altitude without application of  heat from the balloon’s heater. This ascent can be rapid and  may exceed the maximum rate of climb limitations in the  balloon’s flight manual. Since the air mass is also rising with  the balloon, there is no significant pressure against the top  of the balloon. Thus, the top cap will not be pushed open  (commonly referred to as “floating the top”).  Depending on their size, some thermals may have a rotative  motion similar to a small low pressure system. This motion  draws the balloon in and forces it to fly in an uncontrolled  circle. For balloons caught in a thermal, remember the adage  “altitude is your friend.” First, the pilot should insure there  is sufficient altitude to clear potential obstacles. Second,  maintain the temperature in the balloon appropriate for level  flight. Many pilots attempt to descend immediately, but this  may put the balloon, as well as the passengers, at risk of an  uncontrolled descent with possible injury. Most thermals  have a short lift span. In almost all cases, the thermal will  “spit” the balloon out the top after a short time, and the pilot  may descend and land as necessary. When the air near the surface of the Earth flows over  obstructions, such as irregular terrain, (bluffs, hills,  mountains) and buildings, the normal horizontal wind flow  is disturbed. As a result, it is transformed into eddies or  other irregular air movements.  Figure 4-21  shows how the  buildings or other obstructions near a launch site or landing  field can cause turbulence.  shear between the warm and cold air masses. The vertical  currents in the warm air are strongest when the warm air  is moist and unstable. The most severe cases of frontal  turbulence are generally associated with fast moving cold  fronts. In these cases, mixing between the two air masses,  as well as the differences in wind speed and/or direction add  to the intensity of the turbulence.  Wind shear is a relatively steep gradient in wind velocity  along a given line of direction (either vertical or horizontal)  and produces churning motions (eddies) which result in  turbulence. The greater the change of wind speed and/or  direction in the given direction, the greater the shear and  associated turbulence.  Clear-air turbulence (CAT), or sudden severe turbulence  that occurs in cloudless regions, is associated with wind  shear, particularly between the core of a jet stream and the  surrounding air. CAT is not limited to the vicinity of the jet  stream and may occur in isolated regions of the atmosphere.  For example, the turbulence in a mountain wave can also  be classified as CAT because the identifying clouds in the  Frontal turbulence is cause by the lifting of warm air by a  frontal surface, leading to instability and/or the mixing or  Stability seems to be the most important factor in  determining the strength and vertical extent of the mechanical  turbulence. The strength and magnitude of mechanical turbulence  depends on:  Mechanical Wind Shear Frontal •  The speed of the wind, •  The nature of the obstruction, •  The stability of the air, and •  The angle at which the wind moves over the  obstacle. 4-22