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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.Mechanical
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.
The strength and magnitude of mechanical turbulence depends on:
• 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.
Stability seems to be the most important factor in determining the strength and vertical extent of the mechanical turbulence.Frontal
Frontal turbulence is cause by the lifting of warm air by a frontal surface, leading to instability and/or the mixing or 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
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
4-23
wave do not necessarily have to occur for the turbulence to be present.
Sometimes during a climb or descent, a balloon encounters a narrow zone of wind shear with its accompanying turbulence at the top of a temperature inversion. These inversions occur anywhere from just above the surface to the tropopause.
Strong inversions near the ground are an extreme form of wind shear that adversely affect balloon takeoffs and landings. For example, a pocket of calm, cold air forms in a valley as a result of nighttime cooling, but the warmer air moving over it has not been affected appreciably. Due to the difference between the two bodies of air, a narrow layer of very turbulent air may form. A balloon climbing or descending through this zone will usually encounter considerable turbulence, as well as changes in lift.Low Level Wind Shear
Wind shear is a sudden, drastic change in windspeed and/or direction over a very small area. Wind shear can subject a balloon to violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the balloon. While wind shear can occur at any altitude, low level wind shear is especially hazardous due to the proximity of a balloon to the ground. Directional wind changes of 180° and speed changes of 50 knots or more are associated with low level wind shear. Low level wind shear is commonly associated with passing frontal systems, thunderstorms, and temperature inversions with strong upper level winds (greater than 25 knots).
　
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