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Wind shear is hazardous to a balloon for several reasons. The rapid changes in wind direction and velocity changes the wind’s relation to the balloon disrupting the normal flight attitude and performance of the balloon.Obstructions and Wind
As mentioned earlier, obstructions on the ground affect the flow of wind and can be an unseen danger, causing yet another atmospheric hazard for pilots. For example, ground topography and large buildings can break up the flow of the wind and create wind gusts that change rapidly in direction and speed. Obstructions range from manmade structures, such as hangars, to large natural obstructions, such as mountains, bluffs, or canyons. A safe pilot is vigilant when flying in or out of launch or landing sites that have large buildings or natural obstructions located near them.
The intensity of the turbulence associated with ground obstructions depends on the size of the obstacle and the primary velocity of the wind. This can affect the takeoff and landing performance of a balloon, and can present a very serious hazard. During the landing phase of flight, a balloon may “drop in” due to the turbulent air and be too low to clear obstacles during the approach. Disrupted airflow often extends horizontally as much as ten times the height of the object, if the winds are in the eight to ten knot range. Balloon pilots should be aware of this, and make adjustments accordingly when attempting to launch next to an obstruction, or when landing just past one.
This same condition is even more noticeable when flying in mountainous regions. While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar manner. As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater the downward pressure and turbulence become.
Due to the effect terrain has on the wind in valleys or canyons, downdrafts may be severe. Thus, a prudent balloonist is well advised to seek out another balloon pilot with mountain flying experience and get a mountain “checkout” before conducting a flight in or near mountainous terrain.Mountain Wave
A mountain wave is the wavelike effect, characterized by updrafts and downdrafts, that occurs above and behind a mountain range when rapidly flowing air encounters the mountain range’s steep front. The characteristics of a typical mountain wave are represented in Figure 4-22 which illustrates how air flows with relative smoothness in its lifting component as the wave current moves along the windward side of the mountain. Wind speed gradually increases, reaching a maximum near the summit. On passing the crest, the flow breaks into a much more complicated pattern, with downdrafts predominating.
An indication of the possible intensities in the mountain wave is reflected in verified records of sustained downdrafts and updrafts in excess of 3,000 feet per minute (fpm). Turbulence in varying degrees can be expected, with particularly severe turbulence in the lower levels. Proceeding downwind 5 to 10 miles from the summit, the airflow begins to ascend as part of a definite wave pattern. Additional waves, generally less intense than the primary wave, may form downwind. This event is much like the series of ripples that form downstream from a rock submerged in a swiftly flowing river. The distance between successive waves (wavelength) usually ranges from 2 to 10 miles, depending on existing wind speed and atmospheric stability, although waves up to 20 miles apart have been reported.
4-24
Figure 4-23. Multiple lenticular clouds over Mount Shasta, California.
Rotor turbulence
Breaking wave
Figure 4-22. Characteristics of a mountain wave.
Characteristic cloud forms peculiar to wave action provide the best means of identification. Lenticular clouds, formed by mountain waves, are smooth in contour. [Figure 4-23] These clouds may occur alone or in layers at heights above 20,000 feet MSL, and be quite ragged when the airflow at that level is turbulent. The roll cloud forms at a lower level, generally near the height of the mountain ridge. The cap cloud must always be avoided in flight because of turbulence, concealed mountain peaks, and strong downdrafts on the lee slope. The lenticulars, like the roll and cap clouds, are stationary. They are constantly forming on the windward side and dissipating on the lee side of the mountain wave. Rotors
Rotors or eddies can also be found embedded in mountain waves. Formation of rotors can also occur as a result of down slope winds. Their formation usually occurs where wind speeds change in a wave or where friction slows the wind near the ground. These rotors are often experienced as gusts or wind shear. Clouds may also form within a rotor.
Research on mountain waves and rotors or eddies continues, but there is no doubt that pilots need to be aware of these phenomena and take appropriate precautions. Although mountain wave activity is normally forecast, many local factors may affect the formation of rotors and eddies. When planning a flight, a pilot should take note of the winds and terrain to assess the likelihood of waves and rotors. There may be telltale signs in flight, including disturbances on water or wheat fields and the formation of clouds, provided there is sufficient humidity to allow cloud formation.Thunderstorms
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Balloon Flying Handbook(54)
