PAGE 1

Original idea from Joseph Towers

Altocumulus lenticulars, or jet stream clouds of the standing-wave type, are unique clouds that form above or downwind of a hill or mountain as

strong winds produce waves in the air.

Such extraordinary cloud formations are the result of high velocity or jet stream winds being forced over a barrier or mountain. The resultant laminar flow causes the air to move in wave-like layers as shown in Figure 1. Waves develop above the mountain and on the leeward side. If the air contains sufficient moisture, characteristic standing-wave lenticular clouds will form. Mountain wave activity is usually present in the vicinity of lenticular cloud formations. Mountain wave patterns may extend as much as several hundred miles downwind of a mountain range. A wave may repeat itself several times with amplitude and intensity decreasing over distance. Satellite photographs of the Rocky Mountains have shown mountain waves extending as far as 700 miles downwind of the range. Along the east coast area of the United States, such photographs of the Appalachian chain show that mountain wave activity can exist over 100 miles eastward. Mountain wave activity may exist from the surface up through the tropopause and into the lower stratosphere. Rotor clouds often form in the vicinity of lenticular formations and mountain wave activity. These small cumulus-like puffs form below the mountain ridge and are indicative of high velocity rotary air circulation beneath the wave crests. Rotor clouds, like lenticulars, often appear stationary because they are constantly forming in the updraft and dissipating in the downdraft. At times, rotor clouds may blend in with associated lenticular formations. When present, they are a sure indication of extremely turbulent rotary air circulation and highly unstable conditions.

LENTICULAR ALTOCUMLULUS CLOUDS & MOUNTAIN WAVES

PAGE 2

When mountain wave activity and lenticular cloud formations are present, expect turbulence. Dramatic changes in airspeed, altitude, and pitch attitude may be experienced. The associated turbulence can be extremely disconcerting. Personal injury to passengers and crew could result from penetrating such airspace. Crews should be cautious of potential overspeed conditions during cruise. When operating in turbulence, crews should avoid flying at or near the airplane's maximum aerodynamic ceiling. A cruise altitude with an aerodynamic ceiling which allows for a 1.5 g loading should be maintained. Figure 1 shows a schematic illustration of the mountain wave development with standing lenticular and rotor cloud formations. When the air contains sufficient moisture, such characteristic clouds will form above and downwind of a mountain when high velocity winds are present.

Lenticular clouds may not always be present to visually identify potential turbulence and mountain wave activity. If the air is too dry, lenticular clouds will not form. However, crews should anticipate possible mountain wave turbulence when

strong winds of 40 knots or greater are present in the vicinity of a mountain or mountain range, even in clear air. Lenticular altocumulus clouds of the standing-wave type are shown here in the vicinity of Mount Rainier in upstate Washington (see Figure 2). They form orthographically above or downwind of a hill or mountain and are the product of very strong winds which pass around or through the cloud while the cloud formation itself remains stationary. The clouds continuously form in the updrafts and dissipate in the downdrafts. They may change their position, shape, or size in a very short period of time. Flight conditions in the vicinity of such formations are usually extremely turbulent. Cirrocumulus standing lenticular clouds (CCSL) have formed above 30,000 feet and are visible in the upper portion of the photograph (Figure 2). Just below, are altocumulus standing lenticular clouds (ACSL) at about 25,000 feet. Rotor clouds, although difficult to see in this photograph, are present in the form of small cumulus-like clouds in the lower right and left. Rotor clouds usually form below the elevation of the mountain peaks and are often indicative of high rotary air circulation. Lenticular clouds often form in layers above the rotor clouds and may form as high as 40,000 feet. Figure 3 reveals the exquisite and finely sculptured lenticular clouds close up. They are the same clouds shown in Figure 2. Note their smooth appearance and feathered edges. The wind's laminar flow is evidenced by the clouds multi-layered formation. The extreme brilliance of the white light is the direct reflection of sunlight to the observer. In this case, the sun is directly behind the viewer. An observer located on the opposite side of the formation would likely view it as gray and far less spectacular. Although uniquely beautiful, inflight penetration would not be prudent.

Figure 1

Figure 2

REMEMBER ONLY ONE THING :

One of the most dangerous features of mountainwaves is the turbulent area in & below rotorclouds Never fly in this area !

PAGE 3

Moutain wave accident (see NTSB data) :

- United Airlines / Boeing 737-291 / N999UA / Colorado Springs USA / March 3,1991 - USAir / Boeing 737-3B7 / N5513AU / Pittsburgh region USA / September 8, 1994

SUMMARY Lenticular altocumulus clouds of the standing wave type are indicative of very high

velocity winds, potential mountain wave activity and turbulent flight conditions. Such developments may exist at any time of the year. However, as jetstream

velocities become considerably stronger in the mid-latitudes during winter and earlyspring, such activity should be expected with greater frequency - and dealt withappropriately.

Smaller scale turbulence in the vicinity of smooth lee waves is often made visible by

the sawtooth appearance of shearing gravity waves on lenticular clouds. Avoid ACSLwith ragged edges !

To avoid mountain wave activity, change your route. If this is not possible, change

your altitude away from the most likely layers of turbulence When winds are strong, beware of significant turbulence downwind of isolated peaks. In potential mountain wave areas, watch your altimeter, especially at night. Vertical

motions in lee waves may be strong, resulting in large altitude excursions (altimeterreadings may be inaccurate in strong lee waves).