JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
forms particular to wave action provide the best means of identifying possible turbulence, aside
from weather forecasts and PIREPs. Although the lenticular clouds in Figure 4-4 are smooth in
contour, they may be quite ragged when the airflow at that level is turbulent. These clouds may
occur singularly or in layers at heights usually above 20,000 feet. The rotor cloud forms at a
lower level and is generally found at about the same height as the mountain ridge. The cap cloud
usually obscures both sides of the mountain peak. The lenticular clouds (Figure 4-4), like the
rotor and cap clouds, are stationary in position, even though the wind flows through them.
The pilot is concerned, for the most part, with the first wave because of its more intense activity
and proximity to the high mountainous terrain. Extreme turbulence is usually found at low levels
on the leeward side of the mountain in or near the rotor and cap clouds when the winds are 50
knots or greater at the mountaintop. With these wind conditions, severe turbulence can
frequently be found to exist from the surface to the tropopause and 150 miles downwind.
Moderate turbulence can be experienced often as far as 300 miles downwind under those same
conditions. When the winds are less than 50 knots at mountain peak level, a lesser degree of
turbulence may be experienced.
Mountain wave turbulence is dangerous in the vicinity of the rotor clouds and to the leeward side
of the mountain peaks. The cap cloud must always be avoided in flight because of the turbulence
and the concealed mountain peaks.
The following techniques should be applied when mountain wave turbulence has been forecast:
1. Avoid the turbulence if possible by flying around the areas where wave conditions exist. If
this is not feasible, fly at a level that is at least 50% higher than the height of the highest
mountain range along your flight path. This procedure will not keep the aircraft out of
turbulence, but provides a margin of safety if a strong downdraft is encountered.
2. Avoid the rotor, lenticular, and the cap clouds since they contain intense turbulence and
strong updrafts and downdrafts.
3. Approach the mountain range at a 45° angle, so that a quick turn can be made away from the
ridge if a severe downdraft is encountered.
4. Avoid the leeward side of mountain ranges, where strong downdrafts may exist, until certain
turbulence is not a factor.
5. Do not place too much confidence in pressure altimeter readings near mountain peaks. They
may indicate altitudes more than 2500 feet higher than the true altitude.
6. Penetrate turbulent areas at air speeds recommended for your aircraft.
4-6 Version 3.2/Dec 08