The U.S. Aeronautical Information
Manual states, “Your first experience of flying over
mountainous terrain, particularly if most of your flight time has been
over the flatlands of the Midwest, could be a never-to-be-forgotten
nightmare if you are not aware of the potential hazards awaiting … Many
pilots go all their lives without understanding what a mountain wave is.
Quite a few have lost their lives because of this lack of understanding.
One need not be a licensed meteorologist to understand the mountain wave
The most distinctive characteristic
of the mountain wave is the lenticular cloud. This is a "signpost of the
sky" indicating that mountain wave activity is present.
there are several terms for mountain wave:-
ACSL (altocumulus standing
Or just plane "wave"
The wave that forms over the
mountain is more properly called the "mountain wave." The waves downwind
from the mountain are the "standing wave" or "lee wave." Pilots have come
to accept all of these names for wave activity, regardless of position of
the lenticular clouds.
To set up a mountain wave condition three
elements are needed:
Wind flow perpendicular to the
mountain range, or nearly so, being within about 30 degrees of
An increasing wind velocity
with altitude with the wind velocity 20 knots or more near
Either a stable air mass layer
aloft or an inversion below about 15,000
Because of these elements, the
weather service is able to predict the mountain wave condition with over
In figure 1, we have likened an atmosphere with
low stability to a flimsy spring that offers little resistance to vertical
motion. So while the lower coils move easily up and over the mountain, the
jolt received at ground level is not transmitted very far
Figure 2 represents a stable atmosphere that is
similar to a tough, heavy spring. This air, when it strikes the mountains,
tends to suppress internal vertical motion. It is essentially too tough
for oscillations to be set up.
In figure 3 we have an arrangement of a strong
coil sandwiched between two weaker springs to simulate an atmosphere with
a stable layer sandwiched between areas of lesser stability. With this
arrangement it is conceivable that the strong spring will continue to
bounce up and down for some time after the parcel of air has crossed the
mountain ridge. With a stable layer (or inversion aloft) the air stream is
both flexible enough to be set in vertical motion and elastic enough to
maintain that motion as a series of vertical oscillations.
As the air ascends, it cools and condenses out
moisture, forming the distinctive lenticular clouds. As it descends, it
compresses and the heat of compression reabsorbs the moisture. It goes
through this up and down action many times forming a distinctive
lenticular cloud at the apex of each crest, providing there is sufficient
moisture present for the cloud formation.
to wind speed
Intermountain West -
averages 4 miles
Appalachia Wave -
averages 10 miles
in the summer early morning or late afternoon is best for formation
winter is the best time for formation (jet stream, snow covered
ground = no convection, stable layer aloft)
The up-and-down action forms a
trough at the bottom of its flow and a crest at the top of the flow. The
distance from trough to trough (or crest to crest) is called the wave
length. The wave length is directly proportional to wind wind and
inversely proportional to stability.
The wave length is used for
visualization. In the area from the trough to the crest is an area of
updrafts. The area from the crest to the trough is predominately
In the intermountain west the wave
length can vary from about 2 nautical miles to over 25 nautical miles. It
averages 8 miles and extends downrange about 150-300 nautical miles.
Satellite photos have shown the wave capable of extending over
700-nautical miles downwind from the mountain range.
Cap cloud of the Teton mountain range This cloud is mostly
on the windward side of the mountain.
The foehngap exists because moisture
is reabsorbed during the down rush of air.
With sufficient moisture three typical
wave clouds will form, although there are four types of clouds associated
with the wave.
Cap cloud (foehnwall)
Roll (rotor, arcus)
The presence of clouds merely point out
wave activity and not wave intensity at any particular level. Because
moist air takes less vertical distance to reach its condensation level
than does dryer air, the presence of a lenticular cloud is not necessarily
an indication of the strength of the updrafts or downdrafts in a mountain
For example, high altitude lenticulars may
indicate there is sufficient moisture at that altitude to form them, when
in fact the strongest wave lift and sink occurs at a lower altitude where
there isn't enough moisture to form the lenticular clouds. This is one
reason visualization is so important.
The mother-of-pearl or nacreous cloud is a
pancake-shaped cloud that is extremely thin and visible for only a short
time after sunset or before sunrise when the sky is dark. It is normally
seen in latitudes higher than 50 degree north, or over Antarctica. It is
best seen in the polar regions at 80,000 to 100,000 feet when the sun is
below the horizon.
Lenticulars over Montana
Rotor cloud in Alaska
The lenticular cloud appears to be stationary
although the wind may be blowing through the wave at 50 knots or more. The
wave lift can extend into the stratosphere, more than 10 miles above sea
level, so you can't escape wave effects by flying over them.
What are the flight conditions in lenticular
clouds? Generally the lenticular area will be quite smooth. The only
danger is the magnitude of the sustained updrafts and downdrafts. Usually
individual lenticulars are composed of ice crystals, but when they are
composed of super-cooled water droplets watch out for severe icing
Line of rotors - Calgary
Normally the rotor clouds is centred beneath
the lenticular cloud. Most often it extends anywhere from ground level to
mountaintop level, but is frequently observed up to 35,000 feet. Destructive
turbulence from the rotor rarely exists more than 2,000-3,000 feet above
The rotor is described as a "dark,
ominous-looking cloud with a rotating appearance." If it forms near the ground
where it can pick up dust and debris, it is dark and ominous looking, but more
often it looks similar to a fair-weather cumulus. Turbulence is most frequent
and most severe in the standing rotors just beneath the wave crests at or below
mountaintop level (visualization is helpful where there is insufficient moisture
to form the rotor or the lenticular).
The rotor area forms beneath the lee wave
where a large swirling eddy forms. Sometimes with an inversion (normally stable
air), turbulence succeeds in overturning the air in the stable layer. Once warm
air is suddenly forced beneath colder and denser air a vigorous convection is
set up in an attempt to restore normal equilibrium. This makes the roll cloud a
particularly turbulent hazard. If the top of the cloud is rotating faster than
the bottom, avoid the area like the plague.
The most dangerous characteristic of the
standing wave is the rotor. The rotor can be assumed to exist whenever a
mountain wave forms, but a cloud will not always form to alert you to its
presence. Avoid the area where the rotor will form with visualization.
Often the three conditions that must exist to
form a mountain wave will exist (perpendicular wind flow, increasing wind
velocity with altitude, and a stable air mass layer or inversion) ... but there
is insufficient moisture for the wave clouds to form. This is called a dry wave.
All of the updrafts, downdrafts and rotor turbulence exists, you just can't see
the clouds. You must use visualization.
Just because a mountain wave exists, it is not
a sure sign that your flight must be delayed or cancelled. The degree of
stability can be determined from pilot reports or by a test flight.
Mountain wave safety practices
Altitude 50 percent above the terrain -
Turbulence caused by extreme mountain waves can extend into all altitudes that
you might use, but dangerous turbulence can usually be avoided by clearing the
mountains at least half again as high as the height of the mountain. In
Colorado there are 54 peaks over 14,000-foot elevation. Does this mean we have
to fly at 14,000 plus one-half (7,000) or 21,000 feet? No, use the base of the
terrain to begin measuring. For example, if the surrounding terrain is 10,000
feet and the mountaintop is 14,000 feet, use one-half of the 4,000-foot value
and fly 2,000 feet above the mountaintops.
Approach at a 45-degree angle - The
rule-of-thumb of flying half again as high as the mountain is designed to
reduce the risk of entering the turbulent rotor zone, but it does not
necessarily give you a sufficient margin to allow for height loss due to
downdrafts. You must have an escape route.
Avoid ragged or irregular-shaped lenticulars
- Ragged and irregular-shaped lenticulars can contain the same turbulence as
the rotor area.
Climb in lift - Dive in sink - By diving in
sink, rather than trying to maintain altitude, the airplane is exposed to the
effects of the downdraft for a lesser amount of time. Even though the rate of
descent will likely be double or more the rate of climbing at the best
rate-of-climb airspeed, the airplane will loose less altitude overall.
Avoid the rotor - If rotor clouds are not
present, visualize the area of the rotor and avoid it.
Visualize the wave length - When flying
parallel to the wave, fly in the updraft area.