Chapter 16, Mountain Weather 16-1
16 Mountain Weather
16.1 Introduction
This chapter focuses on mountain waves and adverse winds. Other mountain weather phenomena are
discussed in other chapters, which are noted.
Chapter 16, Mountain Weather 16-2
16.2 Mountain Waves and Adverse Winds
The atmosphere is a fluid in motion. Just as the swiftly flowing water in a stream develops waves and eddies
as it passes over and around obstructions, so does the atmosphere contain disturbances that develop as it
interacts with mountainous terrain. These atmospheric eddies can range in size from a few centimeters to
tens or hundreds of kilometers and can present the pilot with relatively smooth air, or with turbulence of
potentially destructive intensity, and the likelihood of loss of control.
When the atmosphere encounters a mountainous barrier, a number of responses are possible. If the wind is
weak or the moving air mass exceptionally dense, the mountains may act as a dam, preventing the motion
of air over the barrier. More frequently, strong winds flow over or around mountains or ridges. If the
surrounding atmosphere is unstable, the vertical displacement of the air will (if sufficient moisture is
present) lead to thunderstorm formation or at least the development of deep convective clouds. H owever,
if the wind is sufficiently strong and the surrounding atmosphere is stable, a wave will develop.
The wave that results from vertical displacement of a stable air mass over a mountain or ridge can generally
take one of two forms: vertically propagating mountain waves or trapped lee waves. Both types of waves
can be hazardous to aviation operations. The particular type of wave or combination of waves that forms
depends on the nature of the mountain range and on atmospheric properties upwind of the mountain. It is
possible for both types of waves to exist at the same time. It also is possible to have hybrid or intermediate
forms (i.e., waves that are only partially trapped).
The most severe mountain wind events usually occur when the large-scale (or synoptic) winds are strongest,
from late autumn to early spring. During the remainder of the year, when the synoptic winds are normally
much weaker, hazardous winds in the vicinity of mountains are more likely to be associated with
thunderstorms and their outflow fields.
The mountain-induced flow fields to be discussed in this section are frequently accompanied by visual
indicators (such as lenticular and rotor clouds or blowing dust). However, this is not always the case, and
extremely severe wind events can occur with little or no visual warning of their presence.
16.2.1 Gravity Waves
In order for gravity waves to develop, the atmosphere must possess at least some degree of static stability.
As stable air is deflected vertically by an obstacle (e.g., when an air mass moves over a mountain ridge), it
resists the displacement because as it rises, it is heavier than the air surrounding it, and gravity is acting to
return it to its equilibrium level. Because of its negative buoyancy, the deflected air begins to return to its
original level once it has cleared the ridge. However, its momentum will cause it to overshoot the original
altitude, warming by compression and now becoming less dense than th e surrounding air. As a result, it
begins to rise back to the equilibrium altitude, overshoots once more, and continues through a period of
oscillations before the resulting wave motion dampens out. This process is depicted in Figure 16-1.
Chapter 16, Mountain Weather 16-3
Figure 16-1. Gravity Wave Oscillations
The described gravity wave will have measurable wavelength, amplitude, phase speed, and period. The
period of this type of atmospheric disturbance is related to the temperature of the air and the “spread”
between the existing lapse rate and the dry adiabatic lapse rate (or, equivalently, the degree of stability
present). In general, the large-scale wind (wind shear) changes in altitude and temperature (lapse rate), the
size and shape of the mountain or ridge over which the air is moving, and the orientation of the wind relative
to the ridge line all work together in determining the character of the disturbance that develops.
16.2.2 Kelvin-Helmholtz (K-H) Waves
When wind shear is very strong, another type of wave is possible. These w aves, called gravity -shear or
K-H waves, can occur when the kinetic energy inherent in the shear can overcome the damping effects of
a stable temperature lapse rate. This effect is illustrated in Figure 16-2.
Figure 16-2. Gravity-Shear Waves
Chapter 16, Mountain Weather 16-4
If the wind shear that penetrates the layer of the atmosphere is weak (some wind shear is nearly always
present), a shear-induced wave motion will not occur. However, if the magnitude of the wind shear exceeds
a critical value, wave motions will begin spontaneously within the shear layer , resulting in a K -H wave.
The amplitude of the resulting wave will grow with the kinetic energy in the surrounding wind field until,
like an ocean wave breaking on the shore, the wave overturns and breaks down into turbulen ce. The
resulting turbulence can have a range of effects on aircraft. The clouds associated with shear -induced
gravity waves can frequently be observed in the atmosphere, as shown in Figure 16-3 and Figure 16-4.
Figure 16-3. Schematic of Clouds Associated with Gravity-Shear Waves
(Photo Credit: University Corporation for Atmospheric Research (UCAR))
Figure 16-4. Clouds Associated with Gravity-Shear Waves
K-H waves are quite common in the atmosphere; they can form in the vicinity of thunderstorms, in shear
layers near the jet stream, and in association with stable regions of the atmosphere that are topped by a
strong wind -shear layer (such as the top of a pool of cold air on the lee side of a mountain). In fact,
K-H instability induced by the wind shear associated with strong winds aloft is likely the chief source of
high-level turbulence away from mountain ranges [clear-air turbulence (CAT)]. The mechanism that causes
this type of disturbance can be compared to that of a flag flapping in a breeze. The flapping is a result of
Chapter 16, Mountain Weather 16-5
instabilities created by the wind shear along the flexible surface of the flag, analogous to the wind shear
through a very stable (but shallow) layer of the atmosphere.
16.2.3 Vertically Propagating Mountain Waves
Figure 16-5 shows a schematic of a vertically propagating mountain wave. This feature is essentially a
standing gravity wave whose energy propagates vertically. For this class of wave, nothing is preventing
vertical propagation, such as strong wind shear or neutrally stable atmospheric layers. The mere fact that a
wave has developed in air moving over a mountain (or other barrier) does not in itself indicate problems
for an aircraft operating in the vicinity. The potential for hazard is a function of the strength of th e wave
and whether or not an area of the wave “breaks” into turbulent motions that, in the extreme, can lead to
structural damage or failure of an aircraft component.
With this type of wave feature, air that is moving nearly perpendicular to the barrier is deflected upward
and accelerated as it passes over the crests and down the lee slopes of the terrain. Notice in Figure 16-5 that
the standing wave has developed vertically above the mountain crest and that the resulting wave tilts upwind
with height. This vertical propagation of the wave means that the effects of the mountain range can be felt
at heights significantly above the actual altitude of the peaks (at times reaching in excess of 60,000 ft). As
a result, aircraft flying at virtually any altitude may have to deal with significant turbulence and
wave-induced altitude excursions. In fact, the amplitude of this type of wave actually increases with height
above the mountain (in the absence of atmospheric features, such as strong inversions or shear layers that
would tend to partially reflect or absorb the upward-moving wave energy). This amplification is a
consequence of the normal decrease in air density with altitude.
The amplitude of the wave will be larger, for the same upstream conditions, the higher the elevation of the
mountain range above the surrounding terrain. Although even very modest terrain relief can cause
appreciable wave activity under the proper conditions. Wave amplitude also will tend to be larger for
stronger cross-mountain wind components at mountaintop level. However, the actual amplitude depends
on complex relationships between upstream atmospheric wind and temperature profiles and the height and
shape of the particular mountain range. Stronger flow across the mountain leads to a deeper wave, given
the same atmospheric stability. However, the greater the background stability, the shallower the resulting
wave, at fixed-wind speed.
As previously noted, the primary concern for pilots with this type of feature is that the vertical motions of
the air moving through the wave may become strong enough to “break” into turbulence.
What is meant by “wave breaking”? Looking again at the streamlines that show the airflow in Figure 16-5,
it shows that high above the ridge there is a region of updraft. With a wave of modest amplitude (in which
the vertical displacement of air moving through the wave is relatively limited), an aircraft flying through
this region would likely experience app reciable “wave action,” with altitude and/or airspeed fluctuations,
but little turbulence. However, with sufficient amplitude, the wave breaks , and localized updrafts and
downdrafts occur. The consequences for a pilot flying through this region include airspeed and altitude
deviations and the possible sudden onset of severe or extreme turbulence. This type of turbulence occurs
typically between 20,000 ft and 39,000 ft MSL and is, therefore, primarily of importance to turboprop and
jet aircraft at cruise as they approach and overfly the mountain range.
Chapter 16, Mountain Weather 16-6
(Source: Durran and Klemp, 1983)4
Figure 16-5. Schematic of a Vertically Propagating Mountain Wave
Often accompanying these high -altitude effects is the occurrence of very strong surface winds that result
from the wave breaking aloft. In this case, strong downslope winds on the lee slopes can reach 100 kt gusts,
creating a low-level turbulence hazard for all aircraft. Further, these extremely strong low-level winds often
abruptly terminate in a “jump” located some distance down the lee slope or well to the lee of the mountains
themselves. These features are indicated schematically in Figure 16-6. The jump region is frequently an
area of extreme turbulence extending to 10,000 ft or more above the surface. The area of the jump is
sometimes marked by a line of ragged rotor clouds exhibiting very turbulent motion. Downwind of the
jump, turbulence decreases in intensity but still may be quite strong.
4 Durran, D. R., & Klemp, J. B. (1983). A compressible model for the simulation of moist mountain waves. Monthly
Weather Review, 111, 2341-2361.