Chapter 16, Mountain Weather 16-13
Flight operations may be conducted in the vicinity of strong horizontal vortices without any encounters
because they are highly localized, short -lived, and generally cloud -free. Conversely, one or more aircraft
may encounter a strong, but invisible, vortex (that might be described as being like a “horizontal tornado,”
even though it is not) and undergo rolling moments and localized turbulence that make it impossible for the
pilot to maintain aircraft control.
16.2.6.3 Intense Vertical-Axis Vortices
Analogous to the horizontal vortices described in the previous section are vertically oriented vortices of
great intensity, similar to a short -lived, tornado-like event. They can form downwind of localized rugged
terrain as well as isolated peaks (see Figure 16-12).
These vortices are not associated with thunderstorms and are therefore not tornadoes, but their wind speeds
can reach 150 kt or more. As is the case with horizontal vortices, there may be no visual indications
(i.e., visible cloud) of the presence of such a strong vertically oriented vortex.
Figure 16-12. Schematic of Vertically Oriented Vortices Generated in the Lee of an Isolated Mountain Peak
16.2.6.3.1 Dust Devils
A common wind phenomenon that occurs throughout much of the world, including the desert Southwest,
is dust devils, sometimes known as whirlwinds. These dust -filled vortices, created by strong surface
heating, are generally smaller and less intense than a tornado. Typical diameters of dust devils range
from 10 to 300 ft, with an average height of approximately 500 to 1 ,000 ft. In most locations, dust devils
typically last only a few minutes before dissipating , although in deserts typical of northern Arizona, dust
devils can reach heights of several thousand feet and last an hour or more. Wind speeds in larger dust devils
Chapter 16, Mountain Weather 16-14
can reach 60 mph or greater. Even though they are generally smaller than tornadoes, dust devils can still be
destructive as they lift dust and other debris into the air. Small structures can be damaged, and even
destroyed, if they are in the path of a strong dust devil.
Dust devils form in areas of strong surface heating, usually at the interface between different surface types
such as asphalt and dirt, or even irrigated fields and dirt roads. Typically, they occur under clear skies and
light winds, when the ground can wa rm the air to temperatures much higher than the temperatures just
above the ground. This is a very unstable condition, since the heated air is less dense and lighter than the
cooler air above it. If the temperature of the ground becomes much warmer than th e air above it, vertical
mixing will take place to release this unstable configuration. Once the ground heats up enough, a localized
pocket of air will quickly rise through the cooler air above it. The sudden uprush of hot air causes air to
speed horizontally inward to the bottom of the newly forming vortex. This rapidly rising pocket of air may
begin to rotate, and if it continues to be stretched in the vertical direction, it will increase in rotation speed.
This increase in rotation speed from vertical stretching is similar to the increased spinning of an ice skater
as they bring their arms in toward their bodies. As more hot air rushes in toward the developing vortex to
replace the air that is rising, this spinning effect is intensified. The air cools as it rises, and will eventually
descend back through the center of the vortex. Under optimal conditions, a balance between the hot air
rising along the outer wall of the vortex and the cooler air sinking in the vortex occurs. The dust devil then
begins to move across the ground, picking up more and more dust, highlighting the vortex and making it
visible to the eye. The dust devil, once formed, is a funnel-like chimney through which hot air moves both
upward and circularly. If a steady supply of warm unstable air is available for the dust devil, it will continue
to move across the ground. However, once the warm unstable air is depleted or the balance is broken in
some other way, the dust devil will break down and dissipate.
(Photo courtesy of NWS Reno)
Figure 16-13. NWS Dust Devils: A Life Cycle
It is important to note that not all dust devils may be easily visible. Some may have no or very little debris.
Pilots should try their best to avoid dust devils. They should not fly through them and should scan takeoff
and landing areas.
Chapter 16, Mountain Weather 16-15
(Photo courtesy of the National Transportation Safety Board (NTSB))
Figure 16-14. Accident Damage Caused by a Dust Devil
16.2.6.4 Boras
The Glossary of Meteorology10 defines a bora as a “fall wind with a source so cold that, when the air reaches
the lowlands or coast, the dynamic warming is insufficient to raise the air temperature to the normal level
for the region; hence it appears as a cold wind. ” Cold air building up on one side of a mountain range will
often be blocked. However, if it deepens sufficiently, it will eventually spill over the mountain barrier and
accelerate down the opposite slope, on rare occasions reaching speeds as high as 80 kt.
The resulting low-level winds and turbulence can be a significant hazard for aircraft that are flying in the
vicinity of the down -rush of air caused by the bora. The danger is heightened by the fact that the exact
timing and location of the air surge is difficult to forecast. There are at least two primary causes of boras:
1) cold fronts aligned parallel to the mountain range and mov ing perpendicular to it, with the cold air
eventually spilling over; and 2) cold outflow, from thunderstorms over or near a mountain range, that builds
up to sufficient depth to spill over and down the opposite slope. The latter phenomenon is short -lived and
very difficult to predict; the strong thunderstorm wind s typically last less than one hour. However, strong
downslope winds accompanying and following cold front passages can persist for several hours. Only the
initial stages of such winds have true bora or fall -wind characteristics; these winds appear to evolve into
severe downslope winds torms associated with breaking waves a loft and, therefore, become potentially
dangerous at all altitudes, not just within a few thousand feet of the surface.
In many areas along the east ern slopes of the Rock y Mountains, and in particular in Colorado, prefrontal
windstorms with very warm lee -side temperatures are known as chinooks; post -cold frontal windstorms
10 American Meteorological Society, cited 2024: Bora. Glossary of Meteorology. [Available online at
https://glossary.ametsoc.org/wiki/bora.]
Chapter 16, Mountain Weather 16-16
with cold lee-side winds are often called bora windstorms, or boras. Thus, the term bora in these areas can
mean both the initial strong burst of a cold downslope wind and any subsequent downslope windstorm. In
the case of eastern-slope boras, the best indicators during the preflight briefing are the presence of a strong
cold front moving through the area (i.e., with much colder air behind the front), with associated rapid frontal
movement (on the order of 30 kt or more). Surface observations (as reported i n a METAR), particularly
special observations of strong, rapidly changing surface winds from the west or northwest, along with
decreasing temperature, may warn of bora activity. The indicators for breaking internal gravity waves
should not be ignored. Western -slope boras are less common and are usually associated with a strong
buildup of extremely cold arctic air on the eastern slopes.
16.2.6.5 Other Phenomena
In addition to the vortex phenomena previously discussed, vortices or strong shear zones may be generated
locally by strong flow past individual mountain peaks and crags, or through gaps and passes across
mountain ranges.
The point is that strong wind flow in the vicinity of irregular terrain can produce a multitude of disturbances
of varying size and strength, many without reliable visual indicators. Their presence should be suspected
when flying downwind of rugged terrain, whenever the wind flow at ridge level exceeds about 20 kt.
16.2.7 Visual Indicators of Orographic Wind Fields
Figure 16-15 provides a schematic of mountain waves and possible associated clouds. The most distinctive
clouds are the sharp-edged, lens-shaped (or almond-shaped) lenticular clouds. When sufficient moisture is
present in the upstream flow, mountain waves can produce interesting cloud formations , including cap
clouds, Cirrocumulus Standing Lenticular (CCSL), ACSL, and rotor clouds (see Figure 16-16). These
clouds provide visual proof that mountain waves exist. However, these clouds may be absent if the air is
too dry.
Figure 16-15. Schematic of Mountain Waves and Associated Clouds
Chapter 16, Mountain Weather 16-17
Figure 16-16. Examples of Mountain Wave Clouds
16.3 Mountain/Valley Breezes and Circulation
See Sections 10.6.4, 10.6.5, and 10.6.6 for information on mountain/valley breezes and circulation.
16.4 Mountain/Valley Fog
See Section 18.1.1.1.1.1 for information on mountain/valley fog.
16.5 Upslope Fog
See Section 18.1.1.1.3 for information on upslope fog.
16.6 Mountain Obscuration
See Section 18.2.2 for information on mountain obscuration.
16.7 Mountain Turbulence
See Chapter 19, Turbulence, for information on mountain turbulence.
16.8 Mountain Icing
See Section 20.3.8 for information on mountain icing.
16.9 Density Altitude
See Section 8.4.1.5 for information on density altitude.