The following smaller-scale phenomena represent specific weather hazards for aircraft operating near mountains.

Occasionally, an extremely strong low-level temperature inversion can occur in mountainous areas, with the inversion top below ridge level (perhaps 900 to 1,000 ft AGL) and a pool of very cold air at the surface. If this phenomenon occurs with strong wind flow above the inversion layer, there will be a concentrated shear zone near the inversion, which can lead to both significant turbulence encounters and abrupt airspeed changes for aircraft that penetrate the inversion on climbout or during descent. This situation is true particularly when significant mountain wave activity is present above the inversion in the strong flow aloft. In this case, the surface-based pool of cold air and the inversion above it shelter the surface from what might otherwise be a damaging windstorm.

The surge of wind across a ridge can initiate a vortex downwind of the ridge (Figure 16-11). The vortex rolls up to maximum strength of rotation as it continues to move downwind away from the ridge and slowly dissipates. In its wake, with a return to steady flow, K-H waves develop at the top of the shear layer.

Extreme gustiness is a characteristic of the surface winds during severe downslope windstorms. The interaction of these gusts with strong large-scale winds moving perpendicular to a ridge may produce strong horizontal vortices of small scale.

(Source: Bedard, 1993)9

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.

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.

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 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 warm 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 the 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)

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.