WX BRIEF

Mountain flying hazards

Clear air turbulence, severe wind conditions, rime and clear ice can accompany spectacular views.


Mountain fogs can form by nocturnal cooling, air being forced to rise up a slope, or mixing of downslope flow. The results can be dangerous, as slopes and peaks may become obscured from view.

At the very least, a good rule of thumb is that pilots should expect wave activity to altitudes of at least 2 times the height of the range, and that the waves will continue downwind at least 2 times the distance in nautical miles as the flow speed in knots.

For example, wind flowing over a 10,000-ft range at 30 kts may, at minimum, affect a region about 90 nm downwind and to an altitude of about 20,000 ft MSL. Isolated higher peaks should be given a wider berth for the same reason.

Around a 14,000-ft peak, for example, the winds will be at least 20–40% stronger than those at 10,000 ft, and the higher elevation will result in a greater displacement and larger waves. Furthermore, with isolated peaks, the air can also flow around the sides with greater ease than around a cluster of peaks.

When winds aloft are strong, from any direction, isolated peaks should be given adequate clearance.

Mountain winds

Not all winds in the mountains form standing waves. In fact, a great many weather-related mountain aircraft accidents are due to smaller-scale wind effects, such as canyon flow, or turbulent shear around a sharp ridge or outcropping.

Most mountain airports are located in valleys. Valleys have both relatively flat terrain compared with the surrounding area, and also tend to have wind flow that moves up the valley during the day, and down the valley at night.

For the most part, a pilot can expect a nice headwind when landing on a mountain valley runway. Unfortunately, approaches and departures to these airports often take an aviator over a ridge or es­carp­ment that presents a sharp transition to the valley flow patterns.

During the day, solar heating of the rocks results in heated air rising up the sides of cliffs or ridges. A pilot may unexpectedly find him/herself ballooning above the glidepath or encountering sudden moderate turbulence while the aircraft is in a less maneuverable configuration.

Conversely, at night, cold dense air will flow like a waterfall over the edge of a cliff, rapidly gaining speed and generating chaotic eddies near the cliff base. Pilots passing over the edge of a cliff should be ready to counter a possible downdraft, while pilots flying low near the base of a cliff should expect possible moderate to strong turbulence. Unfortunately, upper air weather patterns can often set up a cross-valley flow.

Such flows can generate strong 90° crosswinds and moderate to strong turbulence as the air flows over the surrounding valley walls and wraps into the valley from the side. A runway up against one valley wall can complicate matters further because, as a crosswind hits the valley wall, it may spread out toward either end of the runway, meaning that either end of the runway will have a headwind, but a takeoff or landing roll headwind will ultimately turn into a tailwind.

During transition hours (dawn or dusk) or when the overlying weather patterns are set up in certain ways, a mountain airport may actually experience winds out of 2 directions. For example, at dawn, cold air may still be descending from higher elevations and blowing down the runway from upslope.

At the same time, the sunlit lower slope may be heating the air and generating an upslope wind that hits the downslope end of the runway. Arriving or departing aircraft might begin with a tailwind and transition to a headwind about the runway midpoint.

Mountain fog

Mountains are a great place to find fog. In fact, mountain fogs are very common—quite often they close mountain airports and ob­scure valley obstacles or the sides of mountains. There are 3 basic types of mountain fog an aviator should understand.

The first is radiation fog. This is the fog that forms in the valleys under clear nights with light winds. While radiation fog tends to dissipate by a few hours after sunrise, in the mountains the sheltered valleys may not see sufficient sunshine until closer to midday, meaning the fog will not only linger, but will likely thicken as the morning progresses, since the valley air is still cooling.

The second type of fog is advection fog, which forms as cold, dense air descends the slopes toward evening. As the cold air displaces warmer, perhaps more humid air below, the warm air is cooled and may condense its moisture in the form of a thin fog that can blanket the lower slopes of a mountain, creeping downward toward the valley bottom.

In general, this type of fog is thin enough that the terrain remains visible beneath it. The third fog is upslope fog. If warm, humid air flows toward rising terrain, it will cool as it is forced to ascend the slope.

Depending on the amount of cooling and how much moisture is present, the resulting condensation can generate a thick fog that may completely obscure the underlying terrain, appearing as little more than a low stratus cloud.

Upslope fogs are frequently the cause for the mountain obscuration that results in CFIT accidents. Not all mountain obscuration is due to fog. The presence of mountains often means that the terrain pokes well into the altitudes that are normally the domain of clouds.

Air forced up the mountain slopes can quickly cool and saturate, generating stratus decks that occur at altitudes beneath the higher mountain ridges and peaks. As a result, there are few visual cues to give pilots an idea of whether a particular cloud might be hiding a chunk of rock. The forced ascension of air holds another 2 worries for aviators.

If the air being forced to rise enters a colder environment aloft, it has the potential to rise convectively and generate an airmass thunderstorm. While most such storms are not severe, the continual influx of warm, humid air from beneath can sustain a mountain storm for several hours over a particular slope.

Rising, cooling humid air can also result in widespread areas of potential icing on the windward slopes of a mountain range. Unlike the convective lifting inside a thunderstorm, the mechanical lifting of air in these mountain environments is not temperature-dependent—meaning that cool, humid air can be forced to rise, quickly reaching saturation in subfreezing temperatures.

The result is regions of potential icing that may extend several miles upwind of the mountains and from a relatively low altitude to near the ridge crests. Weatherwise, flying in the mountains is not necessarily more dangerous than flying elsewhere.

However, mountain flying requires a realization that the terrain—which changes dramatically over small distances—will have a profound effect on that weather. Pilots who navigate the mountains successfully year after year are generally ones who make sure they are comfortable with the weather conditions on which they’ve been briefed.

They stick to established routes and stay above MEAs, MDAs and MSAs, giving the mountains a wide berth whenever weather conditions mandate it. They know that even in beautiful VFR conditions, the mountains are capable of throwing a curve ball at them if they drop their guard.

Karsten Shein is a climatologist with the National Climatic Data Center in Ashe­ville NC. He formerly served as an assistant professor at Shippens­­burg Uni­versity and was a scientist with NASA’s Global Change Master Directory. Shein holds a commercial license with instrument rating.

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