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Mountain weather

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Complex and hyperlocal conditions create challenges for mountain flying.


By Karsten Shein
Comm-Inst Climate Scientist

Pilatus PC-24 cruises across the Swiss Alps. Mountain flying presents many weather dangers for unprepared pilots.
The copilot had checked the numbers twice. They had taken on less fuel than they wanted due to the full passenger load and the high density altitude at the small mountain airport.

They had to stop to gas up about halfway home, but it was the only way to get the boss and his family home from their vacation in time for work.

Both pilots smiled as the jet left the runway with a few hundred feet to spare, but their mood soured quickly when they realized they were not getting the climb rate they had hoped for. With the throttles full forward, the pilot tried to coax every extra foot from the aircraft as the high ridgeline filled their windscreen.

The airport was in a narrow valley, so there was no way to turn around. With seemingly inches to spare and the stick shaker active, the jet roared over the ridge crest and both pilots let out a huge sigh of relief.

Historically, flying in mountain regions and in or out of montane airports has added an element of danger to aviation operations. The rugged and complex terrain modifies airflow in unexpected ways, enhances turbulence, and reduces visibility.

Airports in these regions are often at altitudes that make density altitude a significant factor, and limited flat ground can mean narrow or short runways that are sometimes surrounded by rising or falling terrain, making handling adverse weather even more of a challenge.

Von Karman vortex street downwind from a volcanic island. Even isolated mountains can disrupt atmospheric flow, creating turbulence and potential hazards to flight.

Standing waves

One of the things pilots are most commonly taught about mountain weather flying is to identify and avoid getting caught in the standing atmospheric waves that mountain ranges often generate.

Variously called standing, lee, or mountain waves, these wave chains can affect flight above and downwind of the mountains. While they pose the greatest dangers near the mountains, they can sometimes propagate hundreds of miles downwind.

Standing waves are formed when the prevailing tropospheric wind flows across a high ridge or the crest of a mountain range. Even an individual peak can produce a lee wave occasionally.

As the air meets the mountain obstacle, it must rise over it, compressing as it does so. Downwind of the mountains, the air descends and sets up into a series of waves.

Not all airflow across mountains produces lee waves. In general, the air in the troposphere should be relatively stable. Instability aloft will cause the orographically lifted air to continue rising, often creating clouds, rain, and even thunderstorms on the upwind slopes. Most mountain waves will form from moderate, steady wind beneath a strong stable layer just above the mountains.

There are 2 main kinds of lee wave. Vertically propagating waves occur with stronger wind and weaker stable layers, with the wind flow pushing vertically to great heights – sometimes into the lower stratosphere.

Lee waves form downwind of mountains as air is forced to flow over and around them in a stable environment. Severe to extreme turbulence is frequently present beneath the wave crests, especially close to the mountains.

Vertically propagating waves often produce the most violent downslope winds on the lee side of the mountains and can create severe to extreme clear air turbulence above and immediately downwind of the mountains.

Trapped lee waves are so called because the moderate air flowing over the mountains is ‘trapped’ between a strong stable surface layer and a weaker stable layer with strong winds above the mountains.

Both vertically propagating waves and trapped lee waves will have horizontal wavelengths of perhaps 10–40 km (5–25 mi) or more. Trapped waves, however, will extend much further downwind.

The greatest dangers for pilots from lee waves are found in the first wave, from its crest over or just downwind of the mountain to the crest of the following wave.

Beneath the wave crests and in the wave trough, pilots may encounter a strong horizontal vortex, or rotor that has enough power to damage an aircraft or cause loss of control. Depending on moisture contained in the rising air, wave crests may be made visible by lens-shaped altocumulus (standing lenticular) clouds.

The violently rotating air beneath the crest may also be betrayed by a tube-shaped ‘rotor’ cloud. Clouds that form around the wave crests often show up in satellite images as cloud streets on the lee side of a mountain range – a clear indicator of an active and dangerous mountain wave.

A glance at winds aloft at a level around the top of the mountain range will often help identify possible mountain wave formation. Look for moderate to fresh winds flowing across the mountains within about +/- 45° of perpendicular to the axis of the range. Look also at the forecast for the mountains and vicinity.

Good weather often means stable air and an increased likelihood of a wave. If forecasts or winds aloft aren’t available, check the surface pressure on either side of the range. If pressures differ by more than 5 mb (5 hPa or 0.15 in Hg), a lee wave is possible.

The dangerous nature of a lee wave means pilots should never approach the lee side of a mountain range at low altitude relative to the terrain if it can be avoided. Instead, pilots have long been advised to approach the mountains at a 45° angle before crossing perpendicularly while maintaining at least 5000 ft above the highest terrain. Naturally, all occupants should be wearing their harnesses and aircraft should not exceed turbulent air penetration speed. If you do encounter a lee wave or associated turbulence, let ATC know.

Standing lenticular clouds over Torres del Paine, Chile. Moisture in air rising into a standing wave crest may condense into lens-shaped clouds that appear stationary. Severe to extreme turbulence often exists beneath such clouds.

Ridge and valley flow

While standing mountain waves predominate when we think of mountain flying, a more dangerous situation by far is the local-scale circulation between and among mountain ridges and valleys – the terrain we must traverse to get in or out of many mountain airfields.

Each ridge or jagged crag in a mountainside represents an obstacle that will modify the direction and speed of the airflow, and which will produce turbulence.

In addition, as valleys and cliff faces are exposed to sun or shadow, they heat or cool the adjacent air, creating micro-scale thermal flow that can come as a surprise to a landing or departing pilot who may not be aware of the unique weather around the airport.

In general, valleys act as conduits for warmed and cooled air. Cold air becomes dense and sinks. As the sun sets behind a valley wall – often many hours before sunset – the local valley air cools, and the air flows down the valley. Conversely, as the sun reaches the valley floor, the warming air will begin to flow up valley.

For many valley airports, especially those that offer only one direction for takeoff and landing, this means that the runway will experience a tailwind for part of the day, and at a certain time of day the wind will abruptly shift 180°. The nighttime cooling in the valley may also generate frequent fog that a shaded valley may not shed until well after sunrise.

In some other valleys, especially those oriented east-west, the sun will heat the valley wall much more than the floor, and the rising air along the wall will create a sideways airflow as the air moving up valley during the day diverts to replace the air rising along the valley edge. At night, air will flow down the valley sides toward the valley center. Airports located to the side of such a valley may often experience such crosswinds.

Critically, strong flow across a narrow valley or one with large or abrupt escarpments on one or both sides can create turbulence as the air breaks over the edge of the valley. At times, such turbulence can be enough to upset an aircraft on approach.

Also, many mountain runways, such as that at TEX (Telluride CO), terminate in a drop-off to a steep slope. Diurnal heating and cooling can create strong up and down drafts at the approach end of the runway that may cause a loss of control for unprepared pilots.

Before flying into or out of a mountain valley airport, it is worth investigating if and when the wind tends to shift. A call to the airport operator or FBO can provide that information. A look at the last 24 hours of metar reports may also reveal those shifts. Also, look at the winds aloft over the valley. If they are perpendicular to the valley, expect low-level shear turbulence of some sort.

LUA (Lukla, Nepal) at 9383 ft MSL has a short, significantly sloped runway ringed by higher peaks and a steep drop-off. Night operations and flights in marginal weather are simply prohibited due to the danger.

Obscuration

Two more challenges of flying in the mountains are a general lack of population providing reference lighting at night or in marginal weather, and the fact that, as air is forced to flow up and over the mountains, it often cools to the point where it produces fog, cloud, and rain.

These challenges mean that controlled flight into terrain (CFIT) is a very real danger. The first of these stems from a reduced frame of reference from artificial lighting, which can help identify approaching terrain at night or in low visibility.

A pilot who loses situational awareness in the mountains may neglect to maintain a minimum safe altitude, or may stray from a flightpath just enough to impact a ridge hidden in the dark or gloom.

The dangers are made greater by terrain obscuration in fog, cloud, or rain. At a broader scale, pilots should expect that humid airflow toward rising terrain may result in clouds that hide the mountains. But such obscuration can occur locally as well. Air is often funneled preferentially by mountain valleys and passes, and that air may locally blanket the upper region of a dead-end valley in cloud, hiding the cliff and ridge, and leaving an errant pilot with no options for retreat.

When planning a mountain flight, look at the winds and the difference between the temperature and dew point at upwind airports. The smaller the temperature difference, the less the air will need to rise before it produces clouds. Unsaturated air cools at a rate of 5.5° F (3° C)/1000 ft 
(9.8° C/km), so, for example, air flowing toward the mountains with a temperature of 20° C and a dew point of 15° C will only need to rise around 1600 ft before obscuring the mountain slopes.

Pilots should also pay very close attention to their position relative to terrain using GPS or other flight management avionics where possible, in order to ensure that they remain clear of terrain and maintain minimum safe altitudes at all times, whether during cruise or on approach or departure. Enhanced flight vision systems may also provide an advantage, but these can sometimes make it difficult to judge distance from the terrain, even if it is visible.

Density altitudes

In addition to frequently being nestled in narrow valleys or on the edges of steep cliffs, many mountain airstrips, including a few major airports, such as LPB (El Alto, La Paz, Bolivia, at 13,325 ft (4061 m) MSL, are situated at high altitudes.

This means that density altitude limitations must be considered in aircraft loading and performance. Many of these airports, especially during the summer months (or year-round for those located in the tropics), will experience days where above-normal temperatures (above ISA for that elevation) exacerbate density altitudes beyond the capabilities of many aircraft.

In addition to being aware of all other challenges of flying in the mountains, pilots arriving or departing these high-altitude airports must plan their flights carefully for the density altitude of the airport. Pilots should be especially aware of climb rates needed to clear the rising terrain that may surround the airport.

More than a few aviators have calculated correctly weight and balance for a flat field departure at a given density altitude only to neglect the performance needed to clear a saddle-back ridge a mile from the departure end of the runway.

To overcome density altitude problems, many cargo and contract carriers plan their operations to and from such airports at night when the normally dry air at these altitudes may reduce density altitude to levels at which they can operate without too great a weight restriction. Of course, night-time operations in the mountains will bring with them the potential for loss of situational awareness in the rugged terrain.

For operations at other times, pilots should monitor the air temperature at the airport closely. A look at a regional weather map will show if there are dominant pressure patterns contributing to warmer air.

If flight planning well in advance, forecast high and low temperatures (normally mid-afternoon and early 
morning, respectively) can help to plan loading and performance, and help determine if a flight should arrive or depart at night.

Because weather forecasting is not yet an exact science, a good idea is to give yourself a margin of safety by adding a few degrees to any forecast temperatures you use in your density altitude calculations.
As always, if you encounter mountain weather your fellow pilots should know about, be sure to file a pirep.


Karsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a Comm-Inst pilot license.