March 25, 2013
BY THOMAS A. HORNE
Why do so many weather-related accidents happen near hilly or mountainous regions? Three reasons come to mind. One is that non-turbine-powered, or non-turbocharged, aircraft lose a lot of power at medium altitudes (10,000 to 20,000 feet M.S.L.): Without the ability to climb away from terrain and turbulence, a pilot's maneuvering options can be severely limited. Another is that pilots can be trapped below ridge lines-in valleys, box canyons, and steep, hostile terrain in general. This is especially true for pilots unfamiliar with the local geography or those electing to fly at night or in adverse weather. Which brings us to the third reason: Bad weather and high terrain go together. Take mountain weather, add an airplane with marginal reserve power, then throw in a pilot who makes a bad judgment call and you've got an accident waiting to happen. It can be a struggle for a reciprocating-engine aircraft, particularly a non-turbocharged model, to reach a minimum safe altitude that will top high terrain. Note that I said minimum safe altitude. Instrument-rated pilots familiar with the area know that the lofty minimum enroute altitudes (MEAs) that cover the western United States are no guarantee of protection against mountain effects. Along many of the airways crossing the Rocky Mountains, MEAs routinely run between 10,000 and 16,000 feet M.S.L.. In mountainous areas like these, MEAs buy pilots on instrument flight plans at least 2,000 feet of vertical separation from the rocks directly below an airway and 4 nautical miles either side. VFR pilots aren't required to fly under such strict guidelines, but it's still a very good idea to make sure you have as much altitude as possible when crossing mountains or other high terrain.
There are some very good reasons why that 2,000-foot cushion may not be enough to prevent you from running into some very nasty weather. The behavior of air flowing over mountains and other high terrain has been compared to the behavior of water in a fast-moving river. It's a good analogy, because water and air are both fluids. When air runs into a ridge or peak, it first rises, then plunges down-just the way water behaves in rapids. This wave-like rising and falling creates updrafts and downdrafts. The problem with this analogy is that air, being less dense, creates rising and falling waves that are much greater in amplitude than those created in water. Anyone who's done any whitewater canoeing, rafting, or kayaking knows that rapids seldom rise more than 3 feet or so above a rock or other obstruction in a stream (unless you're in some serious rapids!).
With air, disturbances can extend to altitudes several times the height of the mountains. A 10,000-foot cliff can cause updrafts and downdrafts at 30,000 feet. And along with those updrafts and downdrafts comes the potential for moderate to severe turbulence, depending on wind speed. The stronger the wind, the rougher the ride. In the Rockies, a good rule of thumb is to stay on the ground if the winds at mountain-top altitude are more than 35 knots. To be on the safe side, lower that limit to 25 knots.
From my office at AOPA headquarters in Frederick, Maryland, I can see a small, 2,300-foot-high ridge of the Catoctin Mountains. It looks tame enough, but a seaplane pilot once rode the updrafts created by that little ridge to an altitude of 20,000 feet. That's quite a testimony to the ratio of updraft effects to terrain height.
Wind effects must also be considered in light of the direction of flight. Most often, the winds aloft in the United States run from west to east. A pilot crossing with the wind will have a smooth ride as the first updrafts are encountered, then hit downdrafts and rough air once on the lee side of the mountains. The onset of bone-jarring, spar-taxing turbulence can be subtle. The first hint of trouble is usually a really smooth ride, accompanied by an uncommanded climb. Sailplane pilots seek out this sort of thing, because they can use the lift in these mountain waves to reach fantastic heights. Some have ridden them as high as 30,000 feet.
If flying from east to west, expect a world of turbulence from the moment you encounter a mountainï¿½s downwind effects. When can the turbulence begin? Satellite imagery, has identified mountain wave activity - marked by altocumulus standing lenticular clouds-as far as 700 nm downwind of the Rockies. The Appalachians' worst mountain waves can extend as far as 300 nm downwind. Lenticular and other high-altitude wave clouds are formed by the condensation of moisture as the waves of disturbed air rise to the tops of their up-and-down cycles.
In lee waves, downdrafts routinely hit the 1,500- to 2,000-fpm rate. Often, they go beyond that. Again, the stronger the winds aloft, the more intense the downdrafts. See why the regulatory minimum of 2,000 feet above a ridge line may not be a good idea?
Rotors are another problem. These violent winds are just what the name implies-horizontal vortices of fast-spinning wind. Some may be marked by distinctive rotor clouds. But in the dry air that so often frequents the scene at high altitude, rotors can be invisible. Fly into one and you'll wish you hadn't. They can be like a horizontal tornado and cause loss of control or structural failure.
As for flight operations on and near airports located in mountainous regions, problems most often center on poor pilot judgment. High density altitude is a big, big weather complication. Common sense dictates that you calculate your airplane's weight and balance carefully prior to any high-altitude takeoff and determine whether a safe climb rate or gradient can be expected. However, accident statistics show that many of us don't do that.
One way to outwit high density altitude may be to plan your takeoffs for early morning or late in the day, when temperatures are lower and climb performance should be better. Another way is to lighten your load. Take your passengers and/or cargo out in stages rather than trying to make a gross-weight takeoff in an airplane with a sick climb rate.
Along with good pilot judgment, a sprightly climb rate is probably the most important factor in safe mountain flying. Turbocharging, or turbine power, can make a big difference when making a high-density-altitude takeoff or trying to outclimb a vicious downdraft.
Even so, it's important to make sure that you have enough maneuvering room once you've departed a high-elevation airport. Most of them are situated in the only flat ground adjacent to river valleys, in canyons, or at the base of high terrain. Another form of the same churning air that can plague the pilot cruising high above can also bring grief to those trying to climb out of a valley or canyon. Turbulence often strikes on the way to cruise altitude. In this phase of flight, the risks can be greater. You'll be flying slower, at Vx or Vy, closer to stall speed, you'll be closer to steep ground, your climb rate may be severely compromised, turbulence may worsen it, and there may be thousands of feet to go before any ridges are topped.
Differential heating of valley walls can compound these kinds of difficulties. Walls exposed to the sun can provide beneficial updrafts. Those in the shade produce sinking air. When you're struggling for altitude and being knocked silly by turbulence, flying closer to the sunny side of a ridge may help save the day, just make sure you can safely turn around in the valley, if things don't work out. Even better, pick an airport in a valley that widens toward lower terrain. That way, you can always turn around and fly down the valley with room to spare, then turn around and attempt a climb without the danger of trying to vault nearby cliffs.
Valley winds are another consideration. By midmorning, rock faces exposed to the sun create rising air and winds that flow up-valley. When the sun goes down, descending air is the rule. It's something to bear in mind, especially when making those late-day takeoffs we talked about earlier. Yes, the density altitude may be more friendly, but can you outclimb any down-valley winds?
By the way, upturned leaves on surrounding foliage can be an excellent clue to the presence and intensity of valley winds. Valley winds also help explain another major mountain weather feature: clouds. Mountains and clouds go together, and valley winds are one reason why. The midmorning winds flowing up valleys merge at their summits, where temperatures are cooler. Cool enough to cause clouds to form at or near mountaintop level. In fact, that's where you'll often find the first clouds of the day when flying near high ground. Forecasters and briefers call it mountain obscuration. You should think of it as a warning to either file IFR or steer clear.
It's also very, very common for nighttime's colder temperatures to create low-lying stratus and fog in mountain valleys. Until the warmth of the day burns these clouds off (assuming that a higher level of clouds doesn't prevent adequate sunlight from doing the job), it'll be low IFR for any valley airports.
The presence of lows or fronts makes mountain cloudiness even more of a certainty. Instead of a few clouds obscuring the highest terrain, entire ridges and chains can be socked into instrument meteorological conditions. Any moist air flowing up mountain sides is almost sure to produce widespread low ceilings and visibilities.
Mountains also have a way of intensifying any nearby bad weather and creating low pressure systems.
Here's the usual setup. A weak low approaches one of America's north-south mountain chains. riding on westerly winds. As the low approaches the high ground, the low's circulation weakens. But once the system crosses the highest terrain, the low re-forms, then moves east with re-invigorated ugliness.
Why? Atmospheric physicists would call this a good example of the conservation of angular momentum. You and I can call this a meteorological version of the dynamics of a spinning ice skater.
We've all seen how skaters can increase their rotational speed by moving their arms and legs closer to their bodies. The weak low moving into a mountain range has its energy sapped because its vertical limits are compressed. Squished between mountaintops and tropopause, the low is like a skater that's spinning on bended knee, with arms outstretched. It slows down and the air's counterclockwise circulation is upset for the time being.
But after it's moved east, to the lee side of the mountains, the low's column of air stretches vertically. Instead of 20,000 feet or more between the base and top of the low, that distance grows by at least a third. Now the low-spinning skater is extended to its full height, its "arms" are closer to the center of rotation, and the low re-intensifies. Pressure gradients steepen, wind speeds pick up, and adjacent air masses are drawn into the low's circulation.
In the west, this means that warm, moist air from the Gulf of Mexico is sucked into the low. In the east, air from the Atlantic is entrained in the low's circulation. In both cases, the result is often a full-blown frontal complex with a cold front aligned northeast-southwest and a warm front spread out along a northwest-southeast line.
In the warmer months, this spells thunderstorms. In the winter, snow and ice will reign. In recent years, the East Coast's miserable spate of ice storms has largely been the work of regenerated lows moving east of the Appalachians, then encountering the warm air over the Gulf Stream. Somewhere near Cape Hatteras, North Carolina, a steep temperature gradient sets in, and a wet, icy mess then typically heads northeast.
With winter coming on, remember that mountains and adverse weather are fellow travelers. Lows come and go with higher frequency in the winter months, thanks to stronger jet-stream flows. Most of them will pass near mountains. The worst icing is always found near mountainous regions, and the odds of meeting up with severe turbulence are greater because of faster winds aloft. If you hear of any lows approaching any mountains near your route of flight, start thinking about alternative plans of action. The best one may be to delay the trip. The weather's going to get worse, perhaps much worse than forecast.
Wind and Gusts,
Safety and Education,
VFR into IMC
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