Regularly fly out of airports in the eastern half of the United States? If so, you'll eventually come to terms with the Appalachian mountain chain — and the weather it generates. While the Appalachians are dwarfs compared to the loftier Rockies, they still pack a meteorological punch. Aviation writers such as Ernest K. Gann and Robert N. Buck tell us of the perils of Appalachian crossings in the 1930s and 1940s, when commercial airliners routinely cruised below 10,000 feet — light general aviation cruising altitudes! — and sometimes encountered dreadful weather.
That was then, you might say. Today, we have more powerful ground-based and airborne weather radars, lightning detection equipment, and a more responsive weather information and dissemination network. True, but those ground-based radars aren't located in mountainous terrain. They're set up in flatter areas, where they can scan larger chunks of sky. Often they're far away from mountains and can have a hard time seeing below ridgelines. This means that the Appalachians can block small but growing storm cells from sight. Airborne weather radar? It's nice to have, but unless you have a large (greater than 18-inch diameter) antenna, radar energy is too weak to allow good storm detection at ranges beyond 40 miles or so. Also, any rain between you and a storm cell's angriest zones may prevent airborne weather radar from seeing — let alone contouring — dangerous signatures. Instead, the radar energy bounces back and gives the impression of a never-ending band of moderate rain. This phenomenon is called attenuation, and it can trick you into pressing on into a thunderstorm's worst neighborhoods. It's also a good reason to rely on lightning detection equipment when trying to avoid thunderstorms.
Low Regeneration
Like the Rockies, the Appalachians can also influence the intensity of any low-pressure systems passing over them. Many times, a low from the west weakens as it first encounters the mountains, then reforms and intensifies if it makes it all the way across. Now the low is stronger than it was before it made the crossing. Next stop: the North Carolina coast, where the low can receive an extra boost of vertical energy from the warm waters of the Gulf Stream. This is the mechanism that causes so many coastal snowstorms and northeasters to plague the mid-Atlantic and New England regions.
Bottom line: For many of us, flying around the Appalachians today is very much like it was for the airline pilots of 50 years ago. When weather moves in we're down low, in the clouds and turbulence, unable to reliably avoid icing and storm cells, and surrounded by hostile geography.
It's the geography that causes the worst Appalachian flying weather. First of all, there's the turbulence.
Any time you have wind blowing across a mountain range you can expect turbulence. This is easy to understand. Just think of water flowing over rocks in rapids. Air, like water, is a fluid, and when it flows past obstacles it's deflected upward, then downward in a wavelike pattern. The pilot recognizes this when the airspeed fluctuates and the vertical speed indicator leaps up and down — and his or her head hits the headliner.
Over the Appalachians the worst turbulence can come with frontal passages. That's when the added lifting action of a front can boost any existing vertical currents. After a front has passed — especially a fast-moving north-south cold front — expect another bout of turbulence as the newly introduced air mass barges its way east.
Mountain waves are another consideration, though for some reason many pilots seem to think mountain waves are solely a Rocky Mountain phenomenon. When winds aloft at ridge altitude are 20 knots or greater, and the wind direction is within 30 degrees perpendicular to the ridge orientation, then expect mountain wave activity. Practically speaking, this means alternating areas of lift and sink — especially when flying downwind of ridgelines. Sometimes these waves can be identified by lenticular clouds; sometimes they can't. I've seen satellite imagery that shows parallel lines of lenticular and other cloud streets that extend from the Appalachians all the way to the Atlantic Coast. Think of it: up to 300 miles' worth of miserable turbulence.
That's saying a lot. When you consider all the airports, airways, and population centers east of the Appalachians, plus all the fronts that move across this terrain, then you get a good idea of how often mountain-induced turbulence and wave action affect flights along the East Coast. Rotors — violent, rotating air masses that roil beneath the ridgeline — also stalk the lee slopes of the Appalachians. Like mountain waves, rotors may or may not be marked by distinctive clouds, and no, the Rockies don't have the corner on rotor probabilities, either.
Over the years I've come to realize that there are specific areas where you can practically count on wave action. One is along Victor 16, between the Holston Mountain (Tennessee) and Pulaski (Virginia) VORs. It seems that every time I fly that route huge sink and climb rates abound. Another big aerial pothole is along Victor 143, between the Lynchburg (Virginia) and Greensboro (North Carolina) VORs. I'll be flying along in smooth air and then, when I near the SLAMMER nondirectional beacon (NDB) the bottom falls out, the stall horn sounds off, I hit my head on the roof, and lose or gain a hundred feet or so. I'm guessing that SLAMMER is near a prison. Anyway, it's about 100 nm east of the Appalachians so this proves that mountain waves have far-reaching effects.
Sailplane pilots know these zones of wave action and ridge lift better than I do. They also know that it doesn't take much of a mountain to produce a lot of lift. Well-known competition sailplane pilots such as John Good, Doris Grove, and Karl Striedeck have soared the ridges of the Appalachians for years, traveling as far as 680 nm.
I've also noticed something about airports in the Appalachians. Namely, that there are a lot of them — for a mountainous region. And most are flanked by ridges or perched on man-made mesas. Many Appalachian airports are located along river banks — often the flattest land around — and this means they're susceptible to valley fogs.
Here's how it works. At night, cold air drains from the nearby ridges and sinks to the river valleys. If temperatures drop to the dew point, fog can fill the valleys. The moisture-rich air over the rivers helps make the fog denser. The fog will persist until the sun burns it off, but this may take a while. It may be hours after official sunup until the sun pops up over the ridges and takes the valley out of shade.
In the winter months, snow showers and icing conditions plague the northern Appalachians. The heaviest and fastest-growing ice accumulations tend to occur in the skies above the ridgelines, where rising air helps concentrate and enlarge supercooled droplets. When crossing the Appalachians, it's hard to follow the rules of icing avoidance. It's seldom warm enough to have ice-free conditions at the typical minimum safe or en route altitudes, there aren't enough suitable airports (i.e., surrounded by flat, obstruction-free terrain and having long, wide runways served by towers and ILS approaches) to serve as precautionary or emergency landing sites, and it's unlikely you'll top any ice-laden clouds.
And yet, some pilots yield to the temptation. Maybe it's because the Appalachians seem so narrow a range. "I can make it through — after all, I only have to go 150 miles," some internal dialogues might go. But a lot can happen in just a few minutes.
It's the flying memories that help to flesh out your weather awareness, and the Appalachians are certainly no exception.
My Appalachian memories include a landing at Burlington, Vermont, after a cold front's passage, and realizing that you don't need the Great Lakes to have lake-effect snow; Lake Champlain does a great job of snow-making, too.
A visit to New Hampshire's Mount Washington (elevation 6,162 feet msl) started out on a clear day. I landed at the Berlin Municipal Airport, New Hampshire, and friends gave me a ride to the Appalachian Mountain Club's Pinkham Notch camp to check in and drop my gear. Two hours after I got to the summit, I was walking around in a cloud, and two cars collided as they tried to make their way downhill. The wind went from calm to 60 mph in 30 minutes. This — and a record 231-mph gust experienced at Mount Washington on April 12, 1934 — teaches that venturi effects near low-level mountain passes can reach jet-stream proportions.
An iced-up instrument approach to ILS minimums in Williamsport, Pennsylvania, taught me that even the thinnest bands of icing can banish all thoughts of a climb. An icy westbound crossing taught me the value of having above-freezing air at the minimum en route altitude.
A fast-building and -moving north-south line of thunderstorms over the Berkshires once chased me all the way to central Connecticut. That proved that even small hills can help lift scattered light rain showers into a cluster of severe thunderstorms. A 1981 study by meteorologist T. Theodore Fujita taught me that central and western Pennsylvania could have tornado outbreaks, just like Kansas and Oklahoma.
Above the ridgelines around the Mcminn County Airport near Athens, Tennessee, I had a smooth ride. But the turbulence in the airport traffic pattern was at the high end of the "moderate" range, convincing me that perhaps those small, roundish cumulus clouds over the valley were rotor clouds.
Sure, the Appalachians can have beautiful weather. But they can teach us all a thing or two about mountain effects. Fly around them long enough and you'll have stories of your own to tell. Been there, done that? Then you know what I mean.
E-mail the author at [email protected].
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