Wx Watch: Winter Rules to Live By

Web site of the month

This month’s Web site is one of my personal favorites. It’s the Aviation Digital Data Service (ADDS) Web site, run by the National Weather Service’s Aviation Weather Center. Because it’s the icing season, be sure to look at the site’s “Icing” tab. Then click on the “Supplementary Icing Information (CIP/FIP)” icon. From the drop-down menu, pick the time and date for icing advisories. From the current day’s icing (CIP) you can select icing probabilities and severities for any number of altitudes on the second drop-down menu to the right. Large-droplet icing predictions are plotted with red dots. When using the CIP and FIP, remember that their icing information does not serve as a substitute for airmets or sigmets—which are also on the ADDS icing page—and that the CIP and FIP are only “authorized for use by meteorologists and dispatchers,” according to the Aviation Weather Center’s disclaimer. Wink, wink.

What if you were at some desolate airport, and had no access to weather information? Could you concoct at least a rudimentary idea of what the local weather held in store? There are ways of determining the weather in the next eight to 12 hours—under certain circumstances. And no, I’m not talking about using the “weather rock” you saw in that tourist trap at the side of the road, among the ridiculously huge cigars, the commemorative spoons, and the paperweights made of scorpions encased in plastic. You know, the “if rock is wet, it’s raining” sort of weather forecasting tool.

The method I’m talking about was first developed in the nineteenth century by Christoph Hendrik Diederik Buys Ballot (1817-1890), a Dutch meteorologist. Buys Ballot’s Law, as it’s come to be known, involves standing with your back to the wind and extending your left arm. You are now pointing at the nearest center of low pressure.

This bit of knowledge comes in handy when you understand the cyclone model, which was the topic of last month’s “Wx Watch” (“ Fronts on the Move,” October AOPA Pilot). The cyclone model explains how the counterclockwise motion of air around a low pressure center creates frontal weather, and divides air masses into cold and warm sectors. So it makes perfect sense that low pressure will always be to your left when your back is to the wind. (Of course, the reverse is true in the southern hemisphere.)

Combined with a knowledge of the typical cloud formations associated with frontal weather, the surface temperature, the barometric pressure, and the strength of the wind, you can come up with a pretty good idea of where your locations fits into the synoptic situation—or “big picture.”

For example, let’s say that the wind’s at your back, you’re pointing to the left, the barometric pressure is dropping (you can tell this by adjusting your altimeter to the field elevation every hour or so, and then watching the pressure tendency in the Kollsman window), the temperatures are unseasonably warm, the wind is picking up speed, and there are cumulus clouds to the west. You can be reasonably confident that you’re in the warm sector of a frontal complex, and a cold front will be arriving sooner rather than later—perhaps bearing convective weather. Similarly, if temperatures are relatively cool, skies are clearing, and barometric pressures are rising, then you’re behind the cold front. Meanwhile, you’re still pointing at the parent low pressure’s general location. Cold temperatures, low clouds, snow or rain, and a strong wind out of the east? Then you’re north of the parent low, and in the coldest sector of the cyclone model.

Wait a minute, I hear you say, what if the wind is calm and skies are clear? Then you’re probably near the center of a high pressure system. But be on the lookout for rising winds out of the south, and warming temperatures. When that happens, you’ll know you’re on the “back side” of a high—and another cold front will no doubt be on the way.

You never know how knowledge like this may come in handy. It may seem unremarkable to us now, but Buys Ballot’s Law was big meteorological news in the mid-nineteenth century. Especially since it unknowingly laid the foundation for modern concepts of frontal movements that wouldn’t come about until a half-century later.

“Upslope” and other geography-related icing

I recently gave a talk about supercooled large-droplet icing at the TBM Owners and Pilots Association’s annual convention at Traverse City, Wisconsin. For one of my PowerPoint slides I used a map of the United States, showing where the worst of this large-droplet icing typically occurs. There are three main areas: the New England states, the states surrounding the Great Lakes, and the Pacific Northwest.

It should be obvious why these areas are home to large cloud droplets. In the Northeast, flows of moist maritime air help build a cloud mass’s liquid water content (the amount of water contained in a cubic meter). As for the Great Lakes region, those bodies of water saturate the air above with plentiful moisture. Ditto the Pacific Northwest, with constant onshore flows of cold, supersaturated air from the North Pacific ocean. Pilots planning winter flights in or near these three potential danger zones should be forewarned: The icing you encounter could well be the world’s worst. Large-droplet icing means abnormally high accretions of runback ice—ice that forms well aft of leading edges, and well aft of the areas protected by de-ice boots. What’s more, those large droplets can create some striking, double-horn ice formations immediately behind the boots. These formations cause lift-robbing separations of airflows over the wings. These airflow separations, called separation bubbles by icing experts, then move fore and aft on the wing—and tail!—surfaces. This causes abnormal control feel, which is a sign of an impending stall. Even worse is that the airflow can reattach aft of the bubble, causing an aileron to be yanked upward, or an elevator to be forced downward. What comes next is an upset.

Don’t believe me? Then you should check out the October 31, 1994, crash of an American Eagle ATR-72 commuter airliner at Roselawn, Indiana. That was the accident that put large-droplet icing on the map. For starters, go to Wikipedia’s online summary of this accident. Perhaps the most regrettable fallout from this accident is that the icing conditions encountered by that ATR-72 are not represented in the icing envelopes (prescribed combinations of temperature, liquid water content, droplet size, and flight duration) used for icing certification. In other words, an airplane can be certified for flight into known icing conditions without its ice protection equipment’s ever having the benefit of being tested in large-droplet icing conditions. As far as I know, the icing certification envelopes are the same today as when they were developed, in 1949.

Mountains make all icing worse, and large-droplet icing is no different. That’s why pilots flying in the Pacific Northwest, Appalachians, and New England had better be wary. Moist, westerly prevailing winds ram into west-facing mountain slopes in these areas. This boosts the upward acceleration of the air mass, which in turn boosts condensation of cloud droplets into the large-droplet state at altitude—along and above the highest elevations. This upslope icing recently became an issue in the establishment of a T-route (T-276) over the high terrain northeast of Portland, Oregon. Intended as a shortcut for bypassing Portland’s busy terminal area, T-276 is smack-dab in a prime area for upslope large-droplet icing. Pilots planning on using T-276 should take this into account.

Back in 2000 I delivered that year’s AOPA sweepstakes airplane—a souped-up Mooney we called the “Millenium Mooney”—to the winner. I flew to Seattle’s Boeing Field to stage the airplane for the award ceremony. I had my choice of flying direct from northern California, or flying along the shore, up Victor 27. Going direct meant high minimum en route altitudes (MEAs) and flight in icing conditions over terrain that a local pilot described as “an icebox—stay outta there.”

So I flew up Victor 27 at 6,000 feet, in and out of clouds that were free of ice. Free, that is, until I had to make that right turn toward Seattle. This meant following V27 from the Hoquiam VOR, step-descending as I flew through lowering terrain. The MEA between Hoquiam and Seattle is 3,000 feet, and I was in the soup the whole time. And, unfortunately, icing up. But it wasn’t the large-droplet kind, and it didn’t last long. By the time I was vectored for the final approach fix at Boeing, I was ice-free at 2,000 feet.

So a word to the wise this winter: Know your icing geography, and be extra wary upwind of soggy high terrain. Remember, you want high cloud bases and low cloud tops, no matter where you fly when icing is advertised.

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