December 1, 2001
By Thomas A. Horne
Hello, winter! Those of us who live in northern climes know what that means. Numb fingers during preflights. Removing snow and ice from parked airplanes. Preheating rituals. Changing engine oil to thinner grades. To the weatherwise, winter also means a whole new set of concerns — with icing at the top of the list. But it's also worth remembering that winter brings more frontal passages and, often, more complex frontal behavior than occur at warmer times of the year. This means that pilots ought to pay more attention to the character and intensity of any fronts that may affect their flight plans. Unfortunately, this information is frequently tucked away in a synoptic discussion — for example, as part of low-level significant prognosis ("prog") charts — where it is easily overlooked or ignored.
All fronts are not created equal. We all remember our early days of ground school, when our instructors and our textbooks depicted fronts as rigid, stereotypical models. Cold fronts always shove thunderstorms ahead of them. Warm fronts always cause widespread stratus clouds, continuous precipitation, and low weather. And so on. Are these and other rules of thumb always correct? Very often, but certainly not always. It's during the winter that we have a great opportunity to see and predict the exceptions to the rules.
As for winter cold fronts, thunderstorms are definitely not the rule. Why? Surface heating and convection are at a minimum in the winter months when days are shorter and the sun rides lower in the sky. The warm sector of a frontal complex (the area behind a warm front and ahead of a cold front) isn't warm at all, so the temperature contrasts along a cold frontal boundary are of a lesser magnitude. Sure, a phenomenon known as thundersnow can occur when convective conditions are just right in a major winter storm, but in the overwhelming number of winter cold fronts thunderstorms simply don't occur. My only encounter with thundersnow happened on March 8, 1984, when a fast-moving cold front loaded with wet snow arrived at Dulles International Airport at the same time I did. I shot the ILS approach to Runway 1R with a ceiling of 300 feet overcast and one-half-mile visibility in blowing snow. It snowed so heavily that by the time I taxied clear of the runway, braking action was nil and the tower couldn't see me. Within a few minutes the airport was closed. As I attempted to tie the airplane down (a general aviation tiedown area known as "southeast parking" was located in front of the tower, in what is now a huge commuter ramp) bolts of lightning hit the airport.
Instead of thunderstorms, pilots need to watch out for turbulence, strong winds, and clear icing conditions when flying in or near winter cold fronts. The clear icing is from the cold front's cumulus clouds, which typically contain large-diameter supercooled water droplets. When these hit an aircraft's leading edges the droplets splatter and run back before freezing. This runback is dangerous because it can cover the whole wing rapidly — compromising lift even on airplanes equipped with ice-protection systems.
The winter cold front's strong winds and turbulence are direct offshoots of the frontal speed. In the winter, jet stream patterns at the 25,000-foot-plus altitudes drop southward, well into U.S. latitudes. The fastest winds in the jet stream cores help intensify and boost the groundspeeds of the low-pressure centers below them — along with the fronts that emanate from these lows.
The winter warm front is where a grab bag of miserable precipitation can originate. As warmer air rides up and over colder air ahead of it, snow occurs if temperatures below the frontal slope remain below freezing. Rain can happen if temperatures below the frontal slope are above freezing, causing snow or ice crystals to melt. Closer to the front's surface location, where temperatures are within just a few degrees of freezing, expect freezing rain. This is really an extremely hazardous form of clear icing that occurs when supercooled raindrops strike a surface — such as your airplane — that's at or below the freezing point.
Fly in the stratus clouds associated with the typical winter warm front and you'll likely encounter rime icing conditions. Rime ice is caused by comparatively small-diameter water droplets. It forms as a whitish ridge along the wing and other leading edges, and can make accretions that project several inches — assuming you dally in icing conditions.
What about occluded fronts and stationary fronts? These can contain a mixture of icing conditions and are favored locations for IFR ceilings and visibilities. An occluded front — which is really two fronts, one at the surface and one or more aloft — usually lives north of low-pressure centers. Like stationary fronts, occlusions move slowly and can cause low ceilings and visibilities for days.
Now to the exceptions. First, the one about larger-droplet icing in cold fronts. As the October 1994 crash of an ATR 72 (see " Airframe and Powerplant: ADs for the Owner," page 99) at Roselawn, Indiana, has shown, cold fronts don't have the corner on large-droplet icing. The ATR crashed in clouds that apparently contained some of the largest water droplets ever recorded. Meteorologists have speculated that cloud droplet diameters could have reached the 400-micron range. (The graphs and rules used for certifying ice-protection systems specify a droplet range between 15 and 50 microns!) So large-droplet icing can lurk in any front's clouds.
What about the textbook wisdom that rigidly links freezing rain with warm fronts? It's a good rule, but not always. Freezing rain can happen whenever warm rain falls into colder temperatures — and that includes occlusions and stationary fronts, where warm frontal slopes can also extend over colder air masses.
Here's something else: The worst winter weather along the East Coast is frequently caused by intense coastal storms that don't even rate much more than a few sentences — at best — in any private pilot textbook. These coastal lows usually form in the Gulf of Mexico or along the lee slopes of the Appalachian Mountains. From there they track to the northeast over three days or so, their central pressures deepening as they go. On winds aloft charts, you'll notice that there's a deep trough aloft with very strong (in excess of 100 knots in some cases) southerly winds aloft helping to support and sustain the'.surface low. When the moist southerly air rams into colder temperatures to the north of the low's warm front, record-setting snows can follow.
Finally, there are northeasters. These are offshore lows that can produce blizzard-like conditions and very high winds. You read or saw Sebastian Junger's The Perfect Storm? That was a northeaster, or "nor'easter" in en.renched New England parlance. Named after the northeasterly winds spun off from the low's counterclockwise circulation, these storms can close coastal airports for days — first for the bad weather, then for the damage cleanup.
Like all weather forecasting prod#cts, your preflight weather briefing will be heavily influenced by conventional wisdom. Just remember that there are exceptions, and that the only guaranteed method of avoiding the worst icing is to avoid all clouds and precipitation. As for strong winds at the surface and aloft, moderate or severe turbulence, or coping with northeasters, these are issues for the go/no-go decision. Here's where one bit of conventional wisdom holds up: If you're uneasy, stay on the ground.
E-mail the author at email@example.com.
Sure, you'll get a thorough preflight briefing. Sure, you'll seek weather updates en route from flight watch (122.0 MHz) and AWOS, ASOS, or ATIS frequencies. But here are a few items to have in mind before you launch on a winter flight.
Lower cloud tops in the winter make on-top flying a more viable option. The climb to on-top conditions won't take as long as in the warmer months, and you'll be in smooth, clear, ice-free conditions. To avoid icing conditions and comply with VFR guidelines, make your climbs and descents in cloud-free areas. If you're IFR-rated and your airplane is certified for flight in known icing conditions, climb at a faster-than-normal airspeed to minimize ice accretions on the underside of the wings and other surfaces.
Did it recently snow, and did a subsequent surface warmup melt some of that snow? If so, watch out for dense, long-lasting fog should a cold front pass through or nighttime temperatures drop to the dew point. The huge amount of moisture trapped in the soon-to-be-cooled soil will act as a great big humidifier. Once colder temperatures set in, expect widespread fog — especially in low-lying terrain.
Watch out for the "northeast corner" of a low. The lowest ceiling and visibilities and the worst icing conditions live here. The northeast corner is bad news because winter lows so often travel from southwest to northeast, shoving more and more moisture ahead and around them. This is especially true around the Great Lakes, which supply the atmosphere with huge amounts of moisture. The ATR 2 crash at Roselawn, Indiana, gives double testimony here: Not only was it in the northeast corner of a low, but it was also about 30 miles from the Lake Michigan shore.
Know the pitch and power combination that will produce V A (maneuvering speed). By having this information memorized, you're spared confusion if turbulence kicks in with a vengeance. Simply reduce power to the proper level, then establish the correct pitch attitude. Hold the attitude as best you can, and you'll hold V A while you sort out your options.
Zero Celsius is bad. That's the freezing point, which of course equates to 32 degrees Fahrenheit. The largest water droplets, and the greatest chance of clear- or other large-droplet icing conditions, occur within just a few degrees Celsius of freezing. The rule of thumb holds that clear ice is possible between zero and minus 10 degrees Celsius, and that rime ice happens between minus 10 and minus 20 degrees Celsius. But the ugliest ice shapes and the most runback of clear ice happen in the range of minus-2 to minus-5 degrees Celsius. Same thing with freezing rain. So if you're in clouds and/or precipitation and you see these temperatures on your outside air temperature gauge, climb, descend, or turn around to reach warmer temperatures or exit the goop.
Climb to escape freezing rain? This is an old debate. Proponents hold that warm, above-freezing air is just 1,000 or so feet away, and that many airplanes can manage a climb to the prom-ised land. The more conservative view advocates a 180-degree turn to exit Nreezing rain, reasoning that: one, warm-er temperatures may well be much more than 1,000 feet above your current altitude; and, two, a climb, which involves flying at an angle of attack higher than that used in cruise flight, will expose the undersides of the lifting surfaces to lift-robbing ice accretions. You have to ask yourself a few questions when you find yourself in freezing rain. Are you sure that warmer air is within reach? Can your airplane manage a climb (turbocharging comes in handy here)? Do you feel lucky? Hopefully, you will have turned around at the first sign of freezing rain, minimizing any ice accretions and flying toward better weather. This underscores the basic rules of flying in the icing season: Avoid clouds and precipitation if at all possible, and escape icing at the first sign. That goes for pilots of just about every general aviation airplane, including many that are approved for flight in known icing conditions. — TAH
Is the cold front on your doorstep a growing, storm-laden monster with 50-knot winds or is it a nonevent, a mere wind-shift in clear skies? Although little advertised, there is a way to decode a front's intensity and trend. It's hidden in the chapter on surface analysis charts in the FAA's Aviation Weather Services, or AC-00-45E. To see the code in action, visit the Web site ( www.awc-kc.noaa.gov/awc/aviation_weather_center.html) and click on the "surface analysis chart" link. The code uses three numbers in sequence, and it's printed near the plotted front's surface position. Here's how to break the code:
The first number indicates the type of front:
The second number indicates the intensity of the front:
The third number indicates what the National Weather Service calls the character of the front:
So, is the front on your trip a 773 or a 411? It's a nice-to-know item in your preflight weather checklist. — TAH
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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