The season of adverse and dangerous weather conditions is upon us.
By Karsten Shein
Comm-Inst Climate Scientist
With high winds and blowing snow, conditions could have been better, but at least the blizzard had passed and, as the sun dropped below the horizon, the pilots had clearing skies to accompany their departure.
Passengers loaded and clearance in hand, the pilot signaled the lineman that they were ready to roll. Toward the end of a crossing taxiway, a sidelong gust of wind hit the business jet.
Unfortunately, the aircraft’s tires were treading on a half inch of ice beneath a dusting of snow. The gust provided just enough force to break the tires free and the jet began to slide uncontrollably toward the edge of the taxiway.
Even though the jet’s forward motion had been slow, it carried the aircraft off the taxiway and into the grass, breaking a taxiway light in the process.
Winter in the middle and high latitudes is frequently a season of dark, cold, and adverse weather that makes flying a challenge. Every winter, dozens of aircraft worldwide run off runways and taxiways, and thousands more flights are delayed or canceled for reasons ranging from blizzard conditions to frozen engine oil.
In a review of the NTSB accident database for Part 135 operations, accidents and incidents involving IMC flying were more than 4 times as common in the winter months than the summer. Of these accidents, a large proportion occur at night, with icing or wet runways, and fog or low ceilings. This combination of factors can result in loss of control, runway excursions, disorientation, and controlled flight into terrain.
Although the specifics may change somewhat, the following probable cause statement from a runway overrun accident sums up a number of wintertime, weather-related aircraft accidents. “[Cause was] the pilot’s failure to stop the airplane on the down-sloping, ice-contaminated runway after landing with a tailwind.
Contributing to the accident was the pilot’s failure to account for the wind conditions and failure to obtain runway conditions.” Unfortunately, a commonality in nearly all of these winter accidents is the investigator’s attribution of pilot error.
Because of the tilt of Earth on its axis, the 1st thing we must contend with in winter at middle and higher latitudes is the reduced daylight. Fortunately, fall is when daylight hours decrease most. After Dec 21 in the northern hemisphere (Jun 21 in the south), day length gradually increases.
Another factor in the length of daylight is where in a time zone we are. The further west one goes in a time zone, the later in the day the sun rises and sets. Of course, the higher the latitude, the fewer daylight hours.
Near the Arctic/Antarctic circles, the winter sun may spend only a few minutes above the horizon each day, while above those latitudes, the sun does not rise at all for several weeks or even months around the winter solstice.
While this may not seem significant, at night we lose depth perception, and while some pilots can turn to enhanced vision systems, not all aircraft have light-gathering or infrared vision systems installed.
After turbulence encounters, ground collisions by moving aircraft with other aircraft or ground vehicles are the 2nd most frequent type of aircraft accident/incident, especially at night and in reduced-visibility situations such as fog.
Operating in darkness only makes it more important that pilots exercise greater situational awareness and caution when operating on crowded tarmacs.
The lower sun angles and decreased day length mean the air in the high latitudes becomes bitterly cold and dense, pushing equator-ward and moving the polar front and jet stream to lower latitudes.
The cooling also increases the temperature differences on either side of the front, frequently generating strong winter cyclones. These storm systems tend to be strongest in the early and late winter when the differences between polar and subtropical air are greatest. The colder air decreases the height of the tropopause, meaning that convective cells don’t rise as high into the flight levels as they do during the summer.
But, simultaneously, the stronger upper air circulation in winter usually produces more disruptive clear air turbulence. At the surface, the colder air holds less water vapor, meaning it becomes far easier to saturate the surface layer of air.
This is partly why winter skies are often characterized by low ceilings, fogs, and terrain obscuration. Consider air at 30° C (86° F) with 70% relative humidity. Under these conditions, the air would need to add around 9 grams of water per cubic meter of air to saturate.
But in winter, air at 2° C (35° F) and 70% relative humidity would only need around 1.6 g/m3 to saturate. Under these conditions, even falling drizzle can provide enough evaporating moisture to saturate the air beneath the clouds, lowering the deck.
In addition, the low water-holding capacity of cold air also means it normally only takes a small decrease in temperature to saturate the air without adding water.
This is one of the reasons that winter warm and stationary fronts, as well as a subtle air flow along gradually rising terrain, frequently produce enough lift to create widespread advection fog and extensive low stratus ceilings.
Winter air at higher latitudes has a freezing level at or near the surface. Given that water droplets in the air can remain liquid to temperatures down to –40° C (–40° F), airframe icing is very prevalent during the winter months.
Although icing can occur any time temperatures are near or below freezing and there is visible water in the air, flying through supercooled rain in temperatures between 0° C and –10° C (32° to 14° F) tends to provide the greatest icing threat. Regardless of whether an aircraft is certified for flight into known icing conditions, it is always best to avoid areas where icing is forecast or reported, or flying in or beneath rain clouds when the OAT is in the icing danger zone.
If you experience icing in flight, deploy anti- and deicing measures, and seek warmer air if possible. On the ground, in situations where ice could accrete, ensure that your aircraft gets a thorough deicing and that you are departing within its effective window.
One of the most characteristic winter weather events is the blizzard. Mid-latitude cyclonic storm systems swirl up around a central low pressure, and drag a strong cold front along the landscape, dumping copious rain and snow in their path. Behind the front, strong pressure changes and cold air produce more snow and howling crosswinds. This is a situation that can lead to runway excursions and overruns.
Pilots should be aware that, while even small airports do their best to keep runways and taxiways open, they only clear the snow. It is rare that an airport will lay down salt or other chemical mixtures due to their corrosive nature with respect to aircraft.
Repeated clearing of a runway can pack grooves in the concrete with snow and ice, rendering them useless. Also, early in the storm, snow falling on the warmer pavement may melt and refreeze beneath subsequent snow.
Snow removal may not get rid of the ice that coats the runway or taxiway itself. Blowing snow is an additional slickness factor. In high winds, blowing snow can quickly recoat a cleared runway, and, because it has not had a chance to bond with the pavement or be compacted, it is often as slippery as ice. An aircraft passing over blowing snow can lose its grip and slide sideways in the wind.
In winter storms, it is wise to continually monitor reported weather conditions, ask ATC about runway conditions, and stay put if you’re on the tarmac and it looks like you may go sledding.
If you are landing on a runway that appears to be contaminated with snow or ice, the best course of action is to try an alternate airport – if possible. Aircraft tires are not generally designed for traction on contaminated surfaces.
If that isn’t an option, land as slowly as possible, and use spoilers to reduce speed. Autobrakes should be set and thrust reversers can be used within limits, but high engine pressure ratios can lead to degraded directional control. Never apply hard pressure on your wheel brakes, maintain a straight path, and, if you must turn (as onto a taxiway), do so at a very slow speed.
If your tires do skid, try to turn into the skid, and apply gradual braking pressure if your aircraft has antiskid brakes, or ease off on the brakes if they do not. This course may provide a chance of recovering before damaging the aircraft. Given the high winds that often follow a winter cold front, there is potential for lateral forces that can push your aircraft off the pavement if it is traveling over snow or ice.
A good guideline is, when there is snow or ice present, reduce your aircraft’s crosswind limit by half. That doesn’t guarantee you won’t leave the pavement, but crosswind limits are established on dry runways.
In a recent accident, an Embraer 145 was pushed off the runway at ORD (O’Hare, Chicago IL) by winds with a 22-kt crosswind component. The aircraft’s crosswind limit is 30 kts. Had the pilots halved that number, they might not have attempted the landing.
Winters can also bring severe clear, with crisp, cold air into which turbines and props can eagerly bite. But the cause of these conditions is strong high pressure that may stretch the limits of pressure altimeters. Occasionally, high-pressure cells over Alaska, Canada and Siberia result in altimeter settings exceeding 31.00 inches of Mercury (1050 hPa) – the record high was 32.01 in Siberia on Dec 31, 1968.
Since pressure altimeters generally only set to a maximum of 31.00 in Hg, they effectively become inoperable and should not be relied on for any IFR operations. When abnormally high pressure occurs, some aviation authorities, such as FAA, will issue a TFR prohibiting aircraft operations (eg, FAR 91.144).
However, in other places, no such restriction exists. In those cases, no matter how high you set your pressure altimeter, your actual altitude will always be greater.
The best course of action is, of course, to postpone flight until the pressure drops back within altimeter limits, but if pilots choose to fly in those extreme high-pressure conditions (as with overflying the region), they are normally advised to set their altimeters to standard pressure (29.92 in Hg, 1013.2 hPa) to maintain altitude clearance from other aircraft, remain above the minimum enroute altitude for the region, and avoid flying in conditions (such as at night) where one’s ability to visually locate company traffic or determine their height above the ground or obstacles is compromised.
Your ability to see and avoid is the most critical tool in these situations.
In places like the Pacific Northwest and many other coastal regions at higher latitudes, ocean currents keep surrounding air temperatures higher than they otherwise would be. This is due to the impressive amount of heat energy that ocean water can store. Because heat flows from hot to cold, the ocean, which in winter is normally warmer than the air above it, continually loses heat to the atmosphere.
The result is that the surface layer of the atmosphere is often well above freezing, even though the air above it is below. Conspiring with this set-up is the strengthened winter polar jet that supports storm development out over the ocean, dropping into the lower middle latitudes, frequently driving these storms onshore. An Arctic (or Antarctic) trough in the jet that sits just offshore can funnel one storm after another into a coastal region.
Similarly, a strong high (ridge) in the jet sitting just offshore can force the jet to split into a northerly and southerly branch. The southerly (northerly in the southern hemisphere) branch may drive an “atmospheric river” of warm, humid air from the tropics directly onshore, where the onshore airflow, coupled with rising terrain, wrings the moisture from the air in the form of prolonged and heavy rainfall.
The “Pineapple Express” that has produced some of the Pacific Northwest’s most extreme floods over the years is one such atmospheric river. This phenomenon normally happens in the winter months, when the jet dips south into the central Pacific, and moisture from the vicinity of Hawaii is able to flow in a narrow band along the polar front into the region stretching from northern California to southern British Columbia.
Atmospheric rivers create several issues for aviation, not the least of which is the potential for standing water on runways and taxiways, and flooded approach roads. Because these systems occur during winter, the air may be only a few degrees above freezing. As the air rises into the inland terrain, it may cool to or below freezing, meaning supercooled rain and cloud droplets and potential icing at low altitudes.
Moreover, aircraft coming down from cruise must penetrate the widespread precipitation shield. After many hours in the flight levels, the fuel in the wing tanks is below freezing, leading to cold soaking. The cold fuel saps heat from the wing surface, so, even though the air may be above freezing, the rain hitting the subfreezing wing skin will still freeze into a glaze. That much moisture running onshore also means saturation at low levels.
Area airports may be socked in by precipitation fog, which forms as the air saturates due to evaporating rain. Clouds will also likely be low-level stratus, meaning that tall towers and nearby terrain are likely to be obscured. Situational awareness is critical during a winter rain event.
Winter in the low latitudes
While middle and higher latitudes tend to be most adversely affected by winter weather systems, the tropics and subtropics fair far better. The intertropical convergence zone (ITCZ) – the belt of convection that rings the planet in the tropics – follows the sun. Therefore, during the northern hemisphere winter, the position of the ITCZ is near or south of the equator in most places.
This leaves the northern hemisphere tropics and subtropics relatively dry. In fact, in most parts of the tropics, the seasons are not described by temperature swings, but rather are divided into wet and dry periods corresponding to the position of the ITCZ relative to the area. Winter and summer monsoons track this relationship as well.
Although we tend to think of monsoon as the rainy season, the word monsoon just means season. In many places, winter monsoons such as the Indian winter monsoon are characterized by cool, dry air flowing in from higher latitudes. This often leads to drought conditions, especially if the monsoon is anchored by a strong high pressure. The same drivers that deliver this dry weather to India also help to produce a wet winter monsoon in eastern Southeast Asia, drawing in humid air from the South China Sea.
Beyond monsoons, the equator-ward shift of the polar front in winter often digs into the subtropics as sharp troughs that can bring brief cold air outbreaks to places such as central Florida, the Mediterranean, and southern China. The atmospheric dynamics generated by these troughs bring the lower latitudes strong cyclones and associated cold fronts with wind, rain, and thunderstorms. In the US, many winter storms develop over Texas and even the Gulf of Mexico, some becoming Nor’Easters that pummel New England.
In rare cases, these systems have produced heavy snow along the Gulf Coast. In Feb 1895, the town of Rayne in coastal Louisiana received a record 24 inches of snow. Regardless of where one flies, winter is a season that demands additional care regarding the weather. As always, if you experience weather conditions your fellow pilots should be aware of, be sure to let them know.
Karsten Shein is cofounder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg University and a climatologist with NOAA. Shein holds a commercial license with instrument rating.