Blizzards

Winter’s icy gales command respect and careful planning.

By Karsten Shein
Comm-Inst
Climate Scientist

Pilots who operate regularly in the middle latitudes understand all too well the ability of a blizzard to mess with even the most cautious flight plans. Every winter, more than a few pilots and their passengers extricate themselves from aircraft that have slid off the runway amid subfreezing temperatures, howling winds, and blinding snow – and the coming winter will be no different.

While reports of such accidents often cite low visibility, excessive crosswinds, and contaminated runways as contributing factors, the cause is almost always that the pilots failed to take those factors into account despite operating in a blizzard.

Although various national weather services provide different definitions of what constitutes a blizzard, all agree that it is a dangerous snowstorm that includes high winds, cold temperatures, and low visibility.

The US National Weather Service gives a more comprehensive definition as a storm which contains large amounts of snow or blowing snow, with winds exceeding 35 mph, and visibility less than 1/4 mile, with those conditions persisting for more than 3 hours. Although storms not meeting those conditions are referred to simply as winter storms, they too should receive the same degree of preparedness, as they can strengthen into blizzards quickly.

The definition of blizzard means that a winter storm doesn’t even require falling snow, as the high winds often whip up fallen snow to create the low visibility. When a blizzard is limited to blowing snow, it may be called a ground blizzard.

But even though it may just be blowing snow, that snow can still be blown hundreds of feet into the air, reducing visibility through some of the most critical parts of an approach.

Unfortunately, blizzards are not uncommon in the vicinity of the polar jet. In fact, most blizzards occur in the northwest section of a northern hemisphere surface low-pressure system (southwest quadrant in the southern hemisphere), and are most likely to occur just before the cold and warm fronts occlude and the low is rapidly intensifying.

The reasons for this are threefold. First, this is the region behind the cold front where the coldest air is streaming down from the Arctic. These cold temperatures normally mean that any precipitation falling there will fall
as snow.

Second, warmer and more humid air from the sector between the warm and cold front is being drawn aloft over the warm front, cooling and saturating to produce precipitation as it is wrapped around the topside of the central low – flowing over the blizzard sector.

Third, upwind of the low, beneath the path of the polar jet, there is normally a corresponding center of high pressure. Aloft, this pattern is a ridge in the jet that trails the trough above the surface low.

The combination of low and high surface pressure sets up an often-intense pressure gradient between the 2, which can be enhanced should the low experience “bombogenesis” – a rapid drop in surface pressure. Of course, pressure gradient force determines wind velocity, with air flowing from the high to the low across the gradient. The greater the change in pressure over a distance, the faster the wind.

Ignoring the 3-hour duration requirement, blizzard conditions can and do occur in situations other than a low pressure cyclone. Katabatic winds – Föhn or Chinook, for example – can produce hurricane-strength straight-line winds.

While these winds usually last less than an hour, they often occur on the lee side of mountain ranges in winter. Although the temperature increase associated with katabatic winds is frequently enough to melt snow, they can still mobilize freshly fallen snow to create blizzard-like conditions.

Challenges for aviation

Aircraft at cruise may need to contend with higher cloud decks and turbulence, since there are vertical air currents associated with major surface pressures and ridge-trough patterns in the jet. But because of the cold air associated with them, blizzards are normally confined to lower altitudes, and actual blizzard conditions may occupy only the lowest few thousand feet of airspace.

In these lower altitudes, much of the in-flight danger from blizzards comes from the high winds and reduced visibility. Visibility in blowing snow can easily drop to zero at any point along the approach path, including just before touchdown.

Snow is among the most effective at reflecting and scattering light, and may direct any light intended to reveal the surface and runway markings – even lights – away from the pilot’s eyes. Often, pilots must rely on reduced minimum approaches, including Cat III approaches, and those who aren’t trained or whose aircraft aren’t adequately equipped may find themselves forced into a hold or diverted to an alternate.

Strong and gusty winds are another factor that pilots on approach and landing need to consider. In the northern hemisphere, blizzard winds favor the northwest, but gusts born from low-level windshear can come from other directions.

Landing with more than a few degrees of crosswind in gusty conditions makes a steady centerline approach challenging, and landing may become impossible without excessive side loading of the landing gear or even wingtip strikes on the concrete.

On the ground, the high reflectivity of blowing snow will often produce whiteout conditions. In low light, taxi and landing lights will also scatter light, sometimes into the cockpit, reducing visibility further.

The high winds producing the blowing snow, coupled with the potential for heavy snowfall from the overlying clouds, not only reduce visibility – they can cover taxiways and runways faster than airport crews can clear them.

Planning for blizzards

Fortunately, blizzards don’t often materialize out of nothing. They are normally  associated with strong winter cyclones, and such large-scale meteorological features are predictable, even to pilots looking at surface weather charts.

Although not all winter storm systems include blizzards, a pilot can look at the surface pressure contours to the northwest of the low. The more closely spaced the contours are, the higher the windspeed. Even without isobars, just looking at the pressure difference between the center of the low and the trailing high can provide a rough estimate.

Observing a wintertime surface low deepening rapidly is another indicator that a blizzard is possible, particularly if the cold and warm fronts have not yet occluded. Once the cold front overtakes the warm front, the inflow of warmer and more humid air is cut off and the chance of a blizzard with intense snow is reduced – although if the system produced a lot of snow, high winds can still create a ground blizzard.

When a blizzard is in the forecast, the wisest action is to avoid arrival or departure operations in the area until the system passes. Blizzards do not normally last for more than a day, and most blizzard conditions will clear up within a few hours as the high approaches and the winds decrease.

If blizzard operations are unavoidable, evaluate whether the airport has the means to keep runways and taxiways clear. Determine what types of approach are available and review missed approach procedures. Have at least 2 alternates in mind, since a blizzard may close multiple airports or make landing difficult across a large region.

On approach, expect a gusty crosswind landing in questionable visibility and anticipate that the runway may be contaminated with blowing snow and icy patches. Once off the runway, if visibility drops to zero, it is best to call a stop rather than accidentally steer off the pavement, turn onto an active runway, or collide with another aircraft.

Often in these situations, a ground stop is issued or follow-me vehicles may need to be requested. Frequently, blizzard visibility fluctuates from one minute to the next, and stopped aircraft can soon continue their journey to the tarmac and a warm FBO.

Karsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.