WX BRIEF

Preparing for wintry blasts

While winter weather can cause widespread flight delays, proper planning can reduce the disruptions.

By Karsten Shein
Comm-Inst, Climate Scientist


Dulles Aviation lineman guides a Millennium Aviation Hawker 700 to a parking spot on a snowy tarmac at HEF (Manassas VA) in Jan 2007. Winter storms make a mess of aviation operations, but are largely predictable.

Many readers will already be experiencing a chill in the air, and maybe some low stratus with icy drizzle. We know that, soon, our preflight walkarounds will be done in parkas and woolen hats, with numb fingers prying open the access panels, and that we'll need to budget extra time to get the wings deiced.

Unless you are among the few pilots who only fly between tropical or subtropical locations, winter months mean gearing up for often unpleasant flying weather. Frequent winter storms can bog down operations at hundreds of airports while plow crews struggle valiantly to keep at least one runway open and aircraft moving.

Rain or snow can change over to freezing rain with little warning, either aloft or at the surface, and wicked, gusty crosswinds threaten to blow us right off our approach. But, while bad winter weather can delay flights by hours or even days, the systems that cause such weather tend to be relatively easy to identify and track.

Unlike the pop-up storms or squall lines with wide gaps that may be present in the summer, winter weather patterns tend to follow the same courses time and again, and pilots can be more or less certain of the conditions a winter storm is likely to bring.

This allows them to modify their plans to minimize the impact these systems will have on operations.

Winter weather

Most of the world's population, and by association, most of the world's airports and flight operations, are in the middle latitudes of the Northern Hemisphere.

Historically, the atmosphere actually played a big role in that distribution-agrarian societies flourished in locations where temperate climates with ample precipitation allowed abundant cereal crops to thrive.

A big part of that success was the presence of a strong winter season. Cold winter temperatures prevented winter plant growth that would sap nutrients from the soil, while heavy winter snows both recharged groundwater and provided a ready reserve of spring water from snowmelt.

The swollen springtime rivers flooded the land, depositing new, fertile soil on the flood plain. In short, the climate of the middle latitudes presented human society with what it needed to succeed, and winter was a critical part of that climate.

The end result was that most of today's population is in the middle latitudes, and our air traffic is always going to be affected by those same winter conditions that helped us prosper. The reason behind the seasons is that as the Earth moves around the Sun, the angle at which Earth's axis is oriented relative to the Sun changes.

On Jun 21, the North Pole is pointed 23.5degrees toward the Sun-on Dec 21, it is oriented away from the Sun by the same amount. During the Northern Hemisphere winter, this axis orientation means long nights and short days at higher latitudes. As the nights lengthen, the Arctic air gets colder and denser.

This in turn allows it to push outward from the Arctic, forcing the polar front south into the middle latitudes. The polar front is the frontal zone that tends to be draped across Northern Hemisphere surface maps between about 40degrees and 60degrees latitude.

It is the boundary between the colder polar air and the warmer subtropical atmosphere, and is the boundary that becomes our warm and cold fronts when a low develops along the polar front. Aloft, we also know this front as the location of the jet stream.

If you compare jet stream maps from January and July, you'll notice that the jet is at much lower latitudes during winter than summer. Also noticeable is the fact that there tends to be a much greater temperature disparity on either side of the surface front during the winter. This is because the warmth of the tropics does not change much during the year.

Therefore, in the summer, when the polar air warms, the temperature difference decreases-during the winter, while the polar air can become very cold, the tropical air remains about the same. Ultimately, this temperature difference means more vigorous cyclonic systems developing along the front in the winter.

Winter cyclones

The low pressures we associate with bad winter weather almost always develop along the polar front. Their formation is a function of both surface circulation patterns and upper air flow. At the surface, inflow from the warm region moves against the boundary and rides up over the cold air.

Winter cyclones go through 4 main phases. An initial low develops along the stationary polar front, and the rotation around the low creates an open wave with warm and cold fronts. When the cold front catches the warm front, the system occludes, finally dying out as a cut-off low.

This forcibly lifted air results in low stratus clouds and cold drizzle that may last for several days because the front is still stationary. At the same time, cold air is pushing against the other side of the boundary, undercutting the warm air and again forcing it aloft.

If there is no support from the upper air, the front is unlikely to move much, and a surface low will not develop. However, under the right parts of the jet stream, the surface gets help from the upper air. Downwind of ridges and troughs in the jet are areas where the air flowing through the jet may diverge enough to draw air up from the surface.

Likewise, areas upwind of ridges and troughs are places where the air may converge, sending some air down toward the surface. With this vertical motion, lows ahead of a jet trough or ridge, and highs behind them, may develop and strengthen, forming a winter cyclone system.

Once a cyclone system develops, the rotation of air around the low at the surface draws the cold air poleward ahead of the low, allowing the warm air to advance along what is now a warm front. Behind the low, the circulation-with the aid of air flowing out of the following high-pressure cell-causes the cold air to advance into the warm sector as a cold front.

The entire system will tend to migrate along the path of the jet, steered primarily by the flow at around 500 mb (18,000 ft). As it moves, the cold front normally circles the low faster than the warm front. After a few days, the portion of the cold front nearest the center of the low catches the warm front.

This occlusion lifts all of the warm air from the surface, effectively killing the inflow of energy into the surface low. Ultimately, the occlusion means that the low gets cut off in the cold air poleward of the front, eventually spinning itself out.

Weathering the differences Each sector of the cyclone has its own unique adverse weather for pilots. Along the warm front widespread low stratus ceilings and IFR conditions are common. The rising warm air cools quickly and, depending on its temperature, may produce rain or snow.

Often the temperature of the cold surface air is below freezing, and the temperature in the warm air above it may not be much warmer-any flight through these altitudes may encounter freezing drizzle.

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