WX BRIEF

Winter weather woes

Cold season flying includes preparing for both severe and subtle adverse conditions.

By Karsten Shein
Comm-Inst, Climate Scientist


Aircraft on the Landmark ramp at IAD (Dulles, Washington DC) during a snowstorm. Winter weather can delay and disrupt aviation, but understanding some of the causes of these adverse conditions can help reduce their impact.

In the Northern Hemisphere, as the midday sun dips lower in the sky and the nights grow longer and colder, arctic blasts routinely invade our airways and visit many of the airports we must frequent in the course of business.

Flying amid these conditions is something all but a few fortunate pilots must contend with each winter. Depending where you fly, winter weather can bring rain, snow, icing, low ceilings and vicious winds, either individually or seemingly all at once.

Of course, knowledgeable pilots have at their disposal a number of tools that can help them overcome many of these adverse conditions—chief among them a knowledge of what conditions favor them, so that their impact can be minimized.

The most common component of winter in most latitudes poleward of the tropics is the cooling of the lower atmosphere. This is simply a result of shorter days with lower sun angles, and longer nights—a function of Earth’s seasonal tilt away from the Sun.

The reduction in solar radiation reaching Earth’s surface means less heating, and therefore less heat available to be transferred to the overlying atmosphere. Pilots may not think of cold air as much of a danger, but, as the temperature drops for prolonged periods, so does the need for additional preflight preparations such as using auxiliary heating or power units prior to start up.

More critically, but perhaps less acknowledged, is the danger of cold air to the flightcrew or passengers. As temperatures drop, the physiological danger to a person increases. Whether the temperature is warm or cold, pilots and line­crew must work outside for extended periods.

Without proper attire, core heat loss to hypothermic levels and possible frostbite can occur in as little as a few minutes. What’s more, while it’s tempting to reduce exposure time, that often requires shortchanging potentially critical safety precautions during preflight inspections or routine maintenance activities.

Idealized schematic of the anatomy of an occluded storm system. The shaded region represents the overall cloud pattern, with its characteristic comma shape, while a trough in the jet stream drives the system. If the strongest winds aloft are found at location A, the system is likely to continue to strengthen and may not move quickly. Strongest winds at location B are a good indicator that the system won’t continue strengthening and may start to move out of the area more quickly.

For example, a lineman wearing gloves might not secure a fuel door properly, and a pilot racing through the preflight could easily miss the error. Temperature forecasts are among the more accurate prognostications made by meteorologists, yet time and again we peer out the frosted-up windows of an FBO to see a pilot preflighting an aircraft in their shirt sleeves when temperatures are below zero.

A quick query on the Internet will tell you the current and forecast temperature at just about any frequented airport worldwide. It’s not a bad idea to keep a set of cold weather gear stowed in the aircraft during the winter.

FBOs may have a loaner parka, but there’s no guarantee of that, and a warm coat, gloves and a hat really won’t add that much to your weight and balance. It’s not all bad, however. The cold winter air tends to increase air density, often decreasing density altitude by hundreds, if not thousands, of feet.

The improvement in performance at any given altitude is usually noticeable, especially with regard to takeoffs and climbs.

Troughs and cold intrusions

The chilling of the air at higher latitudes does something else as well.

As the air becomes colder and denser, it spreads out, pushing the polar front—the general boundary between the colder polar air and warmer subtropical air—toward the Equator. It is along the polar front that most midlatitude storm systems develop and track.

As the front is pushed toward lower latitudes, it brings the core of these storms with it. This means that troughs migrating through the jet stream above will draw frigid arctic air and blizzard conditions down as far as 30 or so degrees latitude.

Troughs are disturbances—short waves in the undulating flow of the polar jet. They appear as “dips” in the jet stream toward lower latitudes—however, in actuality they are simply areas of lower pressure heights due to a cooling of the air beneath them.

(Conversely, a ridge is an area where the pressure heights are increased due to an inflow of warmer air at the surface.) Since air also cools as it rises, due to expansion, smaller-scale troughs may form in regions of convection.

The flow of the air near the base of a trough determines whether the trough will dig (amplify to a more equatorward position) or lift out (become less amplified and migrate downstream along the jet).

Late fall cold front sweeping across the eastern US as seen on an enhanced infrared (IR) satellite image. Enhanced IR differentiates clouds more clearly from surface snow cover.

When looking at an upper air chart, such as a 300-mb map, look at the wind speeds within the trough. If the strongest winds in the trough are found on the upwind side of the trough axis (an imaginary line bisecting the trough at its lowest point), the trough is likely to dig.

However, if the jet streak is to the right of the axis, the trough will most likely begin to lift out and may move very rapidly toward the northeast (or southeast in the Southern Hemisphere).

The speed of the wind through the trough will also help determine whether a significant low pressure develops at the surface just ahead of the trough axis. A jet streak in the axis area means that air will diverge aloft as it exits the trough.

This provides room for lifting air from the surface to higher altitudes. At the surface, the combination of the Coriolis Effect and the force generated by the pressure gradient sets up a counterclockwise spin (clockwise in the Southern Hemisphere) that draws warmer air poleward from the equatorial side of the front and cold air down toward lower latitudes behind the system’s center.

This turns the cyclonic storm system into the classic comma-shaped warm front/cold front appearance. If the temperature differences are significant, as they are with the nor’easters that often roar up the US Atlantic coast, weather conditions can be severe.

1


1 | 2| 3 next