WX BRIEF

Arctic operations

With more use of polar routes, understanding high latitude conditions has never been more important.






Glaciers and a perpetual snow pack cover northern Greenland (about 83°N) even in May. Any melting that may take place would begin around June and continue to September.

As a result of more air being concentrated near the surface, there is a low pressure aloft directly above the surface high. This means that, while surface winds are largely easterly, winds aloft are primarily westerly.

Overall, however, the pressure gradient in the high latitudes is relatively small, meaning that winds are normally fairly weak at any given altitude. In the absence of a cyclone, Arctic surface winds generally don't exceed 20 kts. However, winter cyclone winds over the North Atlantic may occasionally exceed 86 kts.

The Arctic is also a place of varying terrain, from coastal lowlands to large, rugged peaks and high plateaus atop major ice sheets. So, while winds may not usually be excessive, moderate but steady winds blowing across these peaks and plateaus often create standing lee waves that can affect flight below about 10,000 ft.

In addition to the winds, the polar high has an effect on pressure. As the air becomes colder and more compressed toward the surface, aircraft flying a pressure altitude will find their absolute altitude decreasing, often by several thousand feet, as they near the pole.

While not usually a significant problem for aircraft in the flight levels, terrain clearance becomes an issue when aircraft are flying pressure altitudes at lower levels. Mountains and ice packs in the Arctic can attain altitudes between 6000 and 8500 ft msl, so an aircraft flying a 9000-ft pressure altitude could easily be flying below the summit of nearby terrain.

Polar storms

Polar cyclones frequently form along the polar front that circles the Arctic between about 40 and 60° latitude, and may occasionally occlude into the Arctic. Fortunately, these storms are least common during winter. When they occur during the summer months, they are more likely to bring precipitation in the form of rain and even an occasional thunderstorm.

However, fall tends to be the stormiest season, and storm systems become more prone to delivering heavy snowfall and blizzard conditions as winter nears.

Arctic cyclones frequently spin up over Siberia or over open water such as the Bering Sea and the North Atlantic, and usually affect the southern parts of the Arctic more than the interior.

Storm systems are steered by midlevel wind currents, so most of them come into the area from the west, and are also often generated or enhanced by an upper-level trough. A look at the upper-air pressure patterns over the poles usually shows several ridges and troughs.

As with ridges and troughs at lower altitudes, upper air ridges are normally associated with clear skies and light winds. Upper troughs, on the other hand, will often produce wide­spread clouds and precipitation.

Winter also brings frequent temperature inversions. The snow and ice cover extracts heat from the atmosphere, making the surface air colder than the air above it. This situation tends to separate the colder surface layer from the warmer air aloft, meaning that surface winds are often much calmer than the winds above.

Cold air outbreaks are frequently the culprits of these inversions, and when they extend out over open water the air can quickly warm and absorb moisture from the underlying water. These conditions may create thick convective clouds in which visibility is lost and icing potential is high.

Regardless of season, however, an ever present issue is that of the cold. As we take our aircraft to higher and higher altitudes, protecting their systems from the cold becomes increasingly challenging. Even though a summer day at YRB (Resolute Bay NU, Canada) may be above freezing, more often than not pilots will have to start an aircraft that has been sitting in air many degrees below zero.

It is best to be conscious about the temperatures your aircraft may have to endure if you plan to park it for any length of time, and ensure that you have the capabilities to get it fired up and possibly deiced when you are ready to depart again.

Over the next few years, there is likely to be a marked increase in Arctic aviation. But, like many places, the Arctic is not generally a place forgiving of those who don't respect its weather. Fortunately, a lot of atmospheric research was conducted in the Arctic during the Cold War, and we now know a lot more about what we may face.

There are numerous Internet resources for pilots wanting to learn more, and Nav Canada even put together a comprehensive primer on Arctic weather, which can be downloaded at navcanada.ca/contentdefinitionfiles/publications/lak/nunavut/N3637E-W.pdf. And if you go, don't forget your parka.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.



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