WX BRIEF

Arctic operations

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






Surface pressure and temperature across the Arctic. This polar projection clearly shows the extent of cold conditions over the region as well as the strength and position of storm systems migrating through the circumpolar vortex.

There are in fact a number of radars looking at the Arctic, but these are radars designed either for military purposes or to investigate the aurora borealis—not to provide weather information. The only weather radars in North America that have the range to provide weather information above the Arctic Circle are those at Nome AK and Pedro Dome (just north of Fairbanks AK), while elsewhere Norway has 2 Arctic radars near Tromsø (Andenes) and Hammerfest (Hasvik).

Climatology and adverse conditions

But, as with flying anywhere, one key to a successful flight is knowing what you're up against. To understand the possible weather conditions, it is often best to start by understanding the climatology of the poles.

There are 3 main factors controlling atmospheric conditions over the Arctic and Antarctic. The first is the Sun. Directly and indirectly, the Sun also controls the other 2—snow and ice cover, and the general circulation of the atmosphere.

We all know that at the poles there are 6 months of perpetual day and 6 months of perpetual night. But that's only at the poles. At the Arctic and Antarctic circles, the Sun will sit just below the horizon for all 24 hrs only on the respective winter solstice, thereafter making an ever longer appearance above the horizon until it provides 12 hrs of day on the equinoxes and a full 24 hrs of daylight on the summer solstice.

When the Earth is tilted most toward the Sun (the June solstice for the Northern Hemisphere and the December solstice for the Southern), the 24 hrs of sunlight mean that the pole actually receives 36% more solar radiation than the Equator.

The reason why the poles don't heat up more is the snow cover, which can reflect more than 70% of the sunlight that hits it, especially at the lower Sun angles that are ever present in the high latitudes. Even on the solstice, the tilt of Earth's axis ensures that the Sun never gets higher than 23.5° above the horizon. These low sun angles also mean that even moderate terrain can cast long shadows and create localized pockets of cold.

The snow and frozen ground also mean that heat is continually withdrawn from the air in an attempt to melt the ice. Given the cold conditions, it is no surprise that, in general, the air can hold very little moisture. As a result, the region is classified as arid, and places such as the ice-covered interior Arctic and Antarctic are actually considered deserts, receiving fewer than 10 inches of precipitation a year.

However, much of the Arctic is maritime, so, even though air temperatures are cold, the water, which doesn't drop below about –2°C even in winter, imparts some heat to the atmosphere, either directly or through fissures in overlying ice. The presence of so much water also means that where there is open water there is also high humidity. This in turn often means coastal fogs and a coating of frost on aircraft that may be parked overnight, especially where there is an onshore flow.

Winds, temperature and pressure heights at 500 mb (hPa), or around 18,000 ft over the Arctic. This level provides clues about the steering currents for any cyclones that may be working their way across the region.

Some coastal locations are not spared from this in winter, either. Although ice season weather tends to be dominated by clear skies and light winds, stronger offshore winds in places will drive ice continually from the shore, keeping the coastal water ice free through the winter, and supporting fogs and frosts when winds are light.

In general, the ice season in the Arctic begins in mid-September, as the amount of incoming solar radiation is no longer sufficient to sustain the ice melt. After the equinox, the freezing takes hold with a vengeance, and the spring thaw doesn't usually become significant until early June. Snow cover in the Arctic averages between 200 days at the margins to 300 days or more nearer the pole.

In places, summers above the Arctic Circle can be downright balmy. Summertime temperatures throughout the Arctic are often well above freezing except for near the pole and over the Greenland ice sheet. Once the Sun rises for extended periods of the day, the bit of radiation that is not reflected from the icy landscape is absorbed by the snow and ice, melting it.

Meltwater pools atop the ice are darker and absorb even more sunlight, speeding the process. At the same time, the open ocean is warming, melting the ice pack at its fringes and along the coasts, where heating is also being contributed by the newly exposed earth.

By June, a great deal of the land surface, especially near the coast, has become snow and ice free, and the top layer of permafrost may even have melted as the dark earth absorbs more than half of the sunlight that hits it. The melting permafrost may create seasonal bogs and a waterlogged landscape which may leave standing water or soft spots on unpaved runways. It may also buckle hard surfaced runways as they lose support from the melted tundra beneath them.

Highs and lows

Weather in the Arctic is dominated by a set of semipermanent high and low-pressure centers. These cells vary in strength throughout the year and drive much of the adverse weather seen in the region. The Aleutian and Icelandic lows and the Siberian and Polar highs control the general flow of the Arctic atmosphere. They tend to be weakest during the summer and strongest in the winter.

The overall circulation of air around the poles is driven by the Polar high. This high is positioned approximately over the pole during the summer, and migrates to a slightly lower latitude in the winter. The high is generated by the cold dense air packing in over the pole.

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