CH 21 Arctic weather

The Arctic region is strictly defined as the area within the Arctic Circle (66.5° N latitude).
A more general loose definition refers to northern regions.
This chapter will review the following:
- Climate of the Arctic
- Air masses relevant to the Arctic
- Weather fronts and phenomena specific to the Arctic
- Weather hazards for aviation in the Arctic

The Arctic experiences extreme seasonal variations in day length due to the Earth's axial tilt and its orbit around the Sun.
North of the Arctic Circle, the Sun remains below the horizon all day during autumn and winter.
Conversely, during spring and summer, regions may experience 24-hour sunshine.
The duration of these phenomena increases as one approaches the North Pole.
- At the North Pole, there are 6 months of total darkness followed by 6 months of continuous sunlight.
Twilight is prolonged in these regions due to the shallow angle of the Sun's descent below the horizon, allowing for visibility at night.

The distribution of land and water in the Arctic is illustrated in Figure 21-1.
Arctic mountain ranges act as barriers to air movement, leading to stagnation of air masses over inland continental areas, making these regions sources of cold air masses.
The Arctic Ocean is predominantly covered by a thick layer of ice, known as the permanent ice pack.
- This ice undergoes seasonal changes, melting in spring and summer while increasing during autumn and winter.
The ice and the water beneath it retain more heat compared to the surrounding land, thus moderating temperatures.
Coastal areas have milder winter climates and cooler summers compared to inland areas, which experience more significant seasonal temperature changes.

Arctic winters are very cold; however, local terrain and pressure systems may cause unexpected warmth in some areas.
Coastal regions tend to be warmer in winter, while interior regions can see pleasant warmth during the long summer days due to extended sunlight hours.

Arctic cloud cover is minimal in winter, reflecting the widespread ice pack.
Spring sees an increase in cloudiness, which peaks in summer and autumn due to melting sea ice exposing more water in the Arctic Ocean.
Summer afternoons may see the formation of cumulus clouds leading to thundershowers, traveling from northeast to southwest (opposite typical mid-latitude patterns).
Polar low-pressure systems may develop in winter over open ocean, characterized by severe weather, gusty winds, and significant precipitation, dissipating quickly over land.
Precipitation varies widely across regions but is generally light overall, with some areas classified as polar deserts.
- In winter, precipitation is primarily snow; in summer, rain predominates over inland areas while snow falls over ice caps and oceans.
- Annual precipitation is typically less over the ice pack and coastal regions compared to the interior.

Winds in the Arctic are generally light, with stronger winds more common along coastal areas in autumn and winter.
Winds in the continental interior are light year-round, reaching their strongest speeds in summer and autumn.

In winter, air masses develop over the extensive ice pack and nearby snow-covered lands.
- These air masses exhibit extremely cold surface temperatures, low humidity, and significant low-level temperature inversions.
Occasionally, moist cold air from unfrozen ocean regions moves northward, contributing to infrequent winter cloudiness and precipitation.

During summer, the Arctic permafrost's upper layer thaws, leaving moist ground, while open waters in the Polar Basin increase, resulting in a more humid and mild climate.
Summer sees increased cloudiness and precipitation, particularly inland.

Occluded fronts are prevalent in the Arctic.
- Weather patterns associated with occluded fronts mirror those in other regions, producing low clouds, precipitation, and poor visibility, along with sudden fog formation.
Fronts occur more frequently along coastal areas compared to interior locations.

The Arctic frequently experiences low-level temperature inversions, where temperature increases with height, leading to a stable atmosphere.
These inversions can slow surface winds and trap pollutants, resulting in hazy, smoggy conditions until the inversion breaks.
Light rays passing through these inversions can bend, producing a phenomenon known as looming, creating mirage-like effects whereby distant objects appear above the horizon and distorting the shapes of solar and lunar bodies.

Snow-covered surfaces reflect significantly more light than darker surfaces, potentially eliminating shadows.
This can complicate visibility, making distant dark objects recognizable while close features, such as crevasses, may go unnoticed due to reduced contrast.

The illumination from celestial bodies, including the Moon and stars, is stronger in the Arctic than in lower latitudes.
Starlight provides visibility far exceeding that in more southerly regions, with darkness approaching typical lower latitudes only under heavily overcast conditions.

Various weather hazards present challenges, including visibility reduction caused by phenomena like blowing snow, icing, frost, and lack of contrast (whiteout).

Fog is defined as the suspension of water droplets or ice particles in the air at Earth’s surface.
- Water-droplet fog commonly occurs in coastal areas during summer.
Ice fog, also referred to as frozen fog or frost fog, forms through the freezing of supercooled water droplets, consisting primarily of ice particles ranging from 12 to 100 microns in diameter.
Ice fog appears at very low temperatures, typically in clear, calm conditions, especially in high latitudes.
Visibility diminishes, especially toward the Sun, with ice fog becoming rare above temperatures of -30 °C and more prevalent near -45 °C, especially near sources of water vapor such as fast-flowing streams, animal herds, volcanic activity, or combustion byproducts.

Blowing snow refers to snow that is driven by wind, reducing visibility significantly.
Drifting snow arises from an uneven distribution of snow depth or snowfall, commonly associated with blowing snow.
Arctic snow’s fine and dry nature allows light winds to lift it several feet off the ground, leading to sudden and dangerous reductions in visibility.

Frost forms ice crystals on surfaces below freezing temperatures during spring, autumn, and winter in the Arctic.

A whiteout occurs when overcast clouds diffuse sunlight over snow-covered ground, obliterating shadows and depth perception.
- The phenomenon often occurs in spring and autumn when the Sun nears the horizon.
- This effect makes visibility difficult as objects seem to float without defined outlines, and the horizon becomes indistinguishable from the surroundings.