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RESTRICTION TO VISIBILITY. TWO fac-tors make the visibility in the polar regions a very complex matter. Arctic air, being cold and dry, is exceptionally transparent, and extreme ranges of visibility are possible. On the other hand, there is a lack of contrast between objects, particu-larly when all distinguishable objects are covered by a layer of new snow. Limitations to visibility in the Arctic are primarily blowing snow, fog, and local smoke. Local smoke is serious only in the vicinity of larger towns and often occurs simultaneously with shallow radiation fogs of winter.

1. Blowing snow. Blowing snow constitutes a more serious hazard to flying operations in the Arctic than in mid-latitudes because the snow is dry and fine and is easily picked up by moderate winds. Winds in excess of 8 knots may raise the snow several feet off the ground, and the blow-ing snow may obscure surface objects such as run-way markers.

2. Fog. Of all the elements that restrict fly-ing in the Arctic regions, fog is perhaps most im-portant. The two types of fog most common to the polar regions are advection fog and radiation fog. Fog is found most frequently along the coastal areas and usually lies in a belt parallel to the shore. In the winter, the sea is warmer than the land, and relatively warm, moist air is advected over the cool land causing fog. This fog may be quite persistent. In the summer, warm, moist air is advected over sea ice, which is now melting, creating the same situation which is found over land in winter.

3. Ice fog. A fog condition peculiar to Arc-tic climates is ice fog. Ice fog is composed of minute ice crystals rather than water droplets of ordinary fog and is most likely to occur when the temperature is about 45C (50F) or colder but can occur when temperatures are as warm as 30C (20F).

4. Sea smoke or steam fog. The cold tem-peratures in the Arctic can have effects which seem peculiar to people unfamiliar with the area. During the winter months, the inability of the air to hold moisture results in an unusual phenom-enon called sea smoke. This is caused by open bodies of comparatively warm water existing simultaneously with low air temperatures. Actually, this phenomenon is similar to that of steam forming over hot water.

In the case of sea smoke, the temperatures of both the air and the water are quite low, but the air temperature is still by far the lower of the two, causing steam to rise from the open water to form a fog layer. This fog occurs over open water, particularly over leads (navigable passages) in the ice pack and is composed entirely of water droplets. 

5. Arctic haze. This is a condition of reduced horizontal and slant visibility (but good vertical visibility) encountered by aircraft in flight over arctic regions. Color effects suggest this phenomenon to be caused by very small ice par-ticles. Near the ground, it is called arctic mist or frost smoke; when the sun shines on the ice par-ticles, they are called diamond dust.

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