The frequencies of fog and poor visibility are indicated on the Routeing Charts and the subject is treated at length in the relevant Admiralty Sailing Directions. In the region of the Westerlies, overcast skies, with periods of rain or snow, alternate with brief fine spells. Cloud amounts are generally large. The part of the North Atlantic Ocean most affected by fog lies E and S of Newfoundland.
In the vicinity of the coast between Long Island and Nova Scotia, and the Newfoundland Banks, fog is very prevalent in late spring and early summer, being due to the movement of warm, moist air from S or SW over the cold Labrador Current; over a large part of this area fog is experienced on more than 10 days a month. It is also liable to occur at times in other parts of this zone-- usually in spring and early summer and in association with winds from between S and SW. Visibility is good with NW winds, except in showers.
The prevailing wind is from some E point, though, as in the case of the Westerlies, great variations in direction and strength are caused by the passage of depressions across the area.
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