WINDS 



1. Wind direction and velocity are often of great aid in fore- 

 casting visibility conditions, but use of winds in forecasting must 

 be tempered by a thorough consideration of any land mass effects 

 in the vicinity. With a steady flow from the south, poor visibility 

 can be expected in the Canadian archipelago in summer. This 

 is due to cooling of the warm, humid air which moves in from the 

 south to temperatures less than its condensation point, resulting 

 in fog. Even with velocities in excess of 25 knots fog will not 

 dissipate before a steady flow from the south. 



2. Winds slowly veering from north to southeast or south indi- 

 cate the passage of a high pressure ridge across the observing 

 station. This gradual shift is usually accompanied by a decrease 

 in velocity to less than 10 knots as the peak of the wedge ap- 

 proaches. It is an indication that a well developed occlusion is 

 moving into the area when the wind slowly veers to the southeast, 

 and then remains in that direction while gradually increasing in 

 velocity as the barometric pressure decreases. If the wind-veer 

 continues into the southwest, with or without an increase in veloc- 

 ity, it is an indication that a cold front is approaching from the 

 northwest, preceding an arctic air mass outbreak from the Beau- 

 fort Sea area. 



3. A wind shift into the north or northwest with a cold frontal 

 passage is not an indication of clearing behind the front. The air 

 masses behind cold fronts which move across the Canadian archi- 

 pelago are shallow, and thus the clouds behind the fronts are more 

 stratiform than cumuliform, having very little vertical develop- 

 ment. Here also, when occlusions are encountered, the cessation 

 of precipitation is not rapid, due to the fact that the occlusions 

 moving across the archipelago are mature, and the moist air has 

 been transported completely around the northern tip of the oc- 

 clusion. 



CLOUDS 



1. Very often the low stratus overcast prevalent in the mari- 

 time air of the Canadian archipelago obscures all higher clouds, 

 and only by use of radiosonde soundings can their presence be de- 

 tected. However, it is very rare that there is not an occasional 

 break in the low stratus, and through these breaks the higher 

 clouds can be observed for short periods of time, if they are present. 



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