area from the northwest and their surface layers are warmed as 

 they move over the Canadian archipelago. The archipelago, with 

 its maritime characteristics in summer, in much warmer than the 

 ice pack source region of the summer arctic air masses. The 

 warming in the lower layer of air results in a steep lapse rate in 

 that layer, and hence instability. 



PRESSURE 



1. Pressure tendency cannot be relied upon as an aid in fore- 

 casting except in very few instances in the Canadian Arctic. In 

 the western and northern portions of the operating area, weak 

 pressure gradients were found, and these gave little indication as 

 to the orientation of isobars, because of the absence of reporting 

 stations. 



2. In the summer, when pressure gradients are weak, local land 

 mass effects, rather than the general pressure field, are the major 

 influences on wind direction. This does not apply in the cyclonic 

 circulation of northern Baffin Bay, a large area of open water, 

 with a normally steeper gradient than is found to the west. 



3. In autumn, late August and September, the high pressure 

 cell to the northwest of the archipelago, centered over the Beau- 

 fort Sea, becomes more pronounced. This results in a somewhat 

 steeper pressure gradient across the archipelago. Hence, a 

 northerly wind on the east side of the high cell prevails across 

 the archipelago. 



4. In the eastern part of the Canadian Arctic, when migratory 

 storms move from the southwest toward Baffin Bay, the steepness 

 of the pressure tendency provides a good indication of the intensity 

 of the approaching lows. It follows that, in these pressure sys- 

 tems, the sharper the fall and subsequent rise of pressure, the 

 greater will be the velocity of the accompanying winds. 



5. During the summer months, except in the cyclonic circulation 

 of Baffin Bay, rise and fall of barometric pressure was of prac- 

 tically no value in forecasting. In autumn, as the pressure field 

 becomes more pronounced, a certain amount of reliability can be 

 placed on pressure tendency, but not to the extent of that in middle 

 latitudes. 



134 



