preaching storms may be noted in winds aloft in the 10,000- to 

 20,000-foot layer. 



9. Approaching closer than 50 miles to the continent can bring 

 the ship under the influence of drainage flow from the ice cap. 

 This causes an abnormally high wind for the pressure gradient 

 present. It may be due to a tight local pressure gradient, which 

 in turn is due to the rapid increase in depth of the polar air mass 

 between the ship and the continent. This is a large scale kata- 

 batic or foehn wind effect and may be minimized by moving 

 further away from the continent. The total effect is largely de- 

 termined by the local topography of the continent and can be 

 predicted in advance. 



10. With a southerly wind, favorable weather will exist over 

 a coastal area, while off shore 50 to 100 miles low status will psr- 

 sist with resulting poor flying conditions. In general, near the 

 coast a southerly wind will produce to 2 days of good flying 

 weather following the passage of a migratory low. 



PRESSURE 



The use of pressure and pressure tendency as an aid to fore- 

 casting is, at times, misleading south of 75° latitude. This is due 

 to the fact that most of the lows and fronts in this area are dis- 

 sipating. For example, filling lows and associated bad weather 

 approaching the Bay of Whales may result in the pressure at that 

 location remaining steady or rising slightly. There are other 

 instances in the antarctic areas when pressure traces may be of 

 use in forecasting, as follows: 



1. Pressure tendency is an excellent indicator of weather 

 changes in the vicinity of 65°, the location of the track of many 

 of the circum-continental migratory lows. As elsewhere in the 

 world, lows and fronts are preceded by falling pressure and fol- 

 lowed by rising pressure. 



2. When the barometric trace begins to fall off rapidly in the 

 latitudes between 60° and 70°, winds 30 to 40 knots can be ex- 

 pected. These winds will usually continue for a period of 12 to 

 24 hours after pressure begins to rise. When the barometric trace 

 begins to level off after the rise following the passage of the low 

 center, it can safely be expected that the winds will diminish 

 rapidly to 16 to 24 knots. 



3. Good weather conditions were found along the eastern sector 

 of wedge lines where the southeasterly flow of dry air from the 

 continent guaranteed good visibility and negligible cloudiness 

 locally with CAVU weather over the coast line and continent. 



214 



