10,000 feet, except in convergence zones near low pressure areas. 

 These clouds are usually in layers which can be predicted from 

 radiosonde soundings or air mass considerations. Aircraft icing 

 will exist to some degree in all of these layers. 



THUMB RULES FOR LITTLE AMERICA 



1. Ice fogs are common when the temperature takes a sudden 

 drop to approximately 15° F. and the wind is moderate from the 

 southeast. 



2. Antarctic sea-smoke will obscure the front of the barrier 

 when^the temperature falls suddenly and the wind is moderate. 



3. Absolutely cloudless days are experienced at Little America. 



4. Visibility observations are not reliable as there are no dis- 

 tinguishable landmarks. Exceptional visibility of over 100 miles 

 may be expected on clear days. 



THUMB RULES FOR AREAS ADJACENT TO CONTINENT 



1. During the summer months, with normal southeasterly flow 

 of air, temperatures will remain between 27° F. and 34° F. With 

 southerly winds and modified polar continental air reaching the 

 area, temperatures will reach a minimum of between 20° F. and 

 24° F. 



2. Relative humidity is of little aid in practical forecasting. 



3. In summer months, periods of 6 to 9 hours of favorable 

 weather will occur at intervals of 3 to 5 days, with periods of 24 

 to 36 hours of favorable flying weather occurring perhaps once in 

 10 days. These periods depend on the progression of the migra- 

 tory lows and their effect on the high pressure ridges extending 

 from the continent. 



4. Local weather depended entirely i the air trajectory. If 

 within 150 miles of the coast line or shelf ice, evaporation into the 

 lower layers would give nothing more than broken strato-cumulus 

 which would decrease gradually to clear at the coast line. If 

 operating in excess of 150 miles from the coast, considerable 

 shower activity could be expected with occasional closed conditions, 

 depending on the total water travel of the air. If the ice pack was 

 more than 150 miles from the coast with 50 to 75 miles of open 

 water between the ice pack and shelf ice, considerable fog could 

 be expected on the leeward side of the ice pack. 



5. Most favorable weather conditions were found south of the 

 Antarctic Front. 



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