variability can be measured for a 12-hr. period, we may expect these condi- 

 tions to prevail over periods up to one month. The duration will probably 

 be longer in tropical regions, shorter in current regions. In boundary or 

 confluence regions, the usefulness of the envelope will be limited by the 

 time-scale of the changes in location of the boundary . 



ACKNOWLEDGEMENTS 



This paper represents results of research sponsored by the Office of 

 Naval research under Contract NONR 2216 (Ol), (05) . 



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