246 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



lower air (about 1 mile thick) was apparently stable (potential 

 temperature lower) relative to the layers above, a condition adverse 

 to strong convection and the growth of eddies. 



Where descending blasts of wind impinged on steep hill slopes and 

 were concentrated by the topographical configuration, their velocity 

 was greatly increased locally, and trees and structures on exposed 

 slopes or shoulders of hills or cliffs or just beyond were blown down 

 or greatly damaged."* At the Blue Hill Observatory, where such 

 gusts might have been expected to show about their maximum in- 

 crease, velocities of more than 150 miles an hour were almost cer- 



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FiGUBE 2. — Weather sequence at the Blue Hill Observatory September 21, 1938, noon 

 to midnight, eastern standard time (plotted by H. S. Rice and C. F. Brooks). 



The topmost line indicates the hourly cloudiness, except that there was a minor break 

 shortly before 6 : 00 when the wind was shifting most rapidly from southeast to south. 

 The second line indicates the general nature of the lower clouds (upper clouds could 

 only be seen about 1:00 and 5:40). Numerals and arrows roughly indicate cloud 

 velocity in miles per hour and direction of movement (as on a map). The occurrence 

 of rain is indicated by the slant lines under the clouds. 



The third line shows the wind direction by half-hourly Intervals. The Initial swing 

 from southeast to nearly east occurred as the circular portion of the storm approached ; 

 the fairly steady veering from east-southeast to south, as the storm passed on the west. 

 Wind velocity (from a 3-cup anemometer) is shown in miles per hour by 5-minute 

 periods, the maximum having reached 121 between 6 : 11 and 6 : 16. Considerable irregu- 

 larity is evident. The pressure record indicates a rather rapid fall until about 4 : 30, 

 followed by a period of nearly stationary pressure, reaching a minimum of 29.01 inches 

 at 5 : 17, then a rapid rise as the center of the storm passed and the cooler air swinging 

 around south of the storm increased the density of the air column. The next line shows 

 the amount of precipitation. Below is temperature, showing a rather marked drop 

 around 6 p. m., as the main cold front arrives. At the bottom is the humidity trace. 

 The air, though still tropical, was appreciably drier than it had been the previous day. 



"» Further details in Brooks, C. F., The hurricane — Meteorological postscript. 

 Alumni Bull., vol. 41, pp. 1165-1168, July 7, 1939. 



Harvard 



