314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 6 



Colon [3] has computed the probabilities that the track would not 

 deviate more than 10° in direction in the next 24 hours from the path 

 followed the previous 24 hours. He found that during most of the 

 hurricane season straight-line extrapolation gets results this good at 

 least 80 percent of the time in the area south of 20° N. 



Steering. — As early as the beginning of the twentieth century, we 

 find examples in meteorological literature suggesting that hurricanes 

 are steered by the winds around the Bermuda anticyclone. Only 

 slightly later articles were published emphasizing that hurricanes fol- 

 low the course of cirrus clouds moving out in advance of the hurricane 

 center. Edward H. Bowie [1], C. L. Mitchell [12], Gordon E. Dunn 

 [6], and later writers have all suggested that we use the winds around 

 the hurricane to estimate the future movement of the storm. 



Grady Norton, formerly in charge of the hurricane forecasting office 

 in Miami, has been the most successful user of what he called "the high- 

 level steering technique." In picking the "steering level" he would 

 examine the wind charts at successive levels from near the ground up 

 to 50,000 or 60,000 feet. Working up from the ground he chose as 

 the "steering level" the first one at which the vortical circulation as- 

 sociated with the hurricane seemed to disappear. He then would 

 predict that the hurricane would move in the direction in which the 

 winds at this level were blowing. 



Many other "steering techniques" have been devised — e. g., low-level 

 steering, from which it is argued that the hurricane moves with the 

 winds at low levels; and warm-tongue steering (Simpson [20]), on 

 which is based the theory that a hurricane will move parallel to the 

 axis of a warm pool of air that usually protrudes in advance of a hurri- 

 cane. 



AROWA technique. — The most widely discussed new forecasting 

 method of the last two seasons was the one developed at the United 

 States Navy's Project AROWA, under the leadership of Professor 

 Riehl and Commander Haggard [14]. This method also makes use of 

 "steering." To get an estimate of the basic current, the 500-mb. (about 

 19,000 feet) map is analyzed. The mean wind at this level is com- 

 puted for an area that extends 7%° of longitude east and west of the 

 storm center and from 5° south to 5° to 10° north of the storm center. 

 The storm is then forecasted to move at the approximate speed of 

 the wind through this area, corrected by a relatively small empirical 

 factor. This method has been used widely by the various hurricane 

 forecast centers for the last two years. Results have varied from 

 office to office and from storm to storm. Although the technique itself 

 is objective in nature the results are entirely dependent on the quality 

 of the analysis of the weather map at the selected level which differs 

 widely from one analyst to another and from one day to another, de- 



