x 
METEOROLOGY — FOREGASTING 
120 130 Ve 
LEGEND 
HEAVY LINES SEPARATE THE OCEANIC REGIONS. 
MONTHS (DECEMBER, JANUARY AND FEBRUARY). 
RANGE OF DUGT WIDTH IN FEET (5 TO/30 FEET). 
253 
10— 35 
° LD >50 
50\N,>10 |30;—,>20 
5b 65" 
JFMAMJ 
JASOND 
o-15 
5\NW, >50 i¢) 
50iNW,>20]50N, >25 
45 50 
5\NW,>50| 
60;NW,>20 70. >s 
10\—,>40 |15)NW, >30 
65;—,>5 |55) ma 
10\NW, >30 
50;N, >10 
40 
PERCENTAGE OF TIME OUCTS OF WIDTH GREATER THAN 40 
FEET OCCUR (O PER CENT). 
PERCENTAGE OF TIME DUCTS OF WIDTH 20 TO 40 FEET OCCUR 
(30 PER CENT)3 PREVAILING WIND DIRECTION WITH DUCTS OF 
THESE WIDTHS (NORTH), WIND VELOCITY NECESSARY TO CAUSE 
DUCTS OF THESE WIDTHS (GREATER THAN 20 MPH). 
PERCENTAGE OF TIME DUCTS OF WIDTH LESS THAN 20 FEE 
OCCUR (70 PER CENT). 
SiN, >50 
(+) (*) 
301N,>20 | 50:SE,— 
70 50 
() 
90\NE,>5 |55)N,>10 
10 40 
Ficure 20. Summarized results of climatological duct with calculations. 
tion of wind speed only, not allowing for possible 
variation in temperature excess and M deficit. 
In spite of these shortcomings, the calculated re- 
sults show regional and seasonal trends consistent 
both with what might be expected on the basis of 
qualitative physical reasoning and also with a limited 
number of actual observations taken in the Pacific 
(see pp- 254-256). In conclusion we may safely state 
that the results represent to a reasonable approxima- 
tion the average conditions of surface duct width and 
variability. 
User or ComputEep RESULTS 
Since the computations are based on climatological 
data and are limited in exactness, they do not have 
what trapping conditions will be on any particular 
day. They serve merely to indicate in a general way the 
average conditions that might be expected over a pe- 
riod of time. For example, the percentage of time that 
duct widths in excess of 40 ft occur gives an estimate 
of the fraction of time that properly sited S- or X- 
band radars would be able to take advantage of ex- 
tremely large ranges ; the percentage of time that duct 
widths from 20 to 40 ft occur indicates that portion 
of the time in which X-band radars would be needed 
to benefit from surface duct conditions ; the percentage 
of time that no ducts (or exceedingly small ducts) 
oceur points out limitations in the utilization of 
simple surface trapping. Inasmuch as the transmitter 
should be located within the duct (if the maximum 
