APPLICATION OF FORECASTING TECHNIQUES AND CLIMATOLOGY 



127 



nomogi'iims and taking the data for (1) the mean 

 temperature and wet bulb depression of the air over 

 tlie sea and tlie mean sea temperature, and (2) tlie 

 mean temf)erature and rehitive liumidity of the air 

 at a land station (wlien tlie given square was near a 

 coast) and the mean sea temperature. 



3. For each square for eacli month note was made 

 of the various surface wind velocity ranges that oc- 

 curred with each wind direction (eight points of the 

 compass) and the percentage of time the wind lay 

 within each velocity range.' 



4. In terms of the mean temperature excess and 

 mean M deficit, the corresponding duct widtli was 

 computed for each wind velocity range, and, knowing 

 tlie percentage of time that winds of each magnitude 

 occurred, it was possible to compute the percentage of 

 time that ducts of various widths would occur, both 

 for each wind direction (on an eight point compass) 

 and for the overall picture regardless of wind direction. 



Eesults of Compotations 



The results were summarized by lumping individual 

 squares showing similar characteristics into nine re- 

 gions within the whole area. In each region data for 

 the individual months were lumped together on the 

 basis of similarity to divide the year into three or four 

 parts (these varying according to region). Then for 

 each group of months in each I'egion were listed those 

 results which were considered to be the most impor- 

 tant, namely, 



1. The range in duct widtlis, giving an indication 

 of the variability at any one jilace at any one time 

 of year. 



2. The percentage of time that ducts characterized 

 by widths greater than 40 ft occur and, following this, 

 tlie most prevalent wind direction associated with 

 ducts of these widths, as well as the minimum wind 

 velocity necessary to establish them. 



3. The percentage of time that ducts characterized 

 by widths from 20 to 40 ft occur, similarly followed 

 by the associated prevailing wind direction and the 

 minimum required wind velocity. 



4. The percentage of time that ducts do not occur 

 or have a width less than 20 ft. 



Figure 20 contains these summarized results. The 

 numerical listings in the figure make no claim toward 



'This led to a slight error, inasmuch as the wind at 1,000 ft 

 should have been used in place of the surface wind (data 

 were available only for the latter). This error in most oases 

 resulted in calculated duct widths of slightly less magnitude 

 than would have been obtained if the 1,000-ft wind had 

 been used. 



being exact, as is evident in view of the remarks made 

 in Section 8.3.3 on the applicability and limitations 

 of the method, in addition to the slightly erroneous 

 computations of the monthly mean M deficit and the 

 use of the surface winds instead of those at 1,000 ft. 

 ( The effect of the latter two errors is mainly to cause 

 the calculated duct widths to tend toward the conserva- 

 tive side.) In Figure 20, for purposes of consistency 

 throughout the area, only the calculations based on 

 air temperature and humidity records over the sea 

 were used [i.e., only (1) under "Method of Compu- 

 tation"]. The difference between these calculations 

 and those based on land station data is primarily that 

 temperature excesses and M deficits are larger in the 

 latter case (this again tends to make the tabulated 

 results conservative) . 



Other deficiencies of the method which might be 

 cited are the neglect to take into account the occur- 

 rence of fog or rain (which tends to create substandard 

 or standard conditions) and the omission of the in- 

 fluence of local effects such as tlie topography along 

 coast lines (see Section 8.3.5). In some cases the 

 period of record was fairly short, so that the data 

 used in the calculations were not always completely 

 representative. Lastly, the ranges of duct widths were 

 calculated on the assumption that there was a varia- 

 tion of wind speed only, not allowing for possible 

 variation in temperature excess and BI deficit. 



In spite of these shortcomings, the calculated re- 

 sults show regional and seasonal trends consistent 

 Ijotli with wliat might be expected on the basis of 

 qualitative physical reasoning and also with a limited 

 number of actual observations taken in the Pacific 

 (see Section 8.3.5). In conclusion we may safely state 

 that the results represent to a reasonable approxima- 

 tion the average conditions of surface duct width and 

 variability. 



Use of Computed Eesults 



Since the computations are based on climatological 

 data and are limited in exactness, they do not have 

 forecasting value in tlie sense of indicating specifically 

 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 



