Chapter 11 — WATCH ROUTINES (CONTINUED) 



Cover the legend with Scotch tape. When the 

 overlay is turned over, the data in the legend 

 can be entered in grease pencil for the particu- 

 lar fallout diagram to be plotted. 



The RADFO diagram consists basically of 

 two somewhat elliptic contour lines as follows: 



1. Low-Yield Explosion. A RED contour line 

 is used to indicate the limits of an area where 

 radioactive fallout may be expected to exceed 

 200 roentgens in 48 hours when a low-yield 

 nuclear weapon is exploded. 



2. High-Yield Explosion. A BLACK contour 

 line is used to indicate the limits of an area 

 where radioactive fallout may be expected to 

 exceed 200 roentgens in 48 hours when a high- 

 yield nuclear weapon is exploded. 



3. Explosion of Unknown Yield. If there is 

 no available information relative to the amount 

 of yield, use the black (high-yield) contour. 



4. Fallout Advance. Fallout does not occur 

 simultaneously over the entire area outlined on 

 a RADFO diagram. It begins in the vicinity 

 of the burst and advances as a function of 

 time and the wind speed. If desired, the area 

 primarily affected by fallout actively precipita- 

 ting at a specified time after the explosion can 

 be determined and marked on the overlay. 



Plotting Procedure 



The following steps should be followed in 

 constructing a RADFO diagram on the over- 

 lay: 



1. From the coded message, select the first 

 geographic point for which fallout forecast data 

 are to be plotted. 



2. Selsct the template indicated for the low- 

 yield trajectory from this point. 



3. Mark a small x on the overlay and label 

 SZ (surface zero). 



4. Using a protractor, plot the effective fall- 

 out wind through the SZ. This is the fallout 

 axis. 



5. Along the fallout axis, plot the down- 

 wind distance from SZ. 



6. Select the appropriate template. Place SZ 

 of the overlay over SZ of the template. Select 

 the template contour which crosses !he axis 

 nearest the downwind distance. Trace this contour 

 in the appropriate yield color. 



The procedure outlined above can also be 

 repeated on another overlay for a second geo- 

 graphic location. 



Fallout does not occur simultaneously ovyr 

 the entire area indicated on the overlay. It 

 starts in the vicinity of surface zero and moves 

 downwind along the trajectory of the fallout 

 pattern. The rate at which it advunces is a 

 function of time and the wind speeds of the 

 various layers of the atmosphere through which 

 the radioactive particles fall. For fallout pur- 

 poses, these winds are treated in a manner 

 analogous to ballistic winds. Wind speeds and 

 directions in the various layers are weighted 

 and averaged, thereby obtaining a single wind 

 speed and direction which is called the Ef- 

 fective Fallout Wind (EFW), 



The RADFO overlay may be marked to show 

 the approximate zone in which fallout occurs 

 at a specific time following the burst, as shown 

 in figure 11-28. Procedures for determining 

 this zone can be found in ATP-25. 



Use of the Overlay 



Place the overlay on the maneuvering board 

 or on the map or chart for which it was de- 

 signed, so that SZ falls on th? center of the 

 board or on the map location of the point of 

 detonation. Orient the overlay so that the 

 NORTH or SOUTH (as appropriate) parallels 

 the north-south axis of the maneuvering board 

 or chart, and is directed in the proper sense 

 to correspond with the protractor scale used. 

 The contour lines indicate the forecast fallout 

 areas with respect to geographic positions. 



RADIOACTIVE FALLOUT 

 COMPUTATIONS 



If RADFO information is not available, it 

 must be prepared locally. The procedures out- 

 lined in Nuclear Fail-Out Forecasting and Warn- 

 ing Organization ATP 25 (NAVY) (AIR), describe 

 the various methods used to prepare a RADFO 

 plot or message. Depending upon the facilities 

 available, the requirements, and the data avail- 

 ability, one of several procedures may prove 

 to be more suitable than another. 



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