Fallout does not occur simultaneously within the predicted fallout zones. It begins to fall in the vicinity of GZ and moves through the zones at the approximate speed of the effective fallout wind.

After plotting the zone of most hazardous fallout, determine the arrival time of the fall-out at the sea surface, using the following procedure:

1. Multiply the effective downwind speed by the time (in hours) elapsed since the detonation to determine the distance the fallout has traveled.

2. Add and subtract a safety distance. Safety distance is provided on the ships fallout template for the standard yield groups, or it can be

determined from the graph in figure 6-9-14. Safety distance allows for unlimited cloud size, diffusion, and wind fluctuations.

3. Plot the distance obtained in step 1, and the two safety distances obtained in step 2 on the template using GZ as the center. Draw these distances as two arcs across the fallout pattern between the radial lines. In most cases, the area enclosed by the two safety distance arcs and radial lines is the zone of deposited fallout (at the surface) at the specified time after detonation.  

Example:

Determine the area where the fallout will fall 1.5 hours after detonation with an effective down-wind speed of 16 knots. Determine safety distances, using yield group Delta.

16 knots x 1.5 hours = 24 nautical miles (nmi) With GZ as the center and 24 nmi as the radius, draw a dotted arc across the fallout field at 24 nmi. This arc represents the middle of the area within which fallout may be expected to reach the surface at H+1.5 hours. From the template, the safety distance for yield group Delta is 5 nmi. Add 5 nmi to and subtract 5 nmi from 24 nmi:

Draw two arcs across the fallout pattern at these two distances. The area delineated by the two arcs and the radial lines (cross-wind boundaries) of the fallout area defines the approximate area of fall-out deposition at 1.5 hours after the detonation. See figure 6-9-15.

Learning Objective: Compute effective downwind directions and speeds using standard-pressure-level winds.

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