Construction of the Left and Right Radial Lines.
Measure the angle formed by the radial lines drawn from ground zero to
the cloud-top height and two-thirds stem height points on the fallout
vector plot (or the radial lines which have been expanded to include
vectors between the two-thirds stem height and the cloud-top height). If
the angle formed is less than 40 degrees, bisect the angle and expand the
angle formed by the two radial lines to 40 degrees, 20 degrees on each
side of the bisector, (Figure 4-3).
Measure the azimuths, in mils or degrees from grid north, of the final
left and right radial lines and record on lines p and q of the worksheet
(use four digits).
Figure 4-4 on page 4-8 is a completed detailed
fallout prediction worksheet.
Start with GZ on an overlay at the selected map scale, and extend the
radial lines at their proper azimuths to any convenient distance. Mark
grid north on this overlay. (The fallout wind vector plot was originally
drawn to a convenient map scale; for example, 1:500,000. If it is more
convenient, a different map scale can now be used to complete the fallout
Between the two radial lines drawn from ground zero, using ground zero as
center, draw two arcs with radii equal to the Zone I and Zone II downwind
Using ground zero as center, draw a circle around ground zero with a
radius equal to the cloud radius at the selected map scale.
Draw two tangents extending from the ground zero circle to the points of
intersection of the two radial lines with the Zone I arc.
Using ground zero as center, indicate the estimated times of arrival of
fallout by drawing dashed arcs downwind at distances equal to the product
of the effective wind speed and each hour of interest.