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The data on pressure, temperature, density, and viscosity in
100G-ft intervals to 120,000 {t are summarized in Table'1.”
* 2.1.6
Terminal Velocity Computations
The average falling speed through 500U-ft layers was computed
for four particle sizes over an altitude range from @ to 120,000 ft. In
these computations all in-flight transition of the particles from streamline to intermediate flow had to be considered through use of the pis:
shows in Fig. 8.
Four particle sizes (75, 100, 200, and 350 p diameter) were
employed since there was evidence from past tests that the 75-p particle
“defined the limi ing distance of failout of interest and the larger sizes
best described the ‘pattern within this limit.
Table 2 presents the falling
speeds computed for the four sizes. Tables 3, 4, 5, and 6 display the
cumulative time ef fall from a given altitude for these particle diameters.
2.1.7
Meteorological Procedures
It is necessary to have available the best possible description
of the winds aloft in order to determine the arrival points of particles of
various sizes originating at various altitudes, Such data are usually availabel from the normal upper air soundings routinely taken by Weather
Bureau and Military Meteorological stations. Although wind velocity as
a function of height varies continuously it can be described by an average
speed and direction in discrete layers. Such averaging can best de
obtained from the WBAN-20 Form where the original data are recorded.
The technique employed in this report was to divide the atmosphcr> into
layers 5000 “t thick and determine an average speed and direction for
each layer. When the average falling speed of particles through these
5000-ft ayers and the speed and direction of the wind are known, hori_ zontal displacement can be computed. Thus, for each particle size a
vector may be drawn for the average particle displacement in a particular
5000-ft layer, A? ition of such vectors from all layers described the
trajectory projection of a particle of given size. Similar plotting for ail
particle sizes originating at all elevations within the cloud source will
map the fallout on the earth's surface,
Me
This technique is valid for any atmosphere that has negligible
vertical motion and is in a steady state conaition with respect to the
horizontal winds during the time needed for the slowest particle to fall
from the highest altitude to the ground. Such an assumption is not
realistic for situations arising from many of the megatona devizres because 15 to 20 hr are necessary to establish the fallout area. Consequently, wren computing particle trajectories, an attempt should be
*
A great “dealofexcellentupper aut da:a forthe MarshalGeiss was obtaiaed at Operation REDWING
in 1956. Reduction of these data will result ina much better desenption of tae Macks)! Islands
atmosphere thar. has beéa previousls available,
made to consider how the wind varies with time and bow it varies with
distarce frorr. ground zero; what effect vertical motions have on particle
falling speeds and how they vary with space and time. Sach considerations complicate computation of trajectories extremely. In most cases
valia input data describing these variables are not available. This phase
of the problem is discussed below.
2.2
Plotting Technique
The use of “particle.size" and “bcight“ tines in mapping fallout is
a standard technique employed by mos? analytical methods. This tech-
nique simply describes a grid (Fig. 9} on the earth's surface indicating
where fallout particles of certain sizes willarrive and from what alutude
they came. These par2:neters are the basic data for describing the fall-
out pattern.
.
.
Assuming steady state meteorclogics] conditions without vertical
motion oF space variation of the winds, it is very easy to construct a
grid describing arcivai points on the earth's surface for particles of
various sizes oviginating at different altitudes, This grid is constructed
by ignoring the horizontal distribution of partictes in the cloud model
and by ‘plotting those trajectoris s that origicate along the line sMurCce
describing the vertical axis of the cloud.
Plotting trajectories for each particle size at every starting elevathom is the first step in determining the resultant fallout pattern; however,
the drafting involved is tedious and time-consuming. This cffort can be
reduced greatly by plotting from the ground up, as is done: in the construction of a wird hodograph. Such plot is mace by stamicg at ground
zero and working up through the altitude increments to the desired elevation, Aithough this technique does nei plot the trajectory of the particie,
it does define the arrival points on the surface of the earth of particles
starting at each altitude increment (Fig. 10}. Te pict these size-lines
one must make the prelissinary computations of sarticle-falling times
through each altitude increment to obtain the displacement for various
wind velocities as described earlier in the Section on Terminal Velocity
Computations (p 8).
A plotting device (Fig. 11}, described elsewhere, ‘acilitates the
computations required for the size-lines of the fallout pattern. Such
devices were constructed for foux particle sizes: 75, 106, 200, and
350 p in diameter. With these plotters, trajectories cz size-lises can
be plorted from anv elevation up to 126,606 fs for the four particle sizes.
The plotters automatically acccurt for the variable particle falling
speed, They also eliminate tee need for drafting equiptment. After establishing the particle arrival posnts by either the use of sive-lines or
trajectories, heigni? lines can be constracted. These lites, joining
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