WMOD00510

• Title: The Practicability of Dry Ice for On-Top Seeding of Convective Clouds
• Date: 1/1/1978
• Weather Modification - Doc Type: Report

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VOLUME 17

JOURNAL OF APPLIED METEOROLOGY

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DIAMETER

2

3

45678910
CYLINDER LENGTH

mm

20

.
30 40

FIG. 1. Spectra of dry ice pellet samples showing only a slight reduction in
cylindrical lengths with time. The straight line is the spectrum adopted for the
numerical simulations of pellet sublimation.

chest freezer at - 170C. A typical diameter of the
pellets was determined upon acquisition. Each day
thereafter the mass of the remaining dry ice was
measured, the pile stirred, and a handful of pellets
removed for size measurement. Most pellets had
irregular ends but uniform diameters of about 15.5
mm. The pellet length used to calculate the median
values was an estimate of the length the pellet would
have if it were aperfect cylinder of the same mass and
diameter. Similarly, the small fragments were expressed
in terms of the length of perfect disks of 15.5 mm
diameter. The sample in this environment sublimed
in less than 4 days.
Fig. 1 shows the size. spectra measured and the
spectrum adopted as a description of the typical dry
ice pellets. Table 1. shows the changes in mass, diameter
and length with time. It can be seen that, while the bulk
mass decreased rapidly, the pellet diameter remained
fairly constant. Compared to the mass decr~ase, there
was only a slight reduction in pellet length with time.
Visual examination of the stored dry ice showed that
pellets on the outer surface of the pile sublimed first,
while those on the interior of the pile did not change
size until the outer surface receded to that position.
These observations suggest that the mass decrease was
primarily due to changes in the number of pellets,
which resulted from the complete sublimation of

pellets on the outer surface of the pile. Interior pellets
probably remain in a temperature environment of
about - 80oC, and slight changes in length are appar-
ently associated with fragmentation during stirring
rather than sublimation.
It seems reasonably safe to assume that there is
little size change of most individual dry ice pellets
during storage. However, fracturing might reduce
cylinder lengths, perhaps by 50%, while the pellets
are being loaded into containers for use on the aircraft
and while dispensing the pellets from the aircraft.

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b. Dry ice sublimation rate

Fukuta et al. (1971) have theoretically derived a
sublimation rate of the form

dm
--= [(8.16X 10-ad)/ P](0.097T+9.4P+ 137.3)
dt

X (0.097T+9.4P-25.4)

X[1+6.66X 102 (d3/ P)lT-S/12(T-125.9)-1/6], (1)

where T is in kelvins, P in atmospheres, d in centimeters
and dm/ dt in grams per second. They used a pellet
surface temperature of - 100oC.
Mee and Eadie (1963) actually measured the sub-

TABLE 1. Changes in dry ice pellets d.uring storage.

Elapsed time (h) 0 27 48 72 95
Remaining mass (kg) 13.6 6.7 3.55 1.05 none
Typical sample diameters (mm) 15.8 15.0 15.5 15.5
Range of diameters (mm) * 14.5-15.5 14.5-16.0 14.5-15.5
Median cylinder length (mm) * 4.6 4.8 4.7
Sample size (pellets) 21 50 39 71
* Not accurately measured.

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