WMOD00486

• Title: Microphysical Effects of Wintertime Cloud Seeding with Silver Iodide Over the Rocky Mountains - Part III
• Date: 10/10/1988
• Weather Modification - Doc Type: Report

OcTOBER 1988

ARLIN B. SUPER AND BRUCE A. BOE

1177

I,
,..

for recorded intervals, and the same 4.7 X 7.1 cm area
of each sample was photographed. Deliberately pho-
tographing the same area of every sampling plate elim-
inated the possibility that the camera operator might
select the largest or most interesting crystals as subjects.
The 35 mm negatives were projected at a magnification
of 13.5, and each crystal visually evaluated according
to size and habit. The size bins employed were the
same as those used for evaluation of the aircraft 2D-C
probe images. Crystals were classified as either dendrite,
aggregate, graupel-like, hexagonal (compact), or irreg-
ular. Holroyd's (1987) spherical category was included
with the hexagonal category in the photography eval-
uations. Most of the crystals in the former were smaller
than about 0.4 mm, too small to classify more precisely.
The typical photograph contained images of 230 ice
particles collected over a 30 s interval. The average
sample volume per photograph was approximately
15 L.
Precipitation began at the Snow Lab at 1158 and
continued at rates up to 1.1 mm h -1 until 1237 (Fig.
10). Most of the crystals were classified as hexagonal
(plates) or irregular, the latter largely being fragments

of dendrites. Most of the precipitation mass was com-
prised of dendrites between 0.6 and 1.0 mm, aggregates
( of dendrites) up to 10 mm, irregular crystals, and even
some heavily rimed graupel-like snow. The period of
precipitation coincided with the passage of the seeded
zone over the Snow Lab (see Figs. 8, 11). Virtually no
snowfall was recorded within 0.5 h before or after the
passage of the seeded zone. Precipitation amounts up-
wind of the Snow Lab ranged from 0.025 mm at the
Land's End gage site (between 1145 and 1200) to 0.150
mm at the GMO East and GMO West gages (between
1200 and 1245), coincident with the passage of the
seeded zone. The mechanical gages are known to re-
spond for some minutes after the cessation of precip-
itation. No gage indicated precipitation after 1300, and
none at all was recorded at Ward Creek, the only site
downwind (and downslope) of the Snow Lab.

b. Airborne seeding experiment 2

The second arc of AgI was generated 19 km upwind
of the Snow Lab, half the distance of the first. When
seeding was completed at 1252, the orographic cloud

E
E 1.5
w
~
a: 1.0
..J
..J
e:(
~0.5
o
z
(/) 0.0

_2.0
1:

Snowfall at Snow Lab
AIRBORNE SEEDING
Experiment 1
18 March

1200

~ 75
U
z
850
r< ..J
~
g? 25
a:
t u
r 0
..J;;:- 1.0
..J.r:.
e:(E
u..E
~~ 0.5
z~
(/)e:(
a:
0.0
1200

HABIT
!Nii GraupeHike
.. ., Hexagonal/Compact
Dendritic
Aggregates
Irregular

FIG. 10. Characteristics of the snowfall recorded at the Snow Lab by photography of sedimentation slides for the duration
of the first airborne seeding experiment. Sampling intervals are shown by the black rectangles along the abscissa.