WMOD00434

• Title: The Generation of Secondary Ice Particles in Clouds by Crystal-Crystal Collision
• Date: 11/11/1978
• Weather Modification - Doc Type: Report

2172

JOURNAL OF THE ATMOSPHERIC SCIENCES

VOLUME 35

section. In this. section we now want to treat the
inelastic collisions of a particle with a fixed plate.
Eq. (4), a general equation for any collision, may be
rewritten as

e = Vil- vf;/ Vi~ - vgl,

where vft and viI are final velocities, Vi~ and vgl are
initial velocities and e is the coefficient of restitution.
For the case where one of the objects is of very
large mass, say object kl, the velocity after the col-
lision is equal to the velocity before the collision.
If the velocity before the collision is also zero, as in
the case of the fixed plate,. the following relation
holds:

v~l=vil=O.

Therefore, Eq. (20) becomes

e= -vf;/vg.

in momentum can be calculated for the collision of
a particle of known mass and velocity with a fixed
plate if the coefficient of restitution is known.

(20) 3. Experimental procedures

(21)

a. Description of experimental apparatus

The determination of the fragment generation func-
tions were made by observing ice crystal collisions
with a fixed plate. The instrument designed to make
these observations is shown in Fig. 2. It consisted
of a 16 mm movie camera mounted to view parallel
to the surface of a black plexiglass plate. An optically
black background was constructed behind the plate
to allow photography during daylight hours. A high-
intensity light source was mounted to one side of the
plate to illuminate the falling crystals. The entire
instrument was secured to a heavy adjustable tripod
which allowed the instrument to be rotated so that
the wind was perpendicular to the optic axis of the
camera.
The camera used was a model DMB 4, 16 mm high-
speed movie camera manufactured by the D. B.
Milliken Company of Arcadia, California. It had a
film magazine which permitted the use of 400 ft rolls
of film. A f /1.8 lens with a 5 mm extension was used
to observe the crystals. The depth of field was about
1 cm at 10 cm from the front of the lens. This con-
figuration allowed the image size on the film to be
about one-third actual size and permitted a magnifi-
cation of 40X actual size by projection later. The
film used was Kodak 4-X Reversal black and white
film with an ASA rating of 320. Better resolution
could have been obtained with slower film but the
light intensity with a continuous lamp was limited
by the heat produced. At temperatures near freezing,
the heat from ve~y high intensity lamps would cause
the crystals to melt and stick on hitting a warm
surface. Strobes were not attempted because of the

(22)

FIG. 2. Photographic instrument us~d in the fixed-plate experiment.