WMOD00297

• Title: Hygroscopic Seeding in Oklahoma
• Date: 10/31/1971
• State: OK
• Weather Modification - Doc Type: Report

The nozzles used on the project were obtained from Spray Engineering
Co. of Burlington, Massachusetts. They were cast iron 6BM nozzles,
with operation and spray characteristics (but not appearance) virtually
identical to the aluminum nozzles on the AF dispenser shown in Figure
3. The problem in a spray system for hygroscopic seeding or for
creating clouds for icing studies is to deliver the hugh mass of material
required while having it dispensed in the correct size range of tiny
particles. No really quantitative data were available for the output of
the AF unit developed at Wright Field. Discussion with Paul Schumacher
there implied that, with proper adjustments of water and air flow and
pressure, the mean diameter would be in the 15-20 pm range. The largest
particle found in the Wright Field tests was 80 pm dia.

It is a difficult task to measure the droplet size. When dispensing water,
with the relative humidity under 1000/0, the droplets will tend to evaporate
before they are picked up. This is perhaps augmented by the heat of the
air just as it issues from the nozzles (several hundred degrees Fahrenheit),
before that air has expanded to ambient pressures and returned to near
ambient temperatures. A droplet calibrator was developed by the Air Force
and made available to this project. It momentarily exposes an oil-coated
slide, the impacting droplets are then partially protected from evaporating
by the oil, and the slides are photographed through a microscope within 2
or 3 seconds.

For this project we felt that calibrations of water droplet dispensing would.
not be particularly fruitful, even though we believe the size distributions
of water droplets and hygroscopic droplets from this dispenser are probably
similar. The slide collection would have to be done in a low concentration
area to avoid overloading the slide, and if enough time is permitted to let
diffusion of the plume lower the concentration, then droplet evaporation in
the ambient conditions introduces an unknown factor.

The hygroscopic particles are conservative as to the amount of hygroscopic
material in the droplet, so if we can ascertain this amount we are not con-
cerned with the actual size of the droplet on which atmospheric water has
condensed. However, the collected droplets must be examined in a carefully
controlled relative humidity so that the ratio of droplet diameter to hygroscopic
material amount is calculable (the lower the humidity, the more accurate is
the calculation). If the droplets are collected in oil, the appropriate ratio of
water to hygroscopic material depends on the fall time and relative humidity
before impact, which is difficult to ascertain. Without oil, the droplet spreads
on the slide differently from a water drop and so its mass cannot be cal-
culated from its visual diameter. All in all, real calibration under the

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