WMOD00297

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

The Hygroscopic Material

For this program, we used a nitrogen solution fertilizer called "320/0
solution" (measuring 320/0 nitrogen). We obtained it from Nipak, Inc.,
P. O. Box 2820, Dallas, Texas, a subsidiary of Lone Star Gas Co.
lt consists of 44. 10/0 ammonium nitrate (by weight), 35.40/0 urea, O. 10/0
free ammonia, and 20.40/0 water. The specific weight is 1. 33 (10. 5
lbs/ gallon). The liquid is colorless. It appears to spray essentially
the same as water. It salts out if cooled below OOC. Its characteristics
have been examined by Clark, Burkhardt, and Carroz (1971) and
Burkhardt (1970) of Naval Weapons Center.

The material has a chemical ph of 8.5. It is considered corrosive to
some materials. It does not corrode aluminum particularly, and is often
stored in aluminum tanks. On farms, it is also regularly handled by
plastic, cast iron, and other high carbon iron such as water pipe. It is
somewhat corrosive to copper and bronze, and more so to some steels.
We made qualitative tests of its corrosiveness before completing the
C-97 system. We avoided copper wherever possible in our system.
The dispenser system we used in Oklahoma operated reliably; corrosion
within the system was not a problem. Protecting the aircraft from
corrosion was a problem requiring 'us to take various precautions.
Plastic sheet was used many places to seal off areas from potential
spills and we regularly used water for cleaning. Clark, Burkhardt and
Carroz (1971) have carefully investigated the corrosiveness of this
hygroscopic material and also developed effective neutralizers.

This fertilizer mixture is very hygroscopic. It starts picking up water
at below 300/0 relative humidity. For cloud seeding application the
important factor is how much water vapor condenses on a particle at high
relative humidities, say over 950/0. The following are a few generalizations
from the calculations by N. Fukuta cited by Clark, Burkhardt, and Carroz
(1971). These calculations start with a particle having initially only 50/0 water
rather than the 20% we actually used.

At 950/0 relative humidity, at equilibrium the particle has grown 2.2 x in
diameter, 10.6 x in volume, and 8.0 x in mass (for all particles beyond
several pm, where curvature effects on vapor pressure can be neglected).

at 970/0 the numbers are 2.5 x, 15 x, and 11 x.
At 980/0 the numbers are 2.9 x, 24 x, and 18 x.
At 990/0 the numbers are 3.6 x, 47 x, and 34 x.

8