WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

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Particles with molecular spacings and crystallographic arrangements similar to those of ice
(which has a hexagonal structure) tend to have good ice nucleating properties and a high
threshold temperature. Most good ice nuclei are also virtually insoluble. Table 1 gives the
threshold temperatures and the structure of some natural and artificial ice nuclei.

Table 1: Crystallographic structure and threshold temperatures of
some natural and artificial ice nuclei

natural IN artificial IN
Covellite hexagonal -60 C Ag I hexagonal -40 C
Tridymite hexagonal -7D C Pb 12 trigonal -40 C
Kaolinite tric linic -90 C Cu S hexagonal _60 C
Magnetite cubic -90 C Methaldehyde tetr-lo C

Many organic materials are good ice nucleators. The University of Wyoming group has recently
discovered that decayed biological matE'rial contains copious ice nuclei (see Vali et al..
1976). In weather modification pyrotechnics of Ag I and Pb 12 are widely used as seeding
material. In order to disperse them in a cloud. a direct injection technique should be used,
since there is evidence (Federer et al.. 1979) that the nuclei are deactivated in the warm
parts of the cloud due to the blocking of active nucleation sites by impurities of the
pyrotechnics.

Measurement of IN

Several techniques exist for measuring the concentrations of IN. A common method is to
draw a known volume of air into a contcliner and to cool it until a cloud is formed (Fig. 7).
The number of ice crystals forming at CI particular temperature is then measured. In
expansion chambers the cooling is produced by compressing the air and then suddenly expand-
ing it, and in mixing chambers cooling is produced by refrigeration. In these chambers.
particles may serve as freezing, contact or deposition nuclei. In laboratory investigations
on artificial nuclei it is sometimes possible to distinguish between these 3 mechanisms
(Langer et al., 1978). The number of ice crystals which appear in the chamber may be deter-
mined by illuminating a certain volume of the chamber and estimating visually the number of
ice crystals in the light beam. by letting the ice crystals fall into a dish of supercooled
sugar solution where they can be detected and counted by the number of larger crystals that
they produce, or by allowing the ice crystals to pass through a small capillary tube
attached to the chamber, where they produce audible clicks which can be counted electronic-
ally (Langer. 1973). In another technique. a measured volume of air is drawn through a

Sugar solution
In tray

Lid clamp

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Air
intake

Fig. 7: Schematic design of thH U.S. Weather Bureau version of a Bigg-Warner box
(From Byers, 1965).