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602 WEATHER MODIFICATION to the clouds; obviously, droplets released at some distance would be subject to evaporation. The principal reason why water spray seeding has not been adopted as an operational cloud seeding method lies in the very large quantities of water that must be transported to produce any significant rainfall. If one raindrop 2 mm in diameter is produced for each 100-lLm raindrop embryo released, the value of the water pro- duced would not pay for the necessary aircraft operations unless several clouds could be treated successfully on every mission. Langmuir pointed out in 1948 that an artificial raindrop embryo might grow into a raindrop large enough to become hydrodynamically un- stable and break up, yielding several new rain- drop embryos. This process for multiplying rain- drop embryos is called a Langmuir chain reaction. Today it is believed that most rain- drops break up due to collisions before becom- ing large enough to break up spontaneously, but the generalized concept of a chain reaction is still valid. The logistics of seeding to promote coalescence of cloud droplets are improved if one utilizes a hygroscopic solution or powder. For example, an NaCl particle 20 /Lm in diame- ter released in an updraft below a cumulus cloud will grow to nearly 100 /Lm diameter and, hence, will be able to function as a raindrop embryo on reaching cloud base. The properties of a large number of hygro- scopic materials have been investigated for cloud seeding applications. Some possible agents, such as potassium hydroxide (KOH), must be ruled out on the basis of their highly toxic or corrosive properties. Among those that have been used extensively are ammonium ni- trate (NH4N03), urea, and NaCl. The principal concern in the spraying of hy- groscopic solutions is to achieve a fine spray with droplets on the order of 10 to 20 ILm in diameter. A nonuniform spray containing even a relatively small number of large particles, say, 50 to 100 /Lm in diameter, will have a low parti- cle yield per unit mass and hence waste mate- rial. Devices to produce a reasonably uniform spray are available, but required loads are still doubled or tripled by the nonuniformity factor. Corrosion by the seeding agents themselves is a serious problem. Tanks, valves, and other fit- tings must be made of stainless steel, and the solution must be kept off airplane surfaces, es- pecially control cables. The amount of hygro- scopic solution required to modify a significant cloud volume implies a need for multien~ined aircraft with tanks capable of holding as much as 5000 liters of the seeding agent. The use of hygroscopic powders rather than hygroscopic solutions has the advantage that all water is eliminated from the load. This permits a greater amount of hygroscopic agent to be deliv- ered. However, the use of hygroscopic powders poses special problems. It is important that the material be ground uniformly; otherwise, as much as 90% of the mass being transported may consist of particles much larger than the opti- mum size. NaCI and CaCl2 powders can be sieved to obtain a median diameter of 10 to 20 ILm and reasonable uniformity, although the pro- cess is somewhat laborious. The principal diffi- culty with hygroscopic powders is their ten- dency to take on moisture and form lumps (cake) while in storage. Commercial anticaking additives such as soapstone have been added to the powder on some projects. In other projects powdered N aCt has been stored in heated rooms with resultant low relative humidity in an at- tempt to avoid caking. Once the hygroscopic powders are loaded into an aircraft, it is usually impossible to avoid con- tact with the humid ambient atmosphere. (Glass ampules, which were broken in flight to release small amounts of powdered NaCI, were used on a few experiments in West Africa in the 1950s, but are impractical for operations.) However, mechanical stirring devices or the forced flow of air through the powder to produce a fluidized bed can reduce caking and produce a fairly even flow. A quite different problem is found with urea, which is a desirable agent for many applications because it is almost as effective as NaCl and relatively noncorrosive. Urea crystals tend to shatter into submicrometer particles that are far too small to be useful for seeding and are actu- ally detrimental to fog suppression operations. One method to overcome this defect is to mi- croencapsulate the urea in particles of the de- sired diameter, which is typically 25-50 /Lm. Mi- croencapsulated urea has been employed in warm fog suppression trials by the U.S. Depart- ment of Defense. B. EFFECTS OF DRY ICE AND OTHER CHILLING AGENTS Chilling humid air to temperatures below -40oC causes the formation of large numbers of ice crystals. We have already referred to the use of dry ice pellets for this purpose. Many other