How much rain does cloud seeding make?

The rapidity of the increase in size of the embryo raindrop depends on its size. In order for the embryo to reach the size of a raindrop with a diameter of 1,000 microns within a few minutes, the initial size of the large cloud droplet (the initial embryo raindrop) must be at least about 50 microns. The formation of embryo raindrops requires the presence of giant cloud condensation nuclei. These are normally derived from the evaporation of spray from the sea surface. From the point of view of making rain by cloud seeding, the important conclusion from the foregoing discussion is the requirement that giant condensation nuclei must be present in the cloud so that raindrops can form. Therefore, there is a need for cloud seeding to make rain only if there is a deficiency of giant cloud condensation nuclei in the cloud. Furthermore, if this deficiency exists, this can be eliminated by using aircraft to release giant salt condensation nuclei in the cloud.  In this connection, it is important to note that there should be an abundance of such giant nuclei in the atmosphere over the Philippines. This is due to the fact the Philippines is surrounded by the South China Sea and the Pacific Ocean.  In brief, there is no scientific justification for cloud seeding in the Philippines. In other words, the present seeding operations here are highly controversial and are similar to the situation in South Dakota about 30 years ago. At that time, an organization called the Citizens Against Cloud Seeding in the state of South Dakota staged protest rallies against cloud seeding.  Many of its members thought that the program was ineffective and, therefore, a waste of public money. Are the cloud seeding operations in the Philippines also a waste of public money?

The idea of making rain by releasing giant condensation nuclei was introduced about 50 years ago. Since that time, many seeding operations have been conducted.  Some of the well-known operations were those conducted in Africa, Pakistan, India and Thailand.  The cloud seeding in Thailand is very interesting from the point of view of the seeding operations in the Philippines since the technique of seeding used and the atmospheric conditions in both countries are similar. A critical assessment of the cloud seeding in Thailand, Africa and Mexico has been made by Silverman (See May 2001 issue of the Bulletin of the American Meteorological Society). He said that those who are contemplating operational hygroscopic seeding projects for precipitation enhancement based on the statistically positive experimental results in South Africa, Thailand and Mexico should be aware that in the absence of physical evidence required by the proof-of-concept criteria, this cloud seeding technology is unproven. The assessments of cloud seeding are usually made with the aid of radar and rain gauge observations.

In a related policy statement on the use of cloud seeding to enhance precipitation or rainfall, the American Meteorological Society Council on Weather Modification said in 1998: “Precipitation augmentation through cloud seeding should not be viewed as a drought relief measure. Opportunities to increase precipitation are usually few, if any, during droughts; consequently, the cost of mounting a cloud seeding operation will far exceed the benefits that may be obtained.”

Additionally, the World Meteorological Organization (WMO) issued a WMO Statement on Weather Modification at the end of its Executive Congress in 2001. Its statement: “Despite the statistical evidence of radar estimated precipitation changes in the individual cloud systems in both glaciogenic and hygroscopic techniques, there is no evidence that such seeding can increase rainfall over significant areas economically.” Glaciogenic seeding refers to the seeding which involves the release of silver iodide or dry ice. On the other hand, hygroscopic seeding involves the release of salt particles or water droplets.  Hygroscopic seeding is the technique used in cloud seeding operations in the Philippines.

On the basis of the above statements, one concludes that the effectiveness of cloud seeding for making rain is highly controversial. In view of this highly controversial nature, there should be a requirement for future seeding operations in the Philippines. The requirement is the following: Every operation must include a provision for a scientific evaluation of the effectiveness of the rain-making technique using rainfall observations. This requirement should be applied to the planned continuation of seeding in the Cagayan Valley (The Philippine STAR, Sept. 2, 2007) and other future seeding operations.

I end this report by returning to the question: How much rain does cloud seeding make at the targeted area? Current scientific knowledge indicates that the answer is: The amount of rain produced inside targeted areas by the seeding of cumulus clouds is insignificant. Therefore, one can conclude that cloud seeding of cumulus clouds is not cost-effective.

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Mariano A. Estoque is a founding member of the Philippine-American Academy of Science and Engineering. He graduated in 1950 from New York University with a Ph.D. in meteorology. After graduation, he spent the next 35 years, doing research and teaching in various universities in the US and Canada. In connection with cloud seeding, he participated in the cloud seeding research of the Florida Area Cumulus Experiment in 1975-76. He returned to the Philippines in 1985 to become a visiting professor at the UP Department of Meteorology and Oceanography. He was awarded the 1997 International Meteorological Organization Prize by the World Meteorological Organization. More recently, the American Meteorology Society’s Committee on Meteorology and Oceanography of the Coastal Zone awarded him recognition “for his seminal and continuing contributions to the modeling of the atmospheric planetary boundary layer and sea breezes” during the 7th Conference on Coastal Atmospheric and Oceanic Prediction and Processes of the American Meteorology Society last September in San Diego, California. He is currently doing voluntary work as a science consultant in the Philippine Atmospheric, Geophysical, Astronomical Services Administration and the Manila Observatory.

E-mail him at [email protected].