How much rain does cloud seeding make?

This article consists of two parts. Part 1 describes how the atmosphere makes rain naturally without the use of cloud seeding. Part 2 describes the scientific basis of cloud seeding and the results of past cloud seeding operations. Most importantly, Part 2 presents overwhelming scientific evidence that cloud seeding is a waste of government money.

Cloud seeding is the act of releasing some material, usually salt particles, inside a cloud for the purpose of making rain. In the tropics, the clouds, which are normally seeded, are cumulus clouds. Small cumulus clouds look like white cotton balls with flat bases and rounded tops. On the other hand, big cumulus clouds are sometimes massive towers with either round tops or flat tops, which reach heights of many kilometers above the ground. These tall clouds usually produce rain. A targeted area is one where the rain due to cloud seeding is required to fall. An example of a targeted area for cloud seeding operations is the Angat River Basin.

During the period of about three months which ended in the middle of August 2007, a dry spell affected many areas in Northern and Central Luzon. In order to make rain in the affected areas, the National Disaster and Coordinating Council (NDCC) conducted cloud seeding operations. Cloud seeding involves the use of aircraft for releasing the seeding material in the cloud. According to a GMA News report last July 31, the cost of hiring an aircraft for seeding operations is P1.7 million per month. A more recent report (Philippine STAR, Aug. 22, 2007) indicates that P600 million was released by President Arroyo for cloud seeding operations. Many of my colleagues have asked the question: “Are cloud seeding operations cost-effective?” More specifically: “How many cubic meters of rain are produced by seeding operations which cost P1 million?” Before attempting to answer these questions, let me first describe some characteristics of cumulus clouds and how rain from cumulus clouds is produced naturally without artificial cloud seeding.

Cumulus clouds are generated primarily by the heating of the ground during the daytime. The size of these clouds depends on the prevailing atmospheric conditions of temperature and moisture. During the month of April and other dry season months, cumulus clouds are small in both vertical and horizontal dimensions. These clouds are called fair weather cumulus clouds. They do not produce rain. In contrast, during the rainy season, cumulus clouds are large vertically and horizontally. These big clouds, some of which are called thunderclouds, produce heavy rainfall; they are accompanied by lightning and thunder.

A cumulus cloud consists of rising warm parcels of air which originate from the ground. During its ascent, an air parcel expands and cools. The cooling eventually results in the condensation of water vapor into cloud droplets. The condensation requires the presence of cloud condensation nuclei. The typical sizes of the cloud droplets and the condensation nuclei are 10 microns and 0.1 microns, respectively. One micron is equal to a thousandth of a millimeter. The initial size of a cloud droplet at the time of formation depends primarily on the size of the cloud condensation nuclei. Large cloud droplets are formed by correspondingly large nuclei. For convenience in future discussions, we call the large cloud droplets which are large at the time of formation, as embryo raindrops. This terminology is used because embryo raindrops eventually become full-size raindrops. A cloud droplet grows in size into a raindrop primarily by a physical process called accretion, sometimes also called coalescence. Accretion increases the size of a cloud droplet (the collector droplet) to raindrop size while it is being transported up and down by air currents inside the cloud. During its movement, the collector droplet increases in size by colliding and coalescing with smaller droplets. The rate of increase by accretion depends on the size of the cloud droplets. In particular, the embryo raindrops, which are big cloud droplets, grow fast due to their bigger sizes. Under favorable conditions, the embryo raindrops grow to big raindrops with diameters of approximately several millimeters within a few minutes after formation. Subsequently, the full-size raindrops break up into smaller embryo raindrops. These embryo raindrops also grow by accretion and eventually break up. Thus, a chain reaction of raindrop production occurs.

I conclude Part 1 of this article by emphasizing the fact that the atmosphere needs giant cloud condensation nuclei in order to generate rain.

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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 PhD 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 a 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 and Astronomical Services Administration and the Manila Observatory. E-mail him at [email protected]