Cloud Seeding Possibilities; Specific Effects upon the Tucson Area

Brad Durkee

Abstract

Cloud seeding is defined as the addition of seeding agents into supersaturated air to generate the formation and growth of cloud droplets or ice crystals. Simply put, cloud seeding is when rainfall of a cloud is increased by the addition of chemicals. This method's supreme goal is, dissipating fog, enhancing precipitation, or subdue hail. Most of the information was done on the WEB, but the most useful information was the pamphlets, sheets, and speech given to me by Dr. Riley, the professor of the class. Cloud seeding was found to be slightly successful in many areas of the U.S. Test results of an experiment done near Tucson did not reveal a definitive conclusion of whether cloud seeding near Tucson was successful or not. The information collected from across the U.S. and the Tucson experiments made me favor using cloud seeding to prevent the worsening of the Tucson aquifers now and any possibility of a water shortage in Tucson aquifers in the future. If successful, cloud seeding could be used to fill the Tucson aquifers, which helps everyone in the Tucson area by making it possible to not use costly canal water until a later time.

Introduction

My topic is cloud seeding. I am looking into the uses and the stability of cloud seeding that would help Tucson. The increase of rainfall by the use of cloud seeding could increase the water that falls on Tucson, which will increase the buildup of water in Tucson aquifers. These aquifers are the primary source of water for the Tucson area so this process would be highly beneficial. The using of cloud seeding would enable the Tucson area to not use the water that is shipped to the city from the Colorado River through the use of canals. The city may be forced to use this form of water in the future. If cloud seeding were successful, Tucson would be able to continue to use the water from the aquifers to sustain the city's need for water and not have to use and pay a lot for the canal water shipped in from the Colorado River.

Materials and Methods

The Internet was the first area I chose to look for information. On the Internet there were few sites that had information on my topic. While most of those had only biographical data on simply how and why cloud seeding is used. Most of my background information was found through the use of the Internet. However, the biggest help was the articles, pamphlets, and speech that my instructor, Dr. Riley, provided me after I stated I was having trouble finding information specifically on the subject of cloud seeding in Tucson.

Results

"Cloud seeding is the introduction of seeding agents into supersaturated air to induce the formation and growth of cloud droplets or ice crystals. The ultimate goal of dissipating fog, enhancing precipitation, or suppressing hail." Common agents used separately in the process of cloud seeding are dry ice pellets, salt crystals, and silver iodide particles.

Salt crystals have been found to produce the best effects in continental clouds, while dry ice and silver iodide have had the most positive influences on "preexisting clouds of supercooled liquid droplets" which means that they are below zero degrees Celsius. The best temperature for cloud seeding on existent clouds is between the temperatures of minus fifteen degrees Celsius and negative five degrees Celsius. These temperatures have been found to be the most effective in producing precipitation. (sunysuffolk)

The ambient temperature is reduced to below negative forty degrees Celsius when dry ice is introduced. At this temperature, ice crystals form without nuclei. The ice crystal's amount of concentration increases substantially with using either method. (sunysuffolk)

Cloud seeding with salt crystals increases the particle size and settling velocity within clouds. Extremely small droplets compose many continental clouds and this is the main reason that these clouds do not precipitate. The droplet sizes are very similar, so they do not collide very frequently. To form larger droplets, the salt crystals, which are about a ten-micron radius, are introduced in an existing cloud. These so-called "super droplets" coalesce to form larger droplets by settling quicker than the original cloud droplets. Coalescence is the sweeping up of small droplets by bigger droplets. As droplets become larger they fall faster because of their size.

With silver iodide, the cloud's droplets are in a sense tricked into thinking that there are already existing ice crystals so they create even more. What happens in the cloud is that the cloud droplets are attracted to the false ice crystal and accumulate rapidly. As more and more cloud droplets attach themselves to the false ice crystals and real ice crystals are formed and gain weight. A chain reaction is then enacted which takes effect within just a few minutes of the silver iodide crystals addition. The chain reaction consists of the rapid creation of droplets of water within the clouds. The droplets can then become large enough to precipitate rapidly to the earth's surface. (sunysuffolk)

In addition, some information about actual rainfall is that after the cloud drop becomes large enough it will fall to the earth. The falling usually occurs when an ice crystals gain weight from cloud droplets accumulating on them. Before the seeding of the clouds, ice crystal nuclei is in short supply. The air cannot sustain the weight of the formed frozen rain droplet and the droplet begins to fall. The frozen raindrop then drops through the freezing level and melts on the way down. (Riley) The entire process takes around "60-90 minutes." (Biggerstaff) Diagrams in the appendix illustrate the stages of a thunderstorm cloud where silver iodide seeding has been most effective.

Silver iodide is a close structural match to that of ice nuclei. This process begins by again using the large difference in size between the cloud droplets and the ice crystals. There is also a huge difference in the concentration of the two kinds of atmospheric particles. With the fact that the natural ice nuclei amount is usually lower than that of the condensation droplet nuclei, it is known that droplets would only form at the level of supersaturation. Ice crystals could possibly form at forty degrees Celsius and colder.

The introduction of dry ice, which was already stated, lowers the temperature of cloud droplets to the point where they will freeze without ice crystal nuclei. On the other hand, silver iodide, which has a crystalline shape similar to ice, leads to the formation of real ice crystals. This process is most effective in supercooled clouds when there is an absence or shortage of ice nuclei. At warmer temperatures, the amount of concentration of droplets and ice would effect the amount of droplets and ice in cloud formations. It is known that clouds with a temperature between negative five degrees Celsius and negative fifteen degrees Celsius are made largely of supercooled cloud droplets. The formation of ice crystals with the help of silver iodide begins when new nuclei are introduced. (sunysuffolk)

Substantial cloud seeding research has been conducted. There was a testing done of a "Cell idea and System Evolution over the Pacific Ocean Warm Pool region". Thunderstorms develop in stages, which begins with an "updraft rooted in the boundary layer and extending upwards." (Biggerstaff) "These cells are usually large, from 3-5 kilometers in diameter." There is an updraft created by warm and cool air. The warm air rises and creates an updraft. There is also a mature stage, which consists of drafts of both cool and warm air flowing in opposing directions. The draft occurring in this stage is from the boundary layer and extending upwards. There is suggestion that there is smaller existing cells around "1-3 kilometers in diameter". The next stage is the dissipating stage, which consists of a downdraft that breaks up the lower levels of the clouds. From the ground to the mid-level of the cloud, the airflow extends. During this, the cell's diameter increases between "5-10 kilometers". (Biggerstaff)

The evidence that cloud seeding with ice nuclei starts ice crystal formation is said to be "indisputable". Cloud seeding has been done in West Texas for the last twenty-five years. To make the science more sound, the efforts of Texas and Texas Tech University test clouds for updrafts, particle count and amount of liquid water and ice in the cloud. Even though the evidence of widespread enhancement of precipitation is much less conclusive, field studies show the local influence of cloud seeding on the dissipation of clouds with supercooled water droplets. The cause for question is whether the cloud needed the addition of the seeding agents to precipitate or not. The cloud might not have needed the addition to precipitate. Studies show that cloud seeding does enhance the precipitation of the cloud. The Texas programs run with an element of randomness to prove statistically that seeded clouds live longer and rain more. The technique is not a proven fact yet, but it is said that, "area of study has improved in great strides in the past decade." (Nasti)

Cloud seeding is federally funded in states concerned about limited water resources. This is common in the desert climate region of the Southwest. Droughts in areas have also caused states to become involved in cloud seeding. Some fear that cloud seeding will transfer rain from one place to another causing some places to get less rain than it is currently getting. (sunysuffolk)

I did not have to guess whether cloud seeding would work in the Tucson area, because an article with actual testing revealed my conclusion. The study concluded that there was no discernible signs that cloud seeding with silver iodide generated an increase in rainfall around an area north of Tucson. The silver iodide was dropped at a level where the temperature was -6 C. The dropping also was done along a "line perpendicular to the wind at that altitude and located upwind of the mountain target." Battan and Kassander noted that this was done on an already raining clouds. Cloud seeding could be done in Tucson, but it is not definite that it will create additional rain. More money could be used to further test the probability of cloud seeding success around the Tucson area. If the further testing is found to have a definite effect upon the amount of rainfall, then a cloud seeding program should be introduced.

Discussion

The data from the actual testing of the cloud seeding in the Tucson area could be refuted entirely, because the clouds were expected to rain and did on days when the silver iodide was not dropped. The increases in rainfall could be due to climate. However, I believe that the silver iodide could in fact work based on the information found in Tucson and data from tests in Texas and other parts of North America.

Conclusion

In conclusion, the successfulness of cloud seeding seems to be high. Therefore, Tucson should be testing it further now to make it possible for expanded use to improve the abundance of water in the Tucson aquifers in the future. There is evidence cloud seeding works in a variety of places as well as around Tucson, but it has not been found to be fully proved. Further exploration of cloud seeding should be done to reveal the actual extent of the impact of cloud seeding in the Tucson area.

The source of the water in the Tucson aquifers comes from rainfall that falls to the ground on the surrounding area. Cloud seeding could improve the aquifers' fullness by introducing more water while the current use of the water could remain unchanged. This would make it possible for the City of Tucson and the surrounding area to use groundwater without worry. There would also be less pressure for the city t o use costly canal water from the Colorado River. This could be an easy answer the water shortage problem of the future if something is done now to make it possible to know if cloud seeding actually works. The current problem could be solved in the near future if further testing was done with cloud seeding.

References

1. Battan, Louis J., A Richard Kassander, Jr. "Summary of Results of a Randomized Cloud Seeding Project in Arizona" Proceedings of the Fifth Berkeley Symposium on Mathematical Statistics and Probability. University of California Press, June 21 to July 18, 1965 and December 27, 1965 to January 7, 1966

2. Biggerstaff, Dr. Michael I, Svetla Hristova-Veleva." METR 475: Radar and Mesoscale Meteorology"

3. Nasti, Cecelia. "Texas Rainmakers"
Found on http://www.earthsky.com/1995/es950815.html
Found while browsing

4. "Question & answers". Geographical Magazine, Feb 98, Vol. 70 Issue 2, p75
Sabio through EBSCOhost on the WEB
Search word: cloud seeding

5. Riley, Dr. James J., Class Speech

6. Seeding with Dry Ice or Silver Iodide, Cloud Seeding, Seeding with Salt Crystals. www.sunysuffolk.edu/~mandias/honors/students/cloud_ seed/link5.html
Individual articles found under address, but no author found
Found while browsing

This is a student paper written in the Fall of 1998 in a First-Year Colloquia on Water Resources in the Tucson Basin at the University of Arizona. For more information contact: jjriley@ag.arizona.edu