The Science of Cloud Seeding

1. The Science of Cloud Seeding Students Seminar Series 13th October 2003 .by, Vinoj V PhD Student CAOS, IISc

2. Background • By the year 2025 two thirds of the world population will live under severe water stress conditions as determined by studies of the World Meteorological Organization (WMO) • The rapid growth of Agricultural and Industrial sector along with the population explosion makes India particularly vulnerable. • Though India get high amount of rainfall compared to many other countries, the large infrastructure and water intensive agricultural practice makes India vulnerable to even to the minor swings (< 5%) in rainfall • Water related problems within the states • India is at its absolute water limits (Bryson., 1989)

3. Mitigation • Intelligent Utilization of the available Water Resources • Rainwater Harvesting • New source of water? • Artificial Rain Making! - Cloud Seeding? • Remember Atmospheric Vapor is only 0.001% of the total water in the hydrosphere

4. American Indians • The rain dance was a big part of American Indian culture and helped them carry on with their day to day activities. • They believe by pleasing the rain god he would provide rain for their crops.

5. American Indians • Each tribe had a different god or ceremony for their rain needs. • The tribes were similar in the fact that each one of them relied on rhythmic dancing in their events.(http://users.rowan.edu/)

6. Indians • According to Rig Veda - The earliest accepted Gods were • DYAUSH PITA (the sky father) • PRITHVI MATHA (the earth mother) • VAYU (the wind God) • PARJANYA (the rain God) • SURYA (the sun God) • VARUNA (the God of oceans) • AGNI (the fire God) • INDRA (the war God)…

8. The Cloud Formation • Homogeneous nucleation of Water droplets require high Degree of super saturation (~350-400%) • Heterogeneous nucleation in the presence of Cloud Condensation Nuclei (CCN) • Two important processes on how rain is produced • Collision-Coalescence Process • Ice-crystal (or Bergeron) process

9. Typical Cold Clouds Droplet Growth Warm Clouds - Collision and Coalescence Cold Clouds - Ice Crystal Process Typical warm Clouds

10. Seeding – Definition • The deliberate treatment of certain clouds or cloud systems with the intent of affecting the precipitation process(es) within those clouds. • Borrowed from Physical Chemists, “A method used to precipitate solute from the solvent - Pieces of solid material placed in a supersaturated solution to promote precipitation of the dissolved solute”

11. The Beginning • Cloud seeding began by dropping dust particles in to the cloud to help the water condense. • The dust was ejected from crop dusting planes which were already outfitted for the job. • There are now more scientific ways to cloud seed.

12. In 1946, Vincent Schaefer (GE) - a small piece of solid carbon dioxide (dry ice) can generate a large number of ice crystals in a laboratory-generated super-cooled fog. • He then dropped about 1.5 kg of crushed dry ice into super-cooled stratocumulus clouds over western Massachusetts and found that snow crystals did indeed fall out from the cloud. • In the following year Bernard Vonnegut (GE) found that particles of silver iodide can also generate large numbers of ice crystals if the cloud is cold enough. • Seeding causes a large number of small supercooled droplets to condense onto a smaller number of seed crystals. When the seeding works correctly, the grown crystals will fall out as snow and rain, leaving a clear space behind. For the technique to work, the seed material must cause large numbers of ice crystals to form. The Beginning

13. Cloud Seeding • Cloud seeding is actually a very complex process. In the simplest terms, it introduces other particles (CCN’s) into a cloud to serve as cloud condensation nuclei and aid in the formation of precipitation. There are three types of cloud seeding: static mode, dynamic mode, and hygroscopic seeding. • Static mode cloud seeding seeks to increase rainfall by adding ice crystals (usually in the form of silver iodide or dry ice) to cold clouds. • Dynamic mode cloud seeding increases rainfall by enhancing "vertical air currents in clouds and thereby vertically process more water through the clouds." Basically, in this method of seeding, a much larger number of ice crystals are added to the cloud than in the static mode. • In hygroscopic seeding, salt crystals are released into a cloud. These particles grow until they are large enough to cause precipitation to form. Clouds can be seeded from above with the help of airplanes that drop pyrotechnics, or from the ground by using artillery or ground-to-air rockets

14. The Seeding Concepts Static Seeding Alter the microphysical Properties of clouds Dynamic Seeding Attempt to modify the Air motion within the clouds Hygroscopic Seeding Use of hygroscopic material to Obtain the intended result (eg: salt) Static or Dynamic

15. Cloud Seeding Warm Cloud Seeding Cold Cloud Seeding Glaciogenic Seeding Cumulus Clouds having cloud top temp < 0oC Stratiform Clouds To increase population of water drops To increase population of ice crystals Seeding with Silver Iodide Sodium Chloride or Urea

16. Warm Cloud Seeding Formation of Rain through Hygroscopic Seeding Hygroscopic Particles: • Extremely moisture absorbent • Grow in low humidity air below cloud base Examples of Hygroscopic Material • Common Table Salt • Ammonium Nitrate – Urea Fertilizer Objective of Hygroscopic Seeding Introduce large drops into cloud Large drops collide with small cloud droplets and grow into rain drops (Coalescence) Rain drops heavy and large enough to fall out of cloud and reach ground.

17. Ice crystal Process • Bergeron process of rain formation: A process that produces precipitation; involves tiny ice crystals in a supercooled cloud growing larger at the expense of the surrounding liquid droplets • Ice crystals and liquid cloud droplets must coexist in clouds at below freezing • Accretion or riming of ice crystals: Ice crystals grow larger by colliding with the supercooled liquid droplets; the droplets freeze into ice and stick to the ice crystal

18. Mixed Clouds

19. Seeding Materials • The materials used in cloud seeding include two primary categories, tied to the type of precipitation process involved • Glaciogenic (ice-forming)Silver iodide, dry ice and compressed liquid propane • The second major category is focused on cloud systems where the warm (coalescence) process predominates. • Hygroscopic (water attracting)materials such assalt, urea and ammonium nitratecan be utilized. Of the hygroscopic materials, the most commonly used are salts.

20. Silver Iodide • Why Silver Iodide ? • It has a crystalline structure similar to ice crystal, and it acts as an effective ice nucleus at temp. of -4°C and lower • Silver iodide, when burned, creates extra ice crystals in winter clouds to increase snow fall. (The first AgI generator could discharge 1016 particles per gram) (www.wmi.cban.com/) It can be distributed from the ground...

21. Silver Iodide …or from the air.

22. Ground based Generator Automated High-Output Ground Seeding System Cloud Seeding Aircraft Aircraft-mounted flares

23. How Cloud Seeding Works

24. How Cloud Seeding works? • In cold cloud seeding., • the introduction of an ice-forming nucleating agent, (e.g., silver iodide) causes supercooled liquid water droplets to freeze. Once these droplets freeze, the initial ice embryos grow at the expense of the water droplets around them. • The first freezing process is often referred to as a static seeding effect, increasing the efficiency of the precipitation process within the seeded cloud volume. • The second freezing process, resulting from release of additional heat into the cloud, is often called the dynamic effect, whereby the treated clouds are invigorated, thus processing more moisture.

25. Warm Cloud Seeding at 2 to 3 km heights

27. Cold Cloud Seeding

31. A hole in a stratus cloud deck due to cloud seeding with aircraft, using dry ice This is where super cooled cloud droplets are converted into ice crystals , which then precipitate out of the cloud deck.

32. Indian Experiment • IITM, Pune was involved in cloud seeding experiment for a period of 11 years in the seventies • Reported Increase in rainfall due to seeding ~ 24% • Different states have their own operational programs whose success is still being debated

33. Interesting Applications • Hail suppression • Hurricane Modification • Eg: Operation STORMFURY • Fog Dispersion • Defence Applications

34. Suppression of Hail • Cloud seeding can be used to cause rain instead of hail. • However suppressing hail in one area often causes more in another. (twri.tamu.edu)

35. Thank You