The Science of Cloud Seeding

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The Science of Cloud Seeding
Students Seminar Series 13th October
2003 .by, Vinoj V PhD Student CAOS, IISc
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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 (lt 5) in rainfall
• Water related problems within the states
• India is at its absolute water limits (Bryson.,
  1989)

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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

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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.

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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/)

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• 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)

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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

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

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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.

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The Beginning
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Cloud Seeding

• Cloud seeding is actually a very complex process.
  In the simplest terms, it introduces other
  particles (CCNs) 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

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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
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• 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.

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Mixed Clouds
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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 as
  salt, urea and ammonium nitrate can be utilized.
  Of the hygroscopic materials, the most commonly
  used are salts.

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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 -4C 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...

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Silver Iodide

• or from the air.

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Automated High-Output Ground Seeding System
Aircraft-mounted flares
Cloud Seeding Aircraft
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How Cloud Seeding Works
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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.

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Warm Cloud Seeding at 2 to 3 km heights
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Cold Cloud Seeding
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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
• 
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• Different states have their own operational
  programs whose success is still being debated

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Interesting Applications

• Hail suppression
• Hurricane Modification
• Eg Operation STORMFURY

• Fog Dispersion
• Defence Applications

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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)

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Thank You