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. 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)…
Indians
7.
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
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
To increase population
of ice crystals
Seeding with Silver Iodide
To increase
population of water
drops
Stratiform Clouds
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
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 as salt, urea
and ammonium nitrate can 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...
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.
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
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)
