How
often do you look at the skys ! ~ especially when the rain is forecast ? .. clouds are fascinating. Clouds form through the process of
condensation when water vapour, primarily from the oceans, rises into the
atmosphere gets cooled and condensed to cloud formations. When the condensed droplets in a cloud get
large enough, they’ll fall as precipitation. Not all clouds are rain-bearing ! Nimbus
clouds produce drizzle to downpours; more violent relatives of theirs may
unleash rain as part of intense thunderstorms.
Nimbus is an ancient Latin word meaning “rain storm.” The prefix
“nimbo-” or the suffix “-nimbus” designate two prominent kinds of rain clouds,
"nimbostratus” and “cumulonimbus," although rain sometimes falls from
other cloud varieties. Here are clouds seen in Himalayas [photo taken
by me !]
The
Madurai bench of the Madras High Court Friday asked the Central and Tamil Nadu
governments why could not they try modern artificial methods such as 'cloud
seeding' to generate rain to tackle drinking water shortage. Also, the court
asked why were the governments not establishing more desalination plants on
various coastal areas to overcome water scarcity.Justices N Kirubakaran and S Sundar shot these questions while hearing public interest litigation (PIL)
seeking a direction to the governments to store water on a war-footing and
direct the departments concerned to provide drinking water regularly.The judges
wondered as to why the governments could not set up more desalination plants by
tying up with corporates under the corporate social responsibility initiative.With
rainfall scarce and water-table depleting, the petition said, the governments
should use new methods like cloud seeding to tackle scarcity.
Cloud
seeding is perhaps nothing new ! ~ its effectiveness not known – have heard of
this in late 1970s and early 1980s -
when we wondered seeing small aircrafts spraying chemicals on the sky – not
much of rains followed though !
Cloud seeding is a
type of weather modification that aims to change the amount or type of
precipitation that falls from clouds by dispersing substances into the air that
serve as cloud condensation or ice nuclei, which alter the microphysical
processes within the cloud. Cloud seeding also occurs due to ice nucleators in
nature, most of which are bacterial in origin.
The most common chemicals used for cloud seeding include silver iodide,
potassium iodide and dry ice (solid carbon dioxide). Liquid propane, which
expands into a gas, has also been used. This can produce ice crystals at higher
temperatures than silver iodide.
In mid-altitude
clouds, the usual seeding strategy has been based on the fact that the
equilibrium vapour pressure is lower over ice than over water. The formation of
ice particles in supercooled clouds allows those particles to grow at the
expense of liquid droplets. If sufficient growth takes place, the particles
become heavy enough to fall as precipitation from clouds that otherwise would
produce no precipitation. Seeding of warm-season or tropical cumulonimbus
(convective) clouds seeks to exploit the latent heat released by freezing.
Cloud seeding
chemicals may be dispersed by aircraft or by dispersion devices located on the
ground (generators or canisters fired from anti-aircraft guns or rockets). For
release by aircraft, silver iodide flares are ignited and dispersed as an
aircraft flies through the inflow of a cloud. Whether cloud seeding is
effective in producing a statistically significant increase in precipitation is
still a matter of academic debate, with contrasting results depending on the
study in question, and contrasting opinion among experts.
Project
Stormfury was an attempt to weaken tropical cyclones by flying aircraft into
them and seeding with silver iodide. The project was run by the United States
Government from 1962 to 1983.The hypothesis was that the silver iodide would
cause supercooled water in the storm to freeze, disrupting the inner structure
of the hurricane. This led to the seeding of several Atlantic hurricanes.
However, it was later shown that this hypothesis was incorrect. It was determined
that most hurricanes do not contain enough supercooled water for cloud seeding
to be effective. The last experimental flight was flown in 1971, due to a lack
of candidate storms and a changeover in NOAA's fleet. More than a decade after
the last modification experiment, Project Stormfury was officially cancelled.
Hurricane Debbie in
1969 provided the best opportunity to test the underpinnings of Project
Stormfury. In many ways it was the perfect storm for seeding: it did not
threaten any land; it passed within range of seeding aircraft; and was intense
with a distinct eye. On August 18 and again on August 20, thirteen planes flew
out to the storm to monitor and seed it. In the early 1970s, the Navy withdrew
from the project.
Perhaps when it was
tried in Tamilnadu, the technique was neither new nor in vogue! ~ the
experiment was not exactly a success – it was stated that experiment on cloud
can be done only when the cloud is in a
developing stage. It was tried again in
1985 and it was stated that there were 10 days of rain but no scientific means
to establish that they were related to cloud seeding technique.
With regards – S.
Sampathkumar
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