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Cloud seeding is a form of weather modification. It can be used to
disperse fog, suppress hail, or control winds, but is most often used to
increase precipitation. In order to understand the process, however, a
basic understanding of clouds and how precipitation is formed isneeded.
As warm air rises from the Earth, it begins to cool and forms tiny
droplets of water that condense into cloud droplets. Cloud droplets are
formed around particles of dust, salt, or soil (called cloud condensationnuclei) that are always present in the atmosphere. These cloud droplets
group together into clouds, which can form precipitation in one of two
ways. In warm temperatures, the droplets in the clouds merge with many
other droplets and become heavy enough to fall to the Earth as rain. (It
takes millions of cloud droplets to form a single raindrop.) In colder
temperatures, the droplets of water form ice crystals. Other droplets
freeze onto these ice crystals, which grow larger and heavier until they
fall to the ground as rain, snow, or hail.
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Cloud seeding is actually a very complex process. In the
simplest terms, it introduces other particles into a cloud to
serve as cloud condensation nuclei and aid in the
formation of precipitation. There are three types of cloudseeding: 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 ordry 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
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HOW DID CLOUD SEEDING BEGIN?
In the late 1940’s, a discovery was made at
the General Electric labs in Schenectady, NewYork. During an unrelated experiment, it was
noticed that dry ice shavings had the ability to
convert super cooled water droplets (thoseexisting as water at temperatures lower than
freezing) to ice crystals. Later experimentation of
those observations led to a series of laboratorytrials which confirmed the nucleating properties of
various materials in certain cold cloud conditions.
The laboratory results were promising and trials
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The main objective of the ``static mode'' of cloud seeding is to increase the efficiency of
precipitation formation by introducing an ``optimum'' concentration of ice crystals in
supercooled clouds by cloud seeding. It was originally thought that clouds were
deficient in ice nuclei and therefore additions of modest concentrations of ice nuclei
should result in a more efficient precipitation-producing cloud system. All that was
needed was to introduce seeding material from the ground or at the base of clouds
which would then enhance ice crystal concentrations and thereby increase rainfall.
Cotton and Pielke (1995) concluded that physical studies and inferences drawn from
statistical seeding experiments over the last 50 years suggests that there exists a much
more limited window of opportunity for precipitation enhancement by the static-mode
of cloud seeding than was originally thought. The window of opportunity for cloudseeding appears to be limited to:
1. clouds which are relatively cold-based and continental;
2. clouds having top temperatures in the range -10 to -25 C;
3. a time scale limited by the availability of significant super cooled
water before depletion by entrainment and natural precipitation
processes.
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While the fundamental concept of the ̀static mode' of cloud seeding is
that precipitation can be increased in clouds by enhancing their
precipitation efficiency, alterations in the dynamics or air motion in
clouds due to latent heat release of growing ice particles, redistribution ofcondensed water, and evaporation of precipitation is also inevitable.
Alterations in the dynamics of clouds, however, is not the primary aim of
the strategy. By contrast, the focus of the `dynamic mode' of cloud
seeding is to enhance the vertical air currents in clouds and thereby
vertically process more water through the clouds resulting in increased
precipitation. The main difference in implementation of the strategy is
that larger amounts of seeding material are introduced into clouds. A
goal in the static mode of seeding is to achieve something like 1 to 10 ice
crystals per liter at temperatures warmer than -15C. In the dynamic modeof seeding the target ice crystal concentration is more like 100 to 1000
ice crystals per liter, which corresponds to seeding as much as 200 to
1000 g of silver iodide in flares dropped directly into the high super
cooled liquid water content updrafts of cumuli.
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In the 1960's to the 1980's, the hypothesized chain of
physical responses to the insertion of such large quantities
of seeding materials as summarized by Woodley et al.
(1982) included the following: (1) the nucleated ice crystalsglaciate a large volume of the cloud releasing the latent heat
of freezing and vapor deposition, (2) this warms the cloud
yielding additional buoyancy in the seeded updrafts, (3)the
updrafts with enhanced buoyancy accelerate causing thecloud towers to ascend deeper into the troposphere, (4)
pressure falls beneath the seeded cloud towers and
convergence of unstable air in the cloud wil l as a result
develop, (5) downdrafts are enhanced, (6) new towers wi l ltherefore form, (7) the cloud wil l widen, (8) the likel ihood
that the new cloud wil l merge with neighboring clouds wil l
therefore increase, and (9) increased moist air is processed
by the cloud to form rain.
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As a result of their experience in Texas, Rosenfeld and
Woodley (1993) proposed an altered conceptual model
of dynamic seeding as follows:1) NONSEEDED STAGES (i) Cumulus growth stage
The freezing of supercooled raindrops plays a major
role in the revised dynamic seeding conceptual model.
Therefore, a suitable cloud is one that has a warm base
and a vigorous updraft that is strong enough to carry
any raindrops that are formed in the updraft above the
0 C isotherm level. Such a cloud has a vast reservoirof latent heat that is available to be tapped by natural
processes or by seeding.
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(ii)Supercooled rain stage At this stage a significant amount of supercooled cloud and rainwater exists between
the 0 and the -10 C levels, which is a potential energy source for future cloud growth.A cloud with active warm rain processes but a weak updraft will lose most of the water
from its upper regions in the form of rain before growing into the supercooled region.
Therefore, only a small amount of water remains in the supercooled region for the
conversion to ice. Such a cloud has no dynamic seeding potential.
(iii)The cloud-top rain-out stage If the updraft is not strong enough to sustain the rain in the supercooled region until it
freezes naturally,most of it will fall back toward the warmer parts of the cloud without
freezing. The supercooled water that remains will ultimately glaciate. The falling rain
will load the updraft and eventually suppress it, cutting off the supply of moisture andheat to the upper regions of the cloud, thus terminating its vertical growth. This is a
common occurrence in warm rain showers from cumulus clouds.
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(iv) The downdraft stage At this stage, the rain and its associated downdraft reach the surface,
resulting in a short-lived rain shower and gust front.
(iv) The dissipation stage The rain shower, downdraft, and convergence near the gust front weaken
during this stage, lending no support for the continued growth of
secondary clouds, which may have been triggered by the downdraft and
its gust front.
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2) SEEDED STAGES
(i) Cumulus growth and supercooled rain
These stages are the same for the seeded sequence as they are for natural
processes.
(ii) The glaciation stage
The freezing of the supercooled rain and cloud water near the cloud top
at this stage may occur either naturally or be induced artificially byglaciogenic seeding. This conceptual model is equally valid for both
cases.
The required artificial glaciation is accomplished at this stage through
intensive, on-top seeding of the updraft region of a vigorous supercooled
cloud tower using a glaciogenic agent (e.g., AgI). The seeding rapidly
converts most of the supercooled water to ice during the cloud's growth
phase. The initial effect is the formation of numerous small ice crystals
and frozen raindrops
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(i i i ) The unloading stage The greater precipitation mass in the upper portion of the tower eventually moves
downward along with the evaporatively cooled air that was entrained from the drier
environment during the tower's growth phase. When the precipitation descends throughthe updraft, it suppresses the updraft. If the invigorated pulse of convection has had
increased residence time in regions of light to moderate wind shear, however, the
precipitation-induced downdraft may form adjacent to the updraft, forming an enhanced
updraft-downdraft couplet. This unloading of the updraft may allow the cloud a second
surge of growth to cumulonimbus stature.When the ice mass reaches the melting level, some of the heat released in the updraft
during the glaciation process is reclaimed as cooling in the downdraft. This downrush
of precipitation and cooled air enhances the downdraft and the resulting outflow
beneath the tower.
(iv) The downdraft and merger stage The precipitation beneath the cloud tower is enhanced when the increased water mass
reaches the surface. In addition, the enhancement of the downdraft increases the
convergence at its gust front
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.
(v) The mature cumulonimbus stage
The enhanced convergence acts to stimulate more neighboring cloud growth,
some of which will also produce precipitation, leading to an expansion of the cloud
system and its conversion to a fully developed cumulonimbus system.
When this process is applied to one or more suitable towers residing within a
convective cell as viewed by radar, greater cell area, duration, and rainfall are the result.
Increased echo-top height is a likely but not a necessary outcome of the seeding,
depending on how much of the seeding-induced buoyancy is needed to overcome the
increased precipitation loading.
(vi) The convective complex stage
When seeding is applied to towers within several neighboring cells, increased
cell merging and growth will result, producing a small mesoscale convective system
and greater overall rainfall.
This is an idealized sequence of events. Dissipation may follow the glaciation
stage or at any subsequent stage if the required conditions are not present.''
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One method of seeding clouds to enhance
precipitation is to introduce hygroscopic particles
(salts) which readily take on water by vapor depositionin a supersaturated cloudy environment. The
conventional approach is to produce ground salt
particles in the size-range of 5-100 , and release these
particles into the base of clouds. These particles grow
by vapor deposition and readily reach sizes of 25 to 30
in diameter or greater. They are then large enough to
serve as ``coalescence'' embryos and initiate orparticipate in rain formation by collision and
coalescence
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Seeding of tropical cumulus clouds, and indeed any clouds, requires that they contain
supercooled water--that is, liquid water colder than zero Celsius. Introduction of a
substance, such as silver iodide, that has a crystalline structure similar to that of ice will
induce freezing. In mid-latitude clouds, the usual seeding strategy has been based upon
the vapor pressure being lower over water than over ice. When ice particles form in
supercooled clouds, they grow at the expense of liquid droplets and become heavy
enough to fall as rain from clouds that otherwise would produce none.
Seeding of tropical cumuli sought to exploit the latent heat released by freezing as well.
This strategy of "dynamic seeding" assumed that the additional latent heat would add
buoyancy, strengthen the updrafts, ensure more low-level convergence, and ultimately
cause explosive growth of properly selected cumuli.
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The above sketch illustrates an aircraft dispensing
pyrotechnics doped with silver iodide into asupercooled cloud that is invigorated by the latent heat
released as the boundary between liquid and frozen
hydrometeors (blue horizontal line in the cloud on the
left) moves down to the zero Celsius isotherm (green
horizontal line). It was this transformation that the
experimenters hoped to use for construction of an
artificial outer eye wall during Project STORMFURY.
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Historically, there are three main ways to introduce seeding agents:
ground-based generators Clearly, ground-based generators must use
seeding agents that will survive normal ground temperatures, so the
generators typically produce a "smoke" that contains the seeding agent.
The ability of ground-based generators to deliver seeding agents to theappropriate level in the cloud is open to question, except perhaps in
situations where the clouds are being seeded to enhance snowfall in
mountainous regions (where the ground temperatures are already below
freezing and the generators are, effectively, already within the clouds).rockets or artillery shells Again, these delivery methods must use
chemical seeding agents. The Soviets used artillery for many years to
introduce seeding agents as a mechanism for hail suppression. I'll have
more to say on this later.
aircraft Delivery methods based on aircraft are relatively expensive, but
have the clear advantage that it should be possible to put a large fraction
of the seeding agent more or less just where it is intended to go. Using
aircraft, some of the cheaper alternatives to chemical agents become
viable, such as dry ice or crushed water ice.
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