CLOUD SEEDING
This article introduces the concept of cloud seeding through descriptions of the
processes involved in the natural production of rainfall, the affects of hygroscopic and
glaciogenic seeding on these processes, static and dynamic seeding strategies, the
reasons for seeding and problems involved. It concludes that seeding is a viable
method of managing large scale water resources.
26/04/04
Nick Shuttleworth
Nick Shuttleworth
26/04/04
2
Prof. McEwen
For thousands of years people have tried to manipulate water to better match their
needs. This has been done in many ways from the digging of irrigation channels to
performing rituals to the gods. But today the aim is not simply to control the results of
the weather but to harness the power of the source.
Clouds are formed by the condensation of water vapour in the atmosphere. This water
vapour comes predominantly from the oceans but also from transpiration from plant
leaves, surface water and a number of other sources. Water is evaporated by heat from
the sun and the resulting warm moist air rises, expands due to the reduced pressure at
greater altitudes and cools. Winds then carry the clouds across the earth’s surface until
the water is released as precipitation.1
Water droplets within clouds can form by the condensation of water vapour onto
small nuclei in the air, such as sand or dust particles. This process is often
hygroscopic, as the nuclei dissolve to form a small droplet of concentrated solution.
The speed at which droplets form is governed by the concentration of the droplet
solution, higher concentrations accept water more readily, and the droplet size, larger
droplets “grow” more rapidly as it is easier to condense vapour onto a flat rather than
a curved surface. Once the droplets are large enough they fall and grow by coalescing
with other droplets. This is known as the “warm rain” process. The “cold rain”
process occurs when all or parts of a cloud are below 273K.2 Below ~253K cloud
droplets combine with ice nuclei and freeze to form ice crystals. Above 253K
homogeneous nucleation is rare but heterogeneous nucleation occurs using other
nuclei with a similar structure to ice. The supercooled region formed by the ice
crystals draws in moisture from the surrounding droplets and the crystals grow until
their weight causes them to fall. Accretion of supercooled droplets gives hail whereas
accretion of ice crystals gives snow. Whether rain, snow, hail or a combination of the
three falls depends on the temperature at the bottom of the cloud.
The sizes, types and concentrations of nuclei play an extremely important role in
determining the efficiency with which a cloud system forms and ultimately whether it
produces rain. For instance, salt crystals, which are abundant in oceanic regions, can
act as large condensation nuclei, allowing larger cloud droplets to form and the
subsequent coalescence process to initiate rainfall well within the lifetime of the
clouds. Conversely, the atmosphere over continental regions usually contains much
smaller and more numerous condensation nuclei. Medium-sized clouds formed in
these regions normally dissipate before the coalescence mechanism has had a chance
to initiate rain. Likewise, many regions have only a few ice nuclei in the atmosphere
which further reduces the efficiency of the cold rain process.3
In areas where there are too few nuclei for droplets of sufficient size to form and give
rain, cloud seeding may be applied to increase the yield of clouds. Seeding, in its
modern form, dates from the late 1940s when the ability of dry ice shavings to convert
supercooled water droplets to ice crystals was observed. Later consideration of those
observations lead to the discovery of other nucleating materials and to cloud seeding
programs in the early 1950s. Seeding using nuclei can be done in different ways.
Nick Shuttleworth
26/04/04
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Prof. McEwen
Hygroscopic seeding attempts to initiate the warm rain process. Particles such as salt
crystals are released into the updrafts of forming convective clouds to act as
condensation nuclei. These particles allow large drops to form rapidly but in doing so
reduce the total number of droplets and alter the nature of the cloud. Although the
total amount of water in the cloud has not changed, the likelihood of precipitation
occurring has increased dramatically due to the increased drop size. In this type of
seeding the cold rain process is unimportant, as by the time the cloud begins
producing ice nuclei precipitation will have already begun. However, glacioscopic
seeding works solely by “invoking” the cold rain process. The formation of ice
crystals is encouraged by the addition of ice-forming or glaciogenic agents, such as
silver iodide (AgI), calcium chloride (CaCl2) or dry ice (solid CO2). The surfaces of
these materials mimic those of naturally occurring ice nuclei and so ice crystals form
on them. Another method involves cooling an area of the cloud and thereby producing
the correct conditions for ice crystals to form as normal. This supercooling is achieved
by releasing pressurised gas into the cloud, which cools rapidly as it expands and
immediately produces a large number of tiny ice crystals. For both methods the cold
rain process then continues as normal.4
There are two main strategies for increasing the probability that a cloud will produce
precipitation. “Static mode” cloud seeding is intended to increase the efficiency of
precipitation formation by introducing an optimum concentration of ice crystals in
supercooled clouds by seeding. This process is relatively simple as it merely increases
the concentration of ice crystals within a deficient cloud to a level at which the natural
precipitation process (described above) can begin.
The “dynamic mode” of cloud seeding aims to enhance the vertical air currents in a
cloud and thereby vertically process more water through the cloud resulting in
increased precipitation. The main difference in the implementation of this strategy,
compared to the static mode, is that larger amounts of seeding material are introduced
into the high supercooled liquid water content updrafts of cumuli. The stages believed
to take place during dynamic seeding are as follows. The freezing of supercooled
raindrops plays a major role in the dynamic seeding model. Therefore, a suitable
cloud is one that has a warm base and a vigorous updraft, strong enough to carry any
raindrops that are formed in the updraft above the 273K isotherm level. Such a cloud
has a vast reservoir of latent heat that is available to be tapped by seeding. A
significant amount of supercooled cloud and rainwater exists between the 273K and
the 263K levels, which is a potential energy source for future cloud growth. 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. 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. This rapid conversion of water to ice releases the
latent heat of fusion which acts to increase the updraft. The frozen water drops
continue to grow as graupel (“soft hail”) as they accrete any remaining supercooled
liquid water in the seeded volume. These graupel particles will grow faster and stay
aloft longer because their growth rate per unit mass is larger and their terminal fall
Nick Shuttleworth
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Prof. McEwen
velocity is smaller than water drops of comparable mass. This will cause the tower to
retain more precipitation mass in it upper portions. The retention of the precipitation
mass in the cloud's upper portions delays the formation of the precipitation-induced
downdraft and the resultant disruption of the updraft circulation beneath the
precipitation mass. This delay allows more time for the updraft to feed additional
moisture into the growing cloud. The precipitation mass in the upper portion of the
tower eventually moves downward. The precipitation beneath the cloud tower is
enhanced when the increased water mass reaches the bottom surface of the cloud.
When this process is applied to one or more suitable towers within a convective cell
greater cell area, duration, and rainfall are the result.5
The main aim of cloud seeding is generally seen to be the increased production of
precipitation. Whether or not clouds form and rain occurs is strongly dependant upon
the geography of the surrounding area and how this relates to local weather patterns.
This element of consistency caused by effectively constant factors (in the time frame
of local weather) means that if a dry place requires water some kind of intervention is
required and cloud seeding is seen as a viable option to increase rain production.
However, seeding is also used in a number of other applications, such as in the
dispersal of fog. This process uses glacioscopic seeding with the aim of causing
precipitation, using either glaciogenic agents or substances which expand and cool
rapidly when released, such as liquid propane, close to the ground to accelerate the ice
formation process and consume the water vapour. This application is mainly used at
airports and on roadways to improve visibility in areas where thick fog is common.
Another use of cloud seeding technology is in hail damage mitigation. Through the
application of glaciogenic materials to developing cloud turrets growing on the flanks
of mature thunderstorms, the precipitation process can be accelerated, decreasing the
amount of time available for large hail stones to form, which would not have melted
before reaching the ground.
In the field of static mode cloud seeding there have been a number of notable
experiments which have lead, in part, to the current understanding of the static
seeding process. The Climax I and II experiments, which were conducted in the
Rocky Mountains in Colorado, USA, are generally accepted to be the most
compelling evidence that seeding can significantly increase precipitation. The original
analysis of Climax I showed a greater than 100% increase in precipitation on seeded
days and a 24% increase for Climax II. However since, the method of analysis has
been questioned and the results revised to an increase of around 10% for Climax I and
II combined. It has also been seen since the experiments that many concepts on which
the experiments were based are far to simple compared to current knowledge and that
some of the cloud systems seeded were not simple orographic clouds but were part of
major systems. The Israel I and II experiments are also accepted as lending weight to
the view that seeding does increase precipitation, although there are reservations
about the quality of the data. It has been argued that abnormally heavy rain over the
region and the incursion of desert dust into cloud systems caused the data collected to
be inaccurate, as the desert dust contains more active natural ice nuclei and that they
can also serve as coalescence embryos enhancing collision and coalescence among
Nick Shuttleworth
26/04/04
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Prof. McEwen
droplets.6 There have been many other experiments at various sites around the world
but none have so far indicated that seeding significantly increases precipitation
without doubt about the integrity of the data obtained.
Dynamic seeding has so far not been as well investigated as static seeding with the
only recent noteworthy dynamic cloud seeding experiment being performed in west
Texas. Analysis of the seeding of 183 convective cells suggests that seeding increased
the maximum height of the clouds by 7%, the areas of the cells by 43%, the durations
by 36%, and the rain volumes of the cells by 130%. 7 Other experiments have
demonstrated increases in rainfall for “floating” targets but have yet to show the same
for fixed ground targets. Therefore, it can be seen that the potential rainfall increases
are much greater for dynamic seeding than for static seeding, which simply increases
cloud efficiency as opposed to water volume available, but a greater understanding of
the dynamic process is required if dynamic seeding is to become a realistic candidate
for water resource management.
One of the biggest problems that both cloud seeding experiments and practical
planned weather modifications have to overcome is accurately predicting cloud
movements. This is so important to static seeding because it appears that there is a
limited window of opportunity for seeding: clouds must be relatively cold-based and
continental, have tops temperatures in the range 248K to 263K and with a time scale
limited by the availability of significant supercooled water before depletion by
entrainment and natural precipitation processes.8 The ability to forecast clouds and
precipitation is not currently at the spatial and time accuracy necessary for planned
weather modification but the National Weather Service Doppler radar network,
advanced satellite observations, and other measurement networks offer new
possibilities to answer questions about natural cloud systems and how they are altered
through cloud seeding.9 Another problem relating to weather prediction is that the
control data must be correct in order that rainfall increases can be calculated. A
prediction must be made of roughly how much rain would have fallen had seeding not
taken place and this must be done by comparison of the weather conditions at the time
of seeding with those expected and those of previous similar days. Also, due to the
fact that these experiments are difficult to repeat weighting factors, used to allow data
from days with slightly different conditions to be normalised, must be accurate.
There are many important factors to consider when performing large scale projects
which can have potentially far reaching effects but those which concern something as
fundamental as water are especially important. The most obvious matter relating to
precipitation enhancement is that, if the amount of rain is being increased in one
place, and the total amount constant, then theoretically places downwind of where
cloud seeding is taking place will receive less rain. This poses a dilemma as to
whether it is ethically sound to seed clouds as it could be argued that seeding one area
takes water away from another. In practise, however, this is not the case. Of the total
atmospheric moisture passing over any point, the proportion falling as natural
precipitation is typically less than 15%. Cloud seeding-induced increases in
precipitation of the order of 5-30% still result in a small overall proportion of the total
Nick Shuttleworth
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Prof. McEwen
available moisture reaching the ground. Analyses of precipitation data from areas
downwind of several cloud seeding projects have indicated small percentage
precipitation increases extending as far as 100 miles downwind of the intended target
areas.10 Another concern associated with cloud seeding relates to the chemicals
involved. As with almost all large projects the potential hazards involved must be
analysed to ensure that there is no significant environmental danger inherent in the
process. To this end many detailed studies have been conducted, ranging from
chemistry-focused work to comprehensive environmental investigations. Also,
seeding materials are applied in very small amounts relative to the size of the
geographic areas being affected, so the concentrations of the seeding materials in the
resulting rainwater are very low. For example, the concentrations of silver iodide
found in rainwater from seeded cloud systems is around 5000 times less that the US
Public Health Service’s stated acceptable concentration. As a final safeguard
government agencies in many jurisdictions are responsible for regulation of cloud
seeding in the public interest. These agencies commonly require permits for cloud
seeding, to help assure that the projects are properly designed and that those
conducting such operations are properly qualified.11
From a theoretical standpoint cloud seeding is a realistic method of managing large
scale water supplies. However, although dynamic seeding is capable of more
impressive results, it is, as yet, unproven when compared to results of static seeding
experiments which, in some cases, show increased rainfall from seeded clouds.
2B27 Environmental Physics Course, UCL - “The Hydrologic Cycle”
http://www.cmmp.ucl.ac.uk/~jhh/2B27/water.rtf
2
AI Weather Modification
http://www.atmos-inc.com/weamod.html
3
AI Weather Modification
http://www.atmos-inc.com/weamod.html
4
Weather Modification Inc.
http://www.weathermod.com/services/cloud_seed/concepts.php
5
Weather Modification By Cloud Seeding - A Status Report 1989-1997
http://rams.atmos.colostate.edu/gkss.html
6
Weather Modification By Cloud Seeding - A Status Report 1989-1997
http://rams.atmos.colostate.edu/gkss.html
7
Weather Modification By Cloud Seeding - A Status Report 1989-1997
http://rams.atmos.colostate.edu/gkss.html
8
Weather Modification By Cloud Seeding - A Status Report 1989-1997
http://rams.atmos.colostate.edu/gkss.html
9
Progress In Planned Weather Modification Research 1991-1994
http://www.agu.org/revgeophys/czys01/czys01.html
10
North American Weather Consultants
http://www.nawcinc.com/wmfaq.html
11
North American Weather Consultants
http://www.nawcinc.com/wmfaq.html
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