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					                                              Proceedings o f The South African Sugar Technologists' Association-June 1972


                                               By E. HUGHES
                                 "Ellesmere'?,P. 0.Knapdaar, Cape Province.

                           Introduction                    supercooled cloudy air, and silver iodide
       The modern history of rainfall stimulation by       generators provide about 10'3 nulcei per gram of
 means of cloud seeding began in 1945 with the             smoke at -IOOC, only a few grams of silver iodide
 discovery that various substances, notably dry ice,       would be required per cloud to provide one ice
 could cause super-cooled vapour clouds to con-            particle per litre. One litre of cloudy air was
  dense into large droplets. In the following year it      assumed to have about one million cloud droplets
 was found that silver iodide crystals had the same        (10 - 20u diameter) and these should condense to
 effect on super-cooled clouds, causing them to pre-       form one rain drop about Imm in diameter. It
 cipitate ('3). Super-cooled clouds are those which        was assumed that this would upset the colloidal
 contain water vapour and liquid water at                  stability of the cloud. The growth of cloud
 temperatures below OOC, and these occur in all            droplets to precipitation size is explained by
 parts of the world. Great enthusiasm greeted these        reference to the Bergeron Findeison process where
 discoveries and wild claims were made regarding           the ice particles will grow at the expense of water
 man's ability to control the weather and to end           droplets due to the lower vapour pressure over
 droughts. However, several large scale projects           i~e(~,13).  No consideration was given to the
 yielded conflicting results with apparently large         concurrent dynamic effects due to the release of
.increases in precipitation in some cases and              latent heat of fusion or condensation, nor to the
 decreases in others (",'3).                               effects upon the field of motion of the cloud.
      In particular, Project Whitetop, conducted by              The results from operations based upon the
 Roscoe Braham (1966), and a 5 year Arizona                static theory were often disappointing regarding
 experiment, conducted by Louis Batten (1966),             cloud growth and the yield of water (I3). The
 yielded inconclusive results. In the case of Project      dynamic theory involves the releasing of the
 Whitetop, a 20% reduction in precipitation over           maximum amount of latent heat of fusion and
 an area of 100,000 square miles was recorded (13).        condensation as rapidly as possible, thereby
      Public and scientific interest in weather            materially increasing in succession the temperature
 modification waned throughout the world, partly           of the cloud (by 0,5 to IOC), the buoyancy of the
 as a result of the above experiments and also due         cloud and finally the vertical and horizontal
 to the failures of "get-rich-quick" operators, and        dimensions of the cloud. This increases the life-
 the whole subject fell into disrepute.                    time of the cloud and the precipitation from it.
      Interest was revived largely by the successes        More moist air is drawn up into the cloud and
 of commercial operators who had persisted in the          this in turn is condensed and eventually pre-
 field in the U.S.A. and who were supplying extra          cipitated as snow, hail or rain. The technique
 water to large hydro-electric and irrigation dams.        requires the release of about 20 pyrotechnic flares
 Their well-documented operations prompted the             yielding 50 grams of silver . iodide each, for the
 authorities to press. for further research into the       treatment of one cloud. This dynamic approach,
 matter. In 1961-62, the American Congress voted           which was largely developed by Prof. Joanne
 $100,000 for research into "increasing rainfall by        Simpson in Florida, has yielded positive and
 cloud seeding" and this -r-dsearch is now largely co-     convincing seeding results (13,14,'5,16).
 ordinated by the U.S. Bureau of Reclamation, as                The micro-physical : .dynamic theory is the
 Project Skywater('), which is only one of several         most recent and, in terms of extra water
weather modification projects with vast financial          produced, is the most successful explantion of the
 backing. This effort has led to a rapid increase in       precipitation stimulation mechanism. This theory
 knowledge in many fields of weather modification          combines the most recent discoveries in the field
and the confusion and apparent contradictions of           of cloud micro-physics with that of cloud
 previous experiments have now to a large extent           dynamics, and this approach appears to hold
 been elucidated (1,2,3,).                                 great promise and may soon lead to an optimising
         Some theories of super-cooled cloud seeding       of rainfall stimulation techniques (4).
      In the original or static theory, it was
assumed that cloud microphysics should be altered                   The main results of Simpson's work
by the introduction of freezing nuclei into a                   Two important results emerge from Simpson's
supercooled cloud. Since an average cumulus                work and her early use of numerical models.
cloud might 'contain about 10'3 litres of                  (a) Seedability vs seeding effect
Proceedings of The South African Sugar Technologists'Association-June 1972                                      197

      Seedability is defined as "the predicted seeded            climate, but most impressive work has also been
maximum cloud top height minus the predicted                     done by Robert Elliott of North American
unseeded maximum cloud top height in                             Weather Consultants, California, U S A . on
kilometers".                                                     orographic clouds. The records compiled by his
     Seeding effect is defined as "the observed (by              company over 20 years of supplying large
aircraft, airborne camera, radar etc.) maximum                   amounts of water to hydro-electric and irrigation
cloud top height minus the predicted unseeded                    dams in the Sierras have proved invaluable in
maximum cloud top height in kilometers". Results                 documenting the reawakened interest in weather
obtained in 1965 are shown in Figure 1. If the                   modification in the early 1960's ('I,'*).
model and data were perfect, all seeded clouds
(circles) would lie along the straight line with
slope one, since seedability and seeding effect
should be equal for seeded clouds. The unseeded
clouds (squares) should lie along the straight line
with slope zero, since no matter how high their
seedability the control clouds should not grow
above their predicted unseeded tops. In Fig. 1,
both the means and regressions of the seeded and
unseeded populations separate significantly into
two distinct populations. Seeding effect vs seed-
ability correlated ot 0,973 (p-0,01) for seeded
clouds and was about zero for unseeded clouds.
This demonstrates the precision of the model in
predicting seeded and unseeded cloud top heights 1 3 .
(b) The second Important result to emerge from
this work has been the recognition of four basic
regimes which will lead to different behaviour in
seeded clouds. These regimes are:
(i) Explosive growth mode: Seeding will lead to
considerable cloud growth and increased
(ii) Cut o f f tower mode:       Seeding will lead to
decreased precipitation.
(iii) N o growth mode:      Seeding will lead to no
Increase or a decrease in precipitation.
(iv) Disturbed. mode:       Seeding will probably
lead to no increase or a decrease in precipitation
on a naturally rainy day.
      The classification of the day on the above                 Fig I Seedability versus seeding effect for 1965
basis is made on the results of the 0800 hrs.                     Caribbean cumulus seeding experiment (See text
radiosonde observation and a further observation                  for explanation
at 1200 hrs. Should a day be a seed day (mode i)
then a computation of the increased precipitation                       Some theories on warm cloud seeding
is made after measurement of cloud base, height                       Work on warm clouds, i.e. clouds warmer
and diameter (l7,14).                                            than OOC, has not progressed as rapidly as that on
       his work forms the basis for a sophisticated              super-cooled clouds. An intensive effort is now
climatological survey method which has been                      being made to understand the processes involved
developed by Profs. Simpson and Garstang which,                  in the modification of these clouds and numerical
on the basis of analysis of meteorological records               models are being developed which are already
and some field study, gives a reasonably accurate                yielding reliable results. The seeding materials
picture of the weather modification potential for                used for warm clouds are strongly hygroscopic
an area. This is a considerable advance and saving               such as urea or finely ground salt (NaCI). The
in cost, showing the probable amount of water to                 insertion of the hygroscopic seeding material
be produced by seeding, the unit cost of the water               causes the formation of a heirarchy of droplet
and the changes in streamflow due to the extra                   sizes and the mechanism of droplet growth is
water produced (7).                                              thought to be primarily due to coalescence caused
      A stratification of previous experiments (such             by the different rates of fall of different sized
as Project Whitetop) has usually shown that the                  drop~.(~,'O).
decreases which were so baffling were due to                          Some consideration has already been given to
unfavourable synoptic conditions. In general, the                a combination of the "warm cloud/cold cloud"
greatest response to seeding is on "fair" days with              techniques where, in certain cases, a warm cloud
isolated showers (11,14).                                        might be induced to grow into a super-cooled
      Prof. Simpson's work has been done mostly                  cloud and then be treated accordingly ( 9 ) . When
with supercooled cumulus clouds in a maritime                    the techniques for treating warm clouds have been
198                                          Proceedings o f The South African Sugar Technologists' Association-June 1972

developed to the present stage of reliability of                                     TABLE I
those used for treating                     then a          Comparative costs of water produced from natural and
great deal more moisture will be available for                           artificially induced sources
The quantity of water produced by cloud seeding                     Water Supply                   Costs           Source
   and some cost/benefits to be derived from it                                              Rand     Costs         (5)
     The amount of water produced by weather                                                per 1234 per m3
modification will clearly vary from area to area                                               m3     cents
and from year to year. Some typical in-creases
quoted are as follows:                                     Grahamstown Municipality          136.75     11
                                                           Wemmershoek Dam                    40.85    3.3
                                                           Voelvlei Dam                       70.70    5.7
(i)   137 million cubic meters of "new" water per          Sewage Purification High           81.70    6.6
      annum in the Upper Colorado River Basin              Est.      Low Est.                 54.45    4.4
      a t an estimated cost of R1,OO - R1,50 per           Orange River Development
      1234 m3, and a n estimated benefit of R20 -          Project                              7.075      0.574
      R25 per 1234 m3. ( I , Vol. 1 p. 47).                Weather modification High
(ii) an increase of 171,2 million cubic metres per              Est.                            2.15       0.176
      annum in the reservoir to be obtained from a         Water       Low Est.                  1.075     0.088 (1,2,3)
      area of 25 888 sq km. (2, p. 316)
(iii) a 15% increase in stream flow "reasonably
      anticipated" in the Colorado River Basin o r
      11,000 sq miles a t a cost of R2,30 per 1234            Some costs and benefits are quoted in Table 2.
      m3. (2, p. 433)
(iv) 407 220 m3 of extra water could be produced                                    TABLE I1
      per cloud in Miami during one normal
      month (May) and this would produce                   Costs and Benefits of water derived from various sources
                                                                                                                        - -

      165 738 540 m3 over the area, o r a n increase
      of 375mm. and a 23% total increase (13).             Location                 Cost per cubic         Cost benefit
(v) The anticipated increase for the Colorado                                           metre                 ratio
      River Basin Pilot Project is now estimated            I. Colorado River        2295 million            1:lO (9)
      a t 30%. (3, p. 95).                                    Basin                    cubic m.
                                                                                     R1.11234 m  3
                                                           2. Australia              25,000 sq.km   Cost per annum
     In general it would appear that a t this stage,         (Victoria)                "treated"   per acre less than
increases are of the order of 15% - 40% of annual                                                     I penny. (6)
rainfall (4).                                              3. South African         25,000 sq.km    Cost S.A. 1,5c
                   Cloud mergers                           equivalent of 2 above      "treated"       per hectare.
    'It may be noted here that most of the                 4. The costibenefit ratio can be as high as 150.        (9)
experimental work on cumulus clouds has been
done on isolated clouds. Simpson, however, has
considerable documentation on "cloud mergers",                  No critical assessment has yet been made of
where a seeded cloud has merged with a n                   the value of the extra water produced by the
unseeded one and the result has usually been a             "extra-area effect" (discussed in the next section),
spectacular increase in water production. In one           but it clearly is large and must be considered
case a merger produced 8797,6 acre ft of water,            from the start of operations.
and it would have taken 36 isolated clouds t o                  For standing crops and general pastoral
produce the same amount ,of water.                         farming, the calculation of the value of the water
     Work is proceeding actively along the lines of        becomes even more complex. A point which has
inducing mergers by seeding numbers of clouds in           been appreciated by the Australians is that the
rapid succession and so attempting t o induce the          timing of the rainfall is often of great importance.
formation of squall lines, tropical storm bands            It has been calculated that in the Malleen
and giant cumulonimbus systems, which are                  Wimmera wheat growing area of Victoria,
productive of enormous quantities of water. It             Australia, an increase of 25 mm from August 1st
should be noted, however, that at this date no             to October 31st would give a gain of A & I
sure technique has been developed to predict               million at a cost of A & 6 000. (6). Also a n
whether a given day is any more favourable for             increase which might be statistically non-
"mergers" than any other seedable day, or the              significant might yield high financial returns. The
manner in which mergers can be initiated with              rule of thumb .calculation for cane growing is that
certainty.                                                  100 mm of water gives a n extra 9 tons of cane
     Some comparative costs of water from                  per hectare. Consideration of the S-shaped growth
conventional sources a n d f r o m w e a t h e r           curve common to most agricultural crops indicates
modification are given in Table 1. The                     that relatively small increases in precipitation
costlbenefits of seeding will vary greatly with the        around about the mid-point, will give large growth
value and end use of the water produced.                   increases.
Proceedings of The South African Sugar Technologists'Associatio

    Where hailstorms are frequent and damaging,                   3.Persistence effects
the prevention of the formation of hail may be                         Briefly, the persistence effect is the tendency
economically as important as, or more important                                                                           ,
                                                                  for "rain to breed rain and drought to breed
than the production of more water. Severe hail                    drought". This seems to apply to large areas for
forming clouds can now be successfully treated by                 several seasons as well as the tendency for storms
cloud seeding (11,12).                                            to follow the same fairly narrow path during a
                                                                  season, and this phenomenon may be more
Some phenomena assosciated with cloud seeding                     apparent in dry rather than naturally moist
and their effect upon the evaluation of results and               climates. This natural phenomenon was noted in
                 experimental work                                South Africa ( I * ) and Australia (6, 7) but to an
    A number of phenomena are generally                           apparently lesser extent in America. The per-
associated with cloud seeding attempts and where                  sistence effect could prove very valuable in the
those have not been fully appreciated, or ignored,                field of weather modification, but any experiments
they have seriously confused field studies and                    which do not take due note of this may well give
experimental results. In certain cases the                        conflicting results, as happened with early experi-
evaluation methods also were faulty or inadequate.                ments with a randomized cross-over design for an
                                                                  experiment in the Snowy Mountains in Australia
1. Evaluation techniques and methods
     The earliest method of evaluating the success                P).
or otherwise of seeding operations was by means
of a comparison of two supposedly similar climato-
logical areas, the clouds in the one area being                             Conclusions and recommendations
seeded and the clouds (and rainfall) in the other                      The benefits of obtaining greater reliability in
area being left as control. This method is                        what has hitherto been the greatest a n d . most
obviously fraught with many difficulties and                      important variable in agricultural production, the
uncertainties and many years of seeding would be                  rainfall, as well as a reasonable increase in preci-
required before any clear result emerged.                         pitation, are so obvious to the agricultural
Comparisons of stream flow records were used in                   community that they need not be belaboured here.
the early stages and these are still most valuable                It is clear that cloud seeding can, under certain
in the evaluation of seeding results (4,17).                      conditions, and if properly conducted, produce
                                                                  significant amounts of water very economically. In
                                                                  order to take advantage .of this new knowledge in
     The modern methods rely heavily on the use                   the most beneficial manner, the following lines of
of radar, which is calibrated by means of ground                  approach could be the most productive.
observations, to measure rainfall intensity and
duration (     3 4, 5 , 6). The development of                         A potential user of water (farming
numerical computer models and their use for                       community, municipality, etc) could, after a
predictions of cloud growth and precipitation,                    thorough climatological survey of the area had
verified by observations on suitably randomised                   been carried out, consult with weather
clouds, have yielded a considerable amount of                     modification meteorologists and decide upon the
knowledge.                                                        optimum managed climate possible in their area.
                                                                  In all probability this would not differ markedly
2.Extra-area ejrect or large scale dynamic ejj;fcts               from a "good average year". Having jointly
     A considerable problem for a long time has                   decided upon this optimum climate and having
been the question of whether extra water was in                   stated any special considerations regarding floods
fact being produced by seeding or whether water                   or the timing of rainfall which might apply, then
was merely being re-distributed.                                  the cost of attaining this optimum for a period of
                                                                  say 5 years could be calculated.
     While the difficulties in the analysis of this
problem are obviously formidable, it became                            Having reached this point, the whole
apparent many years ago that, while large and pre-                operation could be put on an "insurance" basis
dictable increases in precipitation were found in                 with, in this case, cane growers being com-
the target area, considerable increases in pre-                   pensated should the climate for any one year
cipitation were usually found up to 100 miles and                 deviate too markedly from the optimum.
more downwind of the target area, and that these
increases were as much as 10 times the amount                          In view of the shortage of water in South
that would normally have fallen there (=, 3 , 4, ',).             Africa, and the greater and greater demands being
While the location and magnitude of these                         made on existing supplies, it is clear that many
increases or "extra area effects" cannot at this                  persons and public bodies will have an interest in
stage be predicted with as high a degree of                       weather modification and will want their share of
accuracy as the precipitation in the target area,                 the extra water being produced. This makes it
yet it would appear that the quantity of water                    imperative that potential water users, and in
produced by the extra area effect is considerably                 particular the farming community, should
greater than that produced for the target area.                   carefully examine this new technology and decide
This offers further support for the correctness of                how best to use it, and how best to incorporate it
the dynamic theory of cloud invigoration (4).                     into their farming systems.
200                                               Proceedings of The South African Sugar Techno!ogistsl Association-June 1972

 I.Anon., Atmospheric Water Resources Program "Project             Proceedings of the First National Conference of Weather
   Skywater". 1968 Annual Report. Vol. I and 11. United            Modification sponsored by: Amer. Met. Soc. Albany, New
   States Department of the Interior, Bureau of Reclamation,       York, U.S.A. April - May 1968.
   Division of Atmospheric Water Resource Management,              Proceedings of the Second National Conference on Wea-
   Denver, Colorado U.S.A.                                         ther Modification sponsored by: Amer. Met. Soc. Santa
2. Anon., Atmospheric Water Resources Program, "Project            Barbara, California, U.S.A. 1970.
   Skywater' 1969 Annual Report. (address as in I. above).         Simpson, Joanne., Cumulus Cloud Modification Progress
3. Anon., Atmospheric Water Resources Program, "Project            and Prospects. Repr. "A Century of Weather Progress.
   Skywater" 1970 Annual Report. )address as in I. above).         Amer. Met. Soc. U.S.A. 1970.
4. Anon., Atmospheric Water Resources Program, "Project            Simpson, Joanne., Seeding Cumulus in Florida: "New
   skywater" 1971 Annual Report. (address as in I . above).        1970 Results" from Science.
5. Anon., Construction in Southern Africa. June 1970 Vol.          Simpson, Joanne., On Cumulus Entrainment and One-
    15. No. 3 p. 79.                                               Dimensional Models. Repr. Journill of Atnlospheric
6. Bowen E.G., Cloud seeding Australian Science Journal            Sciences, Vol. 28, No. 3. April 1971. pp 449 - 455.
    1967, C.S.I.R.O., Sydney, Australia.                           Simpson, Joanne., How to Build Clouds and Bring Rain.
7. Bowen E.G., Review of Current Australian Cloud Seeding           Farmers Weekly, Nov. 12 1971.
   Activities C.S.I.R.O. Sydney Australia.                         Sopper, William E., Heimstra, L.A.V., Effects of Simu-
8. Garstang M., Personal Communication Brooks Museum,              lated Cloud Seeding on Streamflow of Selected Watershcds
   University of Virginia, Va. U.S.A.                               in Pennsylvania. Repr. Water Resources Bull., Journal of
9. Heimstra L.A.V., Kunsmatige neerslagvermeerdering: 'n           the American Water Resources Assn. Urbana, Illinois
   % sent per 4546 liters ekstra water? Die Siviele lgenieur       61801, U.S.A.
   in Suid Afrika, Julie 1970.                                     Whitmore, J.S., Technical Bulletin Report No. 25, 1962.
10 Heimstra L.A.V., Kunsmatige Vermeerdering van Reenval.           Dept. of Water Affairs, Patterson Building, Pretoria,
   Departement van Siviele Ingenieurswese Universiteit van          Rep. of South Africa.
   Stellenbosch. Februarie, 1970.

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