Ever since man has begun to understand and investigate the weather, he has tried to find some way of controlling it. From witch doctors and rain dancing we have come a long way to now trying to influence the timing of rain and the location by modifying the weather. Man is now investigating increasing the precipitation, suppressing of hail, dispersion of fog and weakening of hurricanes by cloud seeding.

  Clouds can exist in 3 states depending on their height and temperature. If a cloud is above 0° C then it will be made up of water droplets only. If it or part of it is between 0° C and -40° C it is said to be a mixed cloud. This means that ice and water coexist, the ice as crystals and the water as supercooled water. If a cloud is below -40° C, where even supercooled water cannot exist, then the cloud is completely made up of ice crystals and is said to be glaciated. Within a cloud, heat is released by condensation, freezing or when deposition take place. As the cloud is warmed by this heat, it causes the updraft within the cloud to increase. The updraft is the movement of the air upwards within the cloud, which pulls moist air from below. This will cause the cloud to begin to grow taller and last longer.

In mixed clouds, if an ice crystal is formed then it follows that the water vapour will be supersaturated relative to the crystal. This means that the crystal will grow rapidly by diffusion and sublimation, from the surrounding supercooled water. The crystal will grow about hundreds of times faster than the supercooled water is. Therefore as more and more crystals grow rapidly the cloud vapour will be reduced below saturation and so cause the water droplets to turn to crystals. This process is called the Bergeron-Findeisen mechanism after the Norwegian Bergeron and the German Findeisen. This mechanism is used as the basis for cloud seeding.

In cloud seeding, very small particles of Silver Iodide (AgI) are used as artificial ice nuclei to form ice crystals and initiate the Bergeron-Findeisen mechanism. An alternative method is to use dry ice (solid Carbon Dioxide, CO₂) which as it passes through the cloud cools the surrounding air. This increases the difference in saturation vapour pressure between the ice crystals and the supercooled water and initiates the process.

Silver Iodide is used as its hexagonal crystalline structure is very close to that of ice (see fig 1). The separation of the oxygen atoms in the ice lattice is 0.459nm. The corresponding spacing of silver ions and of iodide in the AgI Crystal is 0.459nm. The AgI crystal does not exactly match the ice crystal but the difference is very small. Vonnegut and Chessin (1971) introduced silver bromide (AgBr) into the AgI crystal to change the lattice structure dimensions. They also tried introducing other chemicals into the AgI lattice, some matched ice more closely and others less closely than pure AgI. From their experiments they showed that epitaxy is indeed important and the ability to act as ice nuclei increased as the lattice structure more closely matched ice. Passarelli et al. (1973-74) found that CuI-3AgI crystals could act as ice nuclei at temperature of -0.5° C to -1° C with near prefect lattice structure. However due to the cost of production and temperature restrictions, silver iodide is still used for cloud seeding. It was also found that the cubic form of AgI was also effective as an ice nucleant.

To introduce AgI into the clouds the particles must be very small and a large quantity easily available. The AgI can be either be introduced at the top of the cloud or at the base using the updrafts of the cloud to inject the AgI. The AgI is usually burnt together with Acetone in generators either connected to planes or stationary on the ground. However planes are more commonly used to seed as they tend to be more accurate and can go to any cloud easily. The generators are used to vapourise the AgI which causes minute particles to be formed when it cools whose diameters are in the order of 0.01m m to 0.1m m. The reason for the ease of generating many tiny particles is due to the high melting and boiling point (Table 1) causing it when burned to turn to gas for a second then cool almost immediately as it come in contact with the air hence causing tiny particles to be formed. One of the first generators yielded 10⁶ particles per gram of AgI (Vonneguts 1947). Other ways of producing AgI particles are by grinding it or spraying solutions of it (AgI in adhyrodrous ammonia). These however are not so economically attractive as vapourising it.

The other method is to use dry ice to cool the air inside the cloud. The size of the dry ice pellets are around 1cm to 1mm in diameter (a lot like sand). This was the first method used, developed by Vincent J. Schaefer and Irving Langmuir (1946), in order to try to make clouds produce precipitation. The dry ice pellets were dropped over the cloud from a plane using a few pounds per mile of the flight. The plane moved around a path a few times, after a time the effects were be seen from the plane. Where the dry ice had fallen, a ‘hole’ had been cut in the cloud, in the ‘hole’ were observed ice crystals floating towards the ground.

Another technique was used by the Russians to try to produce precipitation. This involved building a cage onto the side of the plane and filling it with larger pieces of dry ice than used before. Then as the plane flew through the cloud, the air was cooled and hence ice crystals were formed. The main advantage for dry ice seeding, is it produces ice crystals at all temperatures below 0° C. However unlike AgI which lingers in the air, dry ice evaporates once and is gone forever and can it cannot be dispersed from the ground.

  For a cloud to be suitable for cloud seeding it must pass three tests. Firstly the top of the cloud must be colder than -5° C so that most of the cloud is supercooled. Secondly the cloud must have a steady updraft, so that there is a steady supply of moist air so the ice crystals continue to grow. Thirdly the cloud must not have much ice already existing in it. If ice crystals have already been formed then there is no reason to seed it, as nature has already started the process.

The main problem with measuring the effects due to cloud seeding is that it is impossible to repeat the experiment under the same conditions perfectly without the seeding. To tell how much a difference cloud seeding has had, it is important to be able to measure the results. The usual procedure is to designate two areas of similar landscape and weather. One area is left unseeded and is the control area, the other known as the target area is seeded. Hence by comparing these two areas with the measurements of the usual weather found in these areas it is possible to theorize the weather difference due to cloud seeding. It is therefore important that the clouds and the landscape are as near to the same as possible. Typically, clouds won’t produce rain for 20 to 30 minutes after the have been seeded. Therefore it is important to seed the cloud well up wind of where you want it to rain.

A cloud seeding experiment took place in Tasmania between 1964 and 1970. During this time measurements were made in five separate areas. First, two target areas adjacent to each other, named the west and east target area (TW and TE respectively) where selected. Second, two control areas in the north and south were also selected (NC and SC respectively) and then a third control area, in the north-west (NWC) was added to give a more accurate comparison (see Fig 3). The results were taken during the "on" years (roughly 1964, 66, 68, 70) so to allow the weather systems to recover from the cloud seeding.

The results where taken for 108 periods, which approximately half of them where seeded, during each of the seasons of the year. The results were then analysed ^[1] statistically and using the Double Ratio, it was showed to increase sometimes due to seeding while other times it would decrease (see table 2). In table 2, MWC is the average of the south and the north-west control areas, and MEC in the average of the south and the north control areas. The double ratio calculated from equation 1, where u = unseeded, s= seeded, T=target and C=control. If we expected the rainfall to rise by 50% then the double ratio would be equal to about 1.50, therefore from table 2, cloud seeding has either increased the rainfall by about 40% or decreased it by 12% at the most. The numbers in brackets are the one sided significant levels and indicate the error in the results. From looking at this experiment it is clear that it is hard to show whether or not cloud seeding has truly made a change in the amount of rainfall.

The physical conditions of the cloud are determined from measurements taken from either the surface or from aircraft. By using microwave radiometer, lidars and Doppler radar measurements to observe the cloud and using powerful computer programs, the suitability for cloud seeding can be determined and the effects after seeding are able to be seen. Microwave radiometry can be used for regions of supercooled liquid and radar is used to estimate the rainfall, the height and the reflectivity of the clouds. Bruce Boe of the North Dakota (USA) Atmospheric Resource Board developed and refined a technique that uses Sulphur Hexafluoride (SF) as a tracer gas together with sensitive detectors and a complex computer program to follow AgI through a cloud. SF makes it easier to trace the flow of cloud seeding agents as it can be detected in a second while it takes a minute to trace AgI, even from a plane. At the Desert Research Institute in Nevada, scientists are using Indium Oxide as a tracer, which is able to be detected in snow, to determine the amount of rain fall which is due to seeding.

  Cloud seeding is used in area where there is a shortage of water or where the levels of water tend to fluctuate. In regions which are which are short of moisture, especially semi-arid which are suffering from constant shortages, additional rainfall in the growing season would help to improve the crop’s condition. In areas which suffer from droughts, cloud seeding may lessen the impact which the drought has in the long run. In this case, it would mean a series of seeding operations to improve the ground’s moisture before the drought and may accelerate the recovery of the land by increasing the rainfall as the weather begins to return to normal. However it cannot be used to create clouds as it needs clouds to be already existing and have a large amount of vapour present.

The City of San Angelo (USA) used cloud seeding to try to increase the rainfall and increase the water level in the neighbouring reservoirs and rivers. City officials credited an increase in rainfall of 30% and a decrease in usage (due to less water having to be applied to the ground) of 28%, due to the cloud seeding. However due to the cost it was stopped even though the water levels in the reservoirs jumped from 40,000 acre feet to 230,000 acre feet. In Colorado River (USA) some cotton growers used cloud seeding to try to increase their yield (1971). By comparing the yields from before and after the seeding begun, it was found that cloud seeding had increased their yields by 46% in the target area. Cotton yields in the region have averaged 319 lbs./acre since 1971, compared to that of 312 lbs./acre elsewhere in the state.

Hail is one of the biggest problems facing farmers in certain areas, with over 800 million dollars worth of crops being lost due to hail in the United States in 1975. Hail can be reduced by effectively seeding a thunderstorm which causes most of the supercooled water to freeze and hence halt the accelerated growth of the ice crystals already present. This will decrease the size of the hailstones or cause it to fall as rain, causing less damage. This has been done in France, Italy and Switzerland by firing a rocket loaded with AgI into the cloud and detonating it, so that the AgI would fall into the cloud and seed it, though the accuracy of the method is low.

Hurricanes can also be altered by seeding the clouds in order to alter the release of the latent heat in the cloud. This is done by trying to freeze the water in the clouds through the injection of AgI. Seeding fog or low clouds is useful to increase visibility especially at airports. In this case the fog is cleared by overseeding the clouds, which cause a large number of small ice crystals to be formed, which evaporate rapidly into the unsaturated air, hence dissipating the cloud. Some success has been achieved but the clearing of the fog only lasts for short periods of time.

  Using the nature processes which create clouds and precipitation, man has tried to increase the production of rain by cloud seeding. Cloud seeding uses silver iodide (AgI) which acts just like an ice nuclei to form ice crystal which grow due to the Bergeron-Findeisen mechanism. Alternatively dry ice is used to cool the air and hence create ice crystals. With cloud seeding, man has tried to even out the balance of the water distribution present in nature. In some ways man has won and succeeded to increase the rainfall or decrease the damage caused by hail or powerful weather systems, however there is still a lot of work which has to be done before it is perfected. As scientists concentrate on the processes involved in the weather, we will be able to improve the present method of cloud seeding, and change the weather around us.

1. Miller, A.J., Shaw, D.E., Veitch, L.G. & Smith, E.J. (1979).

Communications in Statistics - Theory & Methods,

`Analyzing the results of a cloud-seeding experiment in Tasmania',

vol.A8(10), 1017-1047.

 2. Henderson-Sellers, A., Robinson, P.J. (1986)

Contemporary Climatology, Longman Group, Essex.

 3. Atkinson, B.W. (1968)

The Weather Business, Aldus Books, London

 4. Battan, L.J. (1965)

Cloud Physics and Cloud Seeding,

 5. Dennis, A.S. (1980)

Weather Modification by Cloud Seeding

 6. Various Sources of Information Using the Internet.