Rainmaking (1968)
By Steve GartnerJune 1st, 1968
Making clouds rain has been the subject of intensive research in several countries since the mid-40s. This study is the story of rainmaking in Australia – from the first tentative experiments with dry ice to the present, large-scale operations using silver iodide as the seeding agent. The film gives an account of the basic theory of cloud-seeding and outlines the techniques used in its practical application.
[Image of an light aircraft preparing to take off appears on screen with the text: A CSIRO film]
[Image changes to show the aircraft taking off and then a view through the clouds looking to the ground]
[Image changes to show the aircraft in the air with the text: Rainmaking]
[Image changes to a man inside the cockpit reviewing a map with a compass]
Narrator: The first real demonstration that man was able to alter the course of the weather came a little over 20 years ago.
[Image changes to show a newspaper article, titled SCIENTIST MAKES A “SNOWFALL”]
In 1946 two scientists in the United States discovered that by dropping pieces of dry ice from an aircraft into a cloud they produced a snowstorm.
[Image changes to show two men packing an RAAF aircraft with bags of dry ice]
The idea was immediately taken up in Australia by the CSIRO Division of Radiophysics and experiments were soon underway to find out whether extra rain could be stimulated by this method.
The trials took place on days when there were a number of separate but similar clouds in the same general area. A check was first made by radar to see whether any of the clouds already contained raindrops.
[Image changes to show the aircraft in the air and then one of the men tipping a bag of dry ice out of the aircraft]
If there were no raindrops present then a hundredweight or two of dry ice was tipped into the top of one of them.
These experiments were successful. When conditions were suitable most of the clouds which had been treated with dry ice rained soon afterwards while the other clouds around did not.
But it was soon realised that this would not be an economical method of making rain over a large area. It would mean that many tonnes of dry ice would have to be taken to high altitudes above the cloud tops.
[The camera pans over stormy clouds above a hilly landscape and then changes to show E.E. Adderley, CSIRO Division of Radiophysics]
E.E. Adderley: It was an American scientist Bernie Vonnegut who first suggested that silver iodide be used instead of dry ice and perhaps much less of it would be needed. To understand how silver iodide can induce a cloud to rain it’s first of all necessary to know something of the nature of clouds themselves.
[Image changes to show Mr Adderley drawing a diagram on a whiteboard]
The sort of cloud that we’re interested in is formed in the first place by air rising up and the moisture cooling and condensing in the form of minute water droplets. If we could take a microscope and have a look at a portion of this cloud we would find that these droplets are so tiny that it requires about a million of them to form one raindrop. They’re so light that they appear to float in the air going up and down with the air currents. The temperatures in these clouds are frequently below freezing but the droplets are still in the form of liquid water.
How then can a raindrop which requires a million or more of these tiny droplets be formed? One way involves the formation of ice crystals.
[Image changes to show Mr Adderley back at a desk viewing an ice crystal under a microscope]
If an ice crystal forms in a cloud which is below freezing point it grows rapidly at the expense of the surrounding water droplets and becomes a snowflake.
[An animation of snowflakes forming and falling plays]
The snowflake grows as it falls through the cloud. Pieces may splinter off and these grow into new snowflakes. In most parts of Australia these snowflakes melt as they fall through the air at lower levels and rain is produced.
Now these ice crystals form in the first place in the presence of ice nuclei, minute particles which have the special property of starting the freezing process. Sometimes there are enough of these nuclei present and the clouds rain of their own accord. If there are not enough natural nuclei present in the atmosphere it is possible to seed the clouds with artificial nuclei.
So far silver iodide crystals have proved to be the most effective and convenient artificial ice nuclei. If we burn a solution of silver iodide enormous numbers of minute crystals are formed.
[Mr Adderley dips a stick into a solution of silver iodide, lights it and puts it in a dark chamber, so the smoke and forming crystals can be seen]
These are so small that they form an invisible smoke. The crystals are similar in structure to ice crystals and act as ice nuclei providing an excellent trigger for the rainmaking process. In a real cloud the ice crystals would continue to grow as they fell through the cloud until they melt and become rain.
Narrator: Some of the first trials with silver iodide were made with burners placed on the ground, the idea being that the smoke would be carried upwards by air currents to the cloud level.
[Image changes to show a ground based burner being prepared and ignited]
This method was unsuccessful because the smoke rose only very slowly and if it ever reached the levels of supercooled clouds it had by that time lost most of its ice nucleating properties due to exposure to sunlight.
This difficulty was overcome by generating the silver iodide nuclei with burners fitted to the wings of an aircraft.
[Image changes to show the burners being fitted to the aircraft, the aircraft taking off and then flying through clouds]
In this way the ice nucleating crystals were carried right into the clouds before they had time to decay. The seeding solution was pumped to the burners on the wings where it was ignited by a spark plug.
[Image has changed back to Mr Adderley]
E.E. Adderley: These trials gave much the same results as with dry ice. When conditions were suitable the seeded clouds gave several times more rain than the unseeded clouds but much less silver iodide was needed and it was not necessary to climb above the cloud.
[Image changes to Mr Adderley pointing out the cloud types on a chart]
With cumulus clouds which are formed in rising currents of air the smoke can be released at the base in the upcurrents and is soon distributed throughout the rest of the cloud and if the temperature at the top of the cloud is cooler than minus ten degrees centigrade then rain usually falls about 20 minutes later.
With stratiform cloud however the updrafts are not nearly so intense. If the smoke were released at the base it would take very much longer for it to be distributed throughout the cloud so seeding is done directly at the minus ten degree centigrade level and the rain usually falls from half an hour to three-quarters of an hour later.
For successful seeding for both types of cloud the depth of the cloud should be at least half the distance from the ground to the base of the cloud otherwise if rain were stimulated it would tend to evaporate in the drier air underneath the cloud. [Image changes to show a field with piles of hay]
Narrator: Rainmaking trials with silver iodide were first carried out over large areas in the 1950s. Specific regions were chosen in various parts of Australia where the rainfall over a seeded area was compared with that over an unseeded area.
These trials showed that rainfall can indeed be increased in areas where the climate is suitable such as on the inland western slopes and during the course of the experiments more was learnt about the physics of clouds but several new questions arose. For instance, there was strong evidence that the effects of seeding tend to persist even after the seeding has ended.
[Image changes to show Mr Adderley with maps in discussion with a colleague and then back to Mr Adderley in front of his cloud charts]
E.E. Adderley: It seems most unlikely that silver iodide smoke will remain in the seeded area for more than a few hours or days at the most and yet the increased rainfall following seeding tends to persist for weeks, even months. We don’t yet fully understand this unexpected result and the causes of it are still being investigated however, there is an old saying that rain begets rain and drought begets drought. This is borne out by the records where it is seen that rainfall patterns tend to persist for a long while so it is possible that increases due to seeding may persist by a similar mechanism.
[Image changes to show Mr Adderley back at a table seated next to a slide projector. The lights are turned off and images of the plateau described below are shown]
In order to investigate these effects more fully a new large-scale experiment was started in Tasmania in 1964. A target area was selected in the Central Plateau region. This was of particular interest as it forms the main catchment of a large hydro-electric scheme. Extra rain here would be of great value as it would add to the water available for generating power.
[Image changes to Mr Adderley pointing out the circled ‘Target Area’ on a map of Tasmania]
Clouds in the target area are seeded or not seeded on a random basis and the rainfall in the target area is compared with that in the two control areas on either side of the central target area. These control areas are never seeded. The comparison of the area rainfalls will then show whether the rainfall in the target area has been increased or not. This whole operation is carried out in alternate years but the rainfall measurements are made in all years so persistent effects of seeding can be investigated as they rise and fall away following the start and the end of seeding.
[Image changes to show the seeding officer on the phone at his desk and then the camera pans over the different maps in the room]
Narrator: The Tasmanian experiments are being conducted by CSIRO in collaboration with the Hydro Electric Commission. During a seeded year the cloud seeding officer in Hobart gets regular forecasts from the weather bureau.
{Image changes to show the aircraft and crew]
A specially fitted aircraft is used and there are two crews, each consisting of a pilot and a cloud seeding officer who also acts as the navigator. If the right supercooled cloud systems are approaching the target area the decision is made for the duty crew to fly.
[Image changes to show the aircraft taking off]
During the climb the cloud seeding officer plots the temperature at various levels and measures the depth of the cloud. Once he knows the direction of the wind and its speed he can calculate where the silver iodide should be released so that the rain will fall within the target area. The silver iodide solution is pumped to the burners on each wing and ignited.
Rain gauges are placed at regular intervals throughout the target and control areas.
[Image changes to show the rain gauge surrounded by a high fence]
This rain gauge at Bronte Park is in the target area. Like the others it is read every morning at nine o’clock. All daily readings are sent to the Bureau of Meteorology. Since no one at the bureau knows when the target area was seeded there’s no chance of bias in working out the total rainfall in each area for any particular period. [Image changes to show the specially fitted aircraft flying above the clouds]
The Tasmanian trials are still going on. When they are complete it will be possible to calculate how much extra rain was caused by seeding. In the meantime the results of 20 years’ research and scientific cloud seeding are now being passed on to state governments for use all over Australia. [Image changes to show Dr E.J Smith, CSIRO Division of Radiophysics, presenting the findings in a lecture theatre]
Dr E.J. Smith: It’s now clear that there are many parts of Australia where clouds suitable for seeding occur reasonably often and it’s quite clear that the right methods of seeding can induce them to increase the rainfall. In this course we aim to give you three things, first, comprehensive account of the background of cloud seeding and its physical basis, second, a description of the methods that we use and third, how you can apply them in various parts of Australia.
[Image changes to show a man charting his findings]
Narrator: New South Wales, Victoria, South Australia, Western Australia and Queensland all conduct cloud seeding operations. It’s often a collaborative affair between several government departments.
[The man charting his findings sits at a desk to answer the phone]
Male: Meteorology.
[Image changes to show the man on the other end of the phone recording what is being said on a chart]
Male: Could I have the cloud seeding forecast for the waterworks catchment area please?
Male: Oh, yes, the southerly stream this morning is quite cold and unstable, suitable cumulus cloud today, base 2,000 to 3,000 feet, tops ten and some as high as 15. The winds at 5,000 feet south-westerly are 30, at ten south-westerly 40 and at 15,000 feet west-southwest 60. The freezing level is 7,000 feet, minus five 10,000, minus eight 11, and minus ten, 12,000 feet.
[Image changes to show a light aircraft taking off]
Narrator: Specially fitted light aircraft can be ready at short notice whenever conditions appear suitable. The targe may be a water catchment, a weed crop, undergrowth on a forest floor. The procedure is the same. The clouds are seeded upwind from the target so that by the time the rain starts it will fall where it is needed.
[Image changes to show the seeding officer preparing the seeding equipment within the aircraft]
Exactly how much the rainfall can be increased is still the subject of critical assessment. In places where suitable cloud systems don’t occur even the best cloud seeding methods cannot produce rain. It’s in the areas of light to medium rainfall that the best prospects for rainmaking lie.
[Image changes to show silos and fields of wheat]
In the wheatlands of the Mallee a half inch increase in the rainfall seems quite possible. Falling during the growing season this could add something like $2 million to the value of the wheat crop, a worthwhile return certainly for an investment of several thousand dollars.
[Image changes to show a hydro-electricity station and then an aerial view of forest]
And there’s no doubt about the value of additional rain for generating hydro-electric power and for damping down forest areas to reduce the hazard of bushfires.
[Image changes to show researchers conducting experiments in a laboratory]
What we already know about cloud seeding is being actively applied but new techniques need to be developed, for example, for treating types of clouds which don’t react to silver iodide and so the research is still going on.
We need to know much more about the natural behaviour of clouds and to understand more fully the basis physics of clouds and rain. Such fundamental research will always be an essential part of any successful method of rainmaking.
[Credits: Produced by the CSIRO Film Unit and the CSIRO Division of Radiophysics with the cooperation of the Bureau of Meteorology, Victorian Department of Agriculture, Tasmanian Hyrdo-Electric Commission. Camera – N.J Alexander & K.Nash. Editing – Alice O’Donnell. Animation – P.Watson & A.Weinberg. Scientific Direction – A.J Higgs and E.J Smith. Film Direction – N.J Alexander. Production – S.T Evans, 1968]