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(Narrator) The first real demonstration that man was able to
alter the course of the weather came a little over 20 years ago.
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.
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.
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.
(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.
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.
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.
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.
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.
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.
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.
(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.
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.
(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.
(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.
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.
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.
(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.
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.
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.
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.
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.
(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.
(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.
(Male) Meteorology.
( 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.
(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.
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.
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.
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.
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.
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