'Is it possible to regulate the rainfall of a country?' This is a question which has interested man throughout the ages. It is a question whose answer is vitally important to nearly every country, especially to Australia which possesses such vast areas of land lying undeveloped, wasting, for lack of that one essential requirement - water!

Today scientists in many countries are endeavouring to find the answer. Much valuable work has been done and much is yet to be done. In Australia the Division of Radiophysics, Commonwealth Scientific and Industrial Research Organisation (C.S.I.R.O.) under the leadership of Dr E. G. Bowen is well to the fore in this work. It is engaged on an extensive program of research into the physics of cloud formation, of natural rain formation and of that ultimate aspect of the artificial production of rain.

But you might ask, what is the connection between radiophysics and rain? The development of microwave radar during the last war gave to the meteorologists a tool of incalculable value to aid him in his work. It allows him to 'see' inside a cloud, to observe the important characteristics of and to measure the magnitude of the rainstorms. The connection therefore, lies in the method of investigation of the mechanisms of rain formation and it is for this reason that a group of 'radiophysicists', familiar with radio and radar techniques, are involved in the particular spere of meteorology.

The link between radiophysics, rain and Qantas Empire Airways is, of course, the aeroplane. It is this important tool which makes it possible to carry radar and associated devices to the cloud which is raining, instead of having to wait for it to come to us. But, in addition, the aeroplane allows us to fly over the cloud, around it but most imporant of all - through it, in order to observe and measure at first-hand what are the meteorological particles involved, be they snowflakes, ice crystals or water droplets; to observe and measure the degree of internal turbulence and in general to collect all the information which has a bearing on the mechanisms involved in natural rain. Such knowledge is an essential step forward in the search for satisfactory methods for the production of artificial precipitation.

Qantas was first approached in August 1950 by the C.S.I.R.O. with a view of chartering a Douglas DC-3 aircraft for continuation of airborne experiments which had previously been carried out jointly with the R.A.A.F. As the flying hours required by the C.S.I.R.O. for such an aircraft were in the region of 10 to 15 hours per week, the use of the aeroplane for part time training of Qantas pilots is feasible and in fact, it has been found that the two requirements can be easily co-ordinated. Thus Qantas Empire Airways supply the flying crew of the 'rainmaking' experiments.

A Douglas DC-3 incorporating the small type passenger entrance door was chosen for the work in order to avoid 'tying up' a large dorr freighter type which is more fexible in its adaptation to airline work. The aircraft chosen was VH-EBJ and the fitting out for the research work required the installation of approximastely 3,000 lbs of equipment.

As the aircraft operates frequently at high altitudes it was necessary to design and install a large capacity oxygen system. The electrical power required for the operation of the research equipment is beyond the 50 amp capacity of the normal DC-3 generators so therefore 200 amp generators had to be fitted.

The main part of the installation in VH-EBJ is a SCR 717B radar set, operating on a wavelength of approximately 10 cm. The antenna of this equipment, mounted in a radome on the nose of the aircraft, is arranged to revolve continuously in a plane at right-angles to the line of flight. As the antenna revolves the position and magnitude of echoes from reflecting objects (e.g. the ground or in a rainstorm) lying in the plane scanned by the antenna, are presented to the observer on a cathode ray tube. The aircraft appears as a spot on the centre of the screen, the echo from the ground appears at the bottom and the echo from the particular rainstorm between the aircraft and ground.

Another cathode ray tube in the recorder enables accurate measurements of the intensity of the echo received by the rainstorm. The special camera and timing equioment completes the installation, allowing the observer to make acontinuous record, suitable for subsequent analysis, of the particular rain phenomena being investigated.

The main mechanisms of natural rain have been established using the radar set in the aircraft, and similar equipment on the ground.

1. Non-freezing rain. Heavy rain has been observed to fall from cumuliform clouds lying wholly below the freezing level. The echo from such a rainstorm has the column-like shape and an intensity which is relatively uniform from the top right down to the ground. The mechabism of natural rain formation thought to be involved in such a case is that some of the cloud droplets grow, first by condensation, then more rapidly by coalesence with other water drops. There is now considerable reason for believing that this mechanism is also at work in some of the larger cumuliform clouds which extend above the freezing level.

2. Freezing or 'Bergeron' rain. This type of rain is named after the scietists who proposed the theory. Ice crystals formed in the sub-freezing region of the cloud fall down through it, grow in the process, melt at a point just below freezing level and finally emerge as rain. When this phenomenon occurs in statiform coulds, a characteristic type of echo is seen on the radar. An intensification of the echo occurs in a layer (called the 'melting' band) just below the freezing level. Such intensification is thought to be due to the melting of the snowflakes or ice crystals.

Important facts on aircraft icing have come out of these airborne investigations. When flying above the freezing level in the presence of snowflakes or ice crystals (e.g. in such a case of 'Bergeron' rain) rim ice builds up on the aircraft. However when flying above the freezing level through rain of the 'non-freezing' type (i.e. no ice present) there is an accumulation of clear ice on the aircraft. The practical application of such knowledge is the possibility of forecasting the type of icing by means of the characteristic echo from the rainstorm as seen on a radar set.

It has become apparent from the investigations into natural rain formation, that information is required on the conditions existing in the cloud before it starts raining. The measurements of the liquid water content in clouds falls into such a category. Two methods are employed:

A. A strip of absrbent paper is moved past a slit in the instrument mounted on the port wing of the aircraft. The electrical resistance across the paper varies according to the amount of water absorbed, and is continuously measured and recorded in the instruments in the main cabin.

B. A porous plug which consists essentially of numerous capilliary tubes, collects the liqid water in the cloud and passes it via a run of tubing to a suitable volume-measuring piece of glassware.

With such equipment, traverses through a suitable cloud at given intervals of altitude provide data on how the amount of liquid water varies with height in the cloud, and also with time, and hence valuable information for correlation with other aspects of rain formation can be made.