Cloud Seeding


by A.A. Tapp


Generators for use on aircraft burn the acetone solution of silver iodide and sodium iodide. Acetone is highly inflammable and it is necessary to handle it with care in appropriately designed apparatus. The same type of equipment as designed by C.S.I.R.O. is used in all our current experiments and drought relief operations. The solution is carried in a tank in the fuselage and fed by a pressure pump to burners under the wings, where it is atomised and ignited by spark plugs. The consumption is two gallons per hour per burner, corresponding to 550 grams of silver iodide, and producing 1017 ice nuclei per hour active at -17° C.


A photograph of a complete cloud-seeding unit is shown in Fig. 1. Electrical and mechanical circuit diagrams appear in Figs. 2 and 3 (not shown).

(a) Solution Tank.

This is fastened to the floor inside the aircraft cabin and consists of a 12-gallon stainless steel tank. D.C.A. require the tank to take pressures up to 5 P.S.I. to withstand any pressurisation with changing altitude in the event of a blockage in the vent lines. This tank is completely surrounded by a sealed tank vented to the outside air. This is a precaution to prevent solution of acetone fumes entering the cabin in the event of a leak in the main silver iodide tank.

Fitted to this outer tank is the instrument panel, solution filter, main supply cock and fuel gauge, as shown in Fig. 1.

(b) Pumps.

These are 28-volt 2-amp. direct-current stainless steel gear pumps capable of delivering the correct amount of silver iodide solution to the burners at a pressure of 25 P.S.I. They are operated through circuit breakers from the aircraft supply. A pressure indicator lamp lights when there is pressure in the fuel lines.

(c) Ignition.

High-energy ignition units, called "banger boxes", (they make an intermittent banging sound when firing), are used to ignite the solution in the burners. As they can give a lethal shock, care must be taken to ensure that the power is off and the high-tension lead short-circuited to discharge any residual energy before any routine maintenance is contemplated.

(d) Silver Iodide Nuclei Generators.

Each unit consists of a streamlined pod containing a inconel metal flame chamber, swirl jet atomiser, thermocouple flame indicator and spark plug. The pods are usually located on pylons approximately 12 inches below the wings, outboard of the engines and are connected to the solution tank via copper pipes fitted in the mainplane.

(e) Flame Indicators.

In some cases mirrors are fitted in the rear of the cabin for visual inspection of the pods in flight. When ignited, about 1 inch of visible flame in daylight can be observed from the rear of the flame tube. A thermocouple switch located on the pod, however, extinguishes an amber "tell-tale" lamp on the instrument panel when the burner is up to temperature. The hottest section of the burner runs at 1100° C (approx.).


The C.S.O. should check the seeding equipment pre-flight in the following sequence:-

(a) Be sure the aircraft is facing into wind. (Acetone attacks the paintwork of aircraft). This will ensure that the solution will blow away from the aircraft, when checking the pumps.

(b) Read your fuel gauge and see you have sufficient silver iodide solution in the tank for your requirements. If you have to refuel, remember to take all the necessary precautions against static electricity and spillage.

(c) Check the solution tank vent lines and ram pressure inlet which are located outside the fuselage. Wasps often block these lines with mud nests.

(d) Inspect the burner pods. A removable plate, with a circular hole (the size of hole depending on the type of aircraft) which acts as a ram air restrictor, is located in the front section of the pod. Make sure this is firmly in position.

(e) Apply power to the burner control panel; switch on the ignition units, one at a time. If normal, you will hear them fire approximately once a second. This must always be the first electrical operation. If there is any acetone-silver iodide solution left in the burner pod from pump testing, you will have a fire. Never try to light the burners on the ground.


(a) Master switch on.

(b) Open solution cock.

(c) Pumps on - speed control on high.

(d) When pressure lamps light - ignite one burner at a time.

(e) Look in mirrors to see if solution has ignited or watch for the amber thermocouple lamp to extinguish.

(f) For correct flow rate of 2 galls./hr., turn the pump speed control back until the pressure lamp just goes out or, using the mirror, adjust until you can see (in daylight) approximately 1 inch of visible flame in the burner.


(a) Switch off fuel pumps.

(b) Wait until thermocouple "tell-tale" lamps illuminate.

(c) Master switch off.

(d) Turn off solution. Failure to close the cock on solution tank will result in loss of silver iodide. In flight, the solution tends to syphon out through the pumps and spray nozzles.


Always clean your flame chambers in the pods immediately after flight. Silver slag tends to build up in the flame chamber after the burners have been operating in excess of 30 minutes. Removal of slag is best accomplished by hosing the flame chambers out with cold water. If no hose is available, immediate use of a bucket of water and a wire brush will save hours of work the next day.

Seeding Equipment - Pre-flight Ground Check
Aircraft facing into wind.
Fuel gauge - if refuelling necessary take precautions for static electricity.
Vent lines and ram pressure inlet.
Ram air restrictor firm.
Apply power to control panel and switch on ignition units one at a time.
Turn on main solution cock.
Switch on fuel pumps and observe solution spray leaving swirl jets.
Check solution lines for leaks, with pumps running.
Switch off pumps, master switch and close solution cock.


Seeding Equipment - Fault Finding
No power - check aircraft main switch and tank circuit breaker.
No spark - "banger box" u/s or check ignition circuit breaker.
Pumps running but no line pressure - pump u/s or tank solution cock closed.
Pump line pressure low:
(i) air leak in system - tighten line couplings.
(ii) blocked filter - clean filter.
(iii) blocked vent line - remove blockage.
Pump line pressure correct but no solution to flame chamber - blocked swirl jet, remove and clean.
Faulty or sticky fuel gauge - replace.
In flight, all systems operate but no flame - speed to high or no restrictor (IAS should be 130k).



The equipment and procedures described in this document, although generic in nature, were applicable to the cloud-seeding operation by Beagle 206S-2 VH-UNL in 1969. Tony Wills, who crewed the aircraft as Cloud Seeding Officer (CSO), has no recollection of mirrors being fitted to the Beagle.


A.A. Tapp (1968) Equipment and Servicing, in 'Fourth Course of Instruction in Cloud-Seeding Techniques', July 22- August2, 1968; pp27a- 27b; CSIRO, Division of Radiophysics, Sydney. (via Tony Wills)