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An Isothermal Cloud Chamber To Test The Efficiency Of Seeding Agents

Published by Prabhat Kumar

Department of Mathematics, MIT World Peace University, Pune, India

These findings are described in the article entitled Towards design and development of isothermal cloud chamber for seeding experiments in tropics and testing of pyrotechnic cartridge, recently published in the Journal of Atmospheric and Solar-Terrestrial Physics (Journal of Atmospheric and Solar-Terrestrial Physics 181, Part B, (2018) 79-93). This work was conducted by P. Kumar from the NICRA Project, ICAR, Ministry of Agriculture, Govt. of India, and MIT World Peace University.

Cloud seeding is the primary step in almost all weather modification campaigns. A seeding agent which produces maximum nucleation particles per gram is an effective agent. And the one which is quick as well as effective, too, in causing condensation or ice nucleation of water vapor in the atmosphere is known as an efficient seeding agent.

Hence, production of an efficient seeding entity is of paramount significance for quality control. Prior to field experiments, therefore, the quality of any entity must be tested in a laboratory which simulates the real atmosphere for maximum efficiency.

This paper presents a cloud chamber which has been designed and developed for testing the efficiency of any seeding agents at the laboratory scale. Chamber pressure is designed to achieve up to 350 hPa, and lower temperature is designed to get -250C. Controlled higher temperature in the cloud chamber is designed to reach ≥ +100C. Humidity could be controlled in the range of 30% to 100%. Also, there are special attachments such as a pyrotechnic cartridge holding system, NaCl dust dispersing, and a nebulizer for spraying atomized brine solution as nucleation surfaces.

The internal capacity of the chamber is a cubical volume of 120x120x120 cm3. It works on the principle of interference caused by a thin horizontal cylindrical (diameter ≈ 1µm) light beam across the chamber width by intervening nucleated surfaces that are artificially generated by the nucleating agents. The higher the nucleation, the higher the interference. A drop in luminosity due to interference is measured by a luxmeter.

If immediately after the release of seeding agents, the luxmeter reading drops down to L0 due to spontaneous nucleation and after 5 minutes it improves to L5, then  it is hypothesized that:

The higher the value of Rate of Nucleation, the higher is the effectiveness of IN released in the chamber in rapidly producing ice crystals/droplets from water vapor.

After 30 minutes, the process of precipitation is almost complete in the chamber; hence, if the Lux meter reading after 15 minutes of the release of IN/CCN is L15, and after 30 minutes the interval is L30, then it may be hypothesized that:

It was found that the AgI Pyrotechnic cartridge formulation as mentioned in table-1 generates efficient seeding entity.

Table 1: AgI Pyrotechnic Cartridge chemical Formulation

Pyrotechnic cartridges composition
MaterialPercent by Weight
Silver iodate45.0
Potassium perchlorate27.0
Magnesium20.0
Nitrocellulose4.0
Triacetin4.0

Through several experiments in the cloud chamber, simulating standard atmospheric conditions, it was noted that for efficient seeding, an optimal atmospheric range of temperature is -3.70 C and -190C and the optimal range of pressure is between 532.86 and 386.76 mmHg (≈ 710 and 514hPa).

Six order polynomial fit displays the highest coefficient of determination values for the rate of precipitation. The cause of sinusoidal variation could be simultaneous variability of pressure and temperature in actual atmospheric conditions. Any drop-cartridge while falling from a higher level to lower levels or a seeding-rocket fired from the ground would encounter a simultaneous change in temperature and pressure in the real atmosphere.

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