Principles of Rainmaking
Clouds are classified into warm and cold clouds based on cloud top temperature. If the cloud temperature is positive, these clouds are called warm clouds and if it is negative, they are called as cold clouds. The nucleus needed for precipitation differs with type of clouds. Hygroscopic materials are necessary as nucleus for warm clouds.
Cloud seeding – It is the process by which the conditions of the cloud, (dimension, life time and size) are modified by supplying them with suitable nucleus at proper time and place. For accelerating the warm rain process, seeding with very large nuclei such as salt crystals can be used. In the case of cold rain process, seeding with ice nuclei such as silver iodide are used.
Cloud seeding is one of the tools to mitigate the effects of drought. It is defined as a process in which the precipitation is encouraged by injecting artificial condensation nuclei through aircrafts or suitable mechanism to induce rain from rain bearing cloud. The raindrops are several times heavier than cloud droplets. These mechanisms are different for cold and warm clouds.
Seeding of cold clouds
This can be achieved by two ways as given below:
1. Dry ice seeding – Dry ice (solid carbon-dioxide) has certain specific features. It remains as it is at –80°C and evaporates, but does not melt. Dry ice is heavy and falls rapidly from top of cloud and has no persistent effects due to cloud seeding. Aircrafts are commonly used for cloud seeding with dry ice. Aircraft flies across the top of a cloud and 0.5-1.0 cm dry ice pellets are released in a steady stream.
While falling through the cloud, a sheet of ice crystals is formed. From these ice crystals, rain occurs. This method is not economical as 250 kg of dry ice is required for seeding one cloud. To carry the heavy dry ice over the top of clouds special aircrafts are required, which is an expensive process.
2. Silver Iodide seeding – Minute crystals of silver iodide produced in the form of smoke acts as efficient ice-farming nuclei at temperatures below –5°C. When these nuclei are produced from the ground generators, these particles are fine enough to diffuse with air currents. Silver iodide is the most effective nucleating substance because; its atomic arrangement is similar to that of ice.
The time for silver iodide smoke released from ground generator to reach the super cooled clouds was offer some hours, during which it would draft a long way and decay under the sun light. The appropriate procedure for seeding cold clouds would be to release silver iodide smoke into super cooled cloud from an aircraft. In seeding cold clouds, silver iodide technique is more useful than dry ice techniques, because, less quantity of silver iodide is required per cloud. There is no necessity to fly to the top of the cloud, if area to be covered is large.
Seeding of warm clouds
1. Water drop technique – Coalescence process is mainly responsible for growth of rain drops in warm cloud. The basic assumption is that the presence of comparatively large water droplets is necessary to initiate the coalescence process. So, water droplets or large hygroscopic nuclei are introduced into the cloud. Water drops of 25 mm are sprayed from aircraft at 30 gallons per seeding on warm clouds.
2. Common salt technique – Common salt is a suitable seeding material for seeding warm clouds. It is used either in the form of 10% solution or solid. The spraying is done by power sprayers and air compressors or even from ground generators. The balloon burst technique is also beneficial. In this case, gunpowder and sodium chloride are arranged to explode near cloud base dispersing salt particles.
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