Write a note on the role of aerosols on cloud seeding.
Answers
Cloud seeding has been the most commonly used technique in weather modification. In addition to cloud seeding, many other terms are also used, including man-made precipitation enhancement, artificial weather modification, rainmaking and so on. Cloud seeding sprays small particles, such as silver iodide, upon clouds in order to increase precipitation. It has been used in a variety of drought-prone countries, including the United States, China, India and Russia. In the Rocky Mountains, Sierra Nevada and other mountainous areas of the United States, cloud seeding has been employed since the 1950s. China has had a perceived dependency upon cloud seeding in dry regions (see Section 7.4 for more details).
Rainmaking or precipitation enhancement began in 1946 when the American scientists Vincent Schaefer and Bernard Vonnegu at General Electric successfully seeded a cloud with dry ice and then watched snow fall from its base. Until recent times it was thought that rain might be induced by explosions, updrafts from fires, or by giving the atmosphere a negative charge. In general, three methods were developed:
1.
Spraying water into warm clouds.
2.
Dropping dry ice into cold clouds (where the dry ice freezes some water into ice crystals that act as natural nuclei for snow).
3.
Wafting silver iodide crystals or other similar crystals into a cold cloud from the ground or from an airplane over the cloud (Cited from UWRL (1971)).
The types of clouds are also important to the success of cloud seeding. There are significant differences between continental clouds and clouds formed over sea. Since larger droplets are needed if rain is to form, this meant that continental clouds were much less likely than maritime ones to be a good source of rain. The temperature of the upper levels of the cloud was found to be another crucial factor. In the case of both cumulus and stratiform clouds, provided this temperature is lower than −7°C, seeding would inevitably be followed by precipitation within 20–25 min (Ryan & King, 1997).
Frankly speaking, existing cloud seeding techniques have been only moderately successful. To judge the viability of a rainmaking program, it is important to establish that the seeding made a difference – that is, it results in rain from clouds that would not otherwise have yielded it naturally. However, it is difficult to determine whether fluctuations in rainfall that occur at the time of cloud seeding were produced by seeding or would have occurred naturally. Besides, the over-seeding can dissipate a cloud sometimes. Research conducted in China shows that even the best efforts of China’s rainmakers could produce an increase in rainfall of only 10%–15% (China Meteorological Bureau, 2004, p. 31).
(source: science direct {google})