Hu Zhijin, an expert with the Institute of Artificial Rain under the China Meteorological Administration, said that in the northern regions of the country, precipitation is produced mainly by stratiform clouds in spring and convective clouds in summer.

"Observations of airborne PMS (a kind of micro camera used to probe and evaluate the moisture in clouds) combined with radar, satellite technology, microwave radiometers and time-lapse photography has shown that the concentration of cloud droplets is relatively large and belongs to the continental type, and the concentration of ice is large," said Hu.

Artificial rain activities carried out by the Chinese Meteorological Department are aimed at initiating better rain or snow in the targeted areas.

"In the cloud seeding process we only try to initiate the natural rain process or try to make clouds more efficient by providing additional condensation nuclei by seeding the clouds with different chemicals, depending on the type of clouds," Hu explained.

He said not all clouds are suitable for seeding. Thin layers of clouds or those without much moisture content are not good prospects.

High clouds are also not good because even if the rain process is initiated through seeding, the raindrops will not reach the ground, Hu said.

Sometimes precipitation only occurs in small amounts, or not at all, because certain required conditions are not present. Of prime importance for determining both the initiation and amount of precipitation from the cloud system are the vertical and horizontal dimensions of the clouds, the lifetime of the clouds and the sizes and concentrations of cloud droplets and ice particles.

Under proper conditions, one or more of these factors can be favourably modified by seeding the cloud with the appropriate nuclei, Hu said.

There were two basic mechanisms by which precipitation forms in clouds. These are called the "warm rain" and the "cold rain" process.

Rain forms in warm clouds when larger drops collide with and absorb smaller cloud droplets in a process known as coalescence, Hu said. The "cold rain" process occurs when temperatures in all or parts of the cloud are colder than 0 C. The ice nuclei present in the colder region of the clouds initiate the process and the ice crystals grow rapidly. Then these falling ice crystals reach the ground as snow or rain.

The sizes, types and concentrations of nuclei present in the atmosphere play an important role in determining the efficiency with which a cloud system forms and ultimately produces rain or snow.

"In cloud seeding we assist the rain process by furnishing appropriate types and numbers of nuclei through seeding the clouds at the proper time and place," Hu said. More than 20 countries including the US, Japan, Russia and Israel are using this technology.

Compared with other developing countries, China is in a leading position in its application.

According to Hu, dry ice, liquid nitrogen and silver iodide are used in airplanes for seeding stratiform clouds.

The improved silver iodide acetone burner can produce greater amounts of ice much quicker. Aritillery shells containing silver iodide are also used. Silver iodide pyrotechnics were developed for airborne flares and rockets.

Particles of calcium chloride and urea are used in airplanes for warm cloud seeding. The seeding dosage is much greater than ice forming agents, Hu explained.

(China Daily July 24, 2004)