The Beijing Olympics inaugural ceremony held on 8 August 2008 was spectacular, not just in terms of the show but also because Chinese scientists promised and delivered on having a clear day. This was achieved by firing over 1100 small rockets into the sky over Beijing to disperse clouds that could potentially have caused rain. China continues to be a serious advocate of cloud seeding for weather modification, spending between $60 and $90 million dollars a year on projects to increase rain over the arid northern China region where Beijing is located.
Cloud seeding, that was discovered by Vincent Schaefer in a GE lab in 1946, has been tried by many countries around the world with mixed results. In 2003, the US National Academy of Sciences said in a report that in over 30 years of studies on cloud seeding, there is no convincing evidence of its efficacy. However, the US Met Department disagrees and says that an increase of up to 10% of precipitation is possible with cloud seeding. The Beijing city Met officials claim an increase of 13% precipitation with their cloud seeding experiments.
With such conflicts of opinion, it is worthwhile to examine this subject in some depth.
1. Cloud formation
Clouds are formed by water evaporation from the sea and other water bodies, condensing on dust particles in the atmosphere. As the cloud rises into the atmosphere, it gets cooled and more water condenses. Once the weight of the condensed water particles becomes large enough, rain falls. In colder climates, the precipitation is as snow and in certain circumstances as hailstones.The clouds formed over the oceans are moved by winds. Once they reach the landmass, the atmospheric temperature is cooler than over the ocean and there are increased dust particles in the air. This hastens water vapor condensation and precipitation. Rainfall is also caused when a cloud-mass rises over a mountain range and gets cooled. This is the reason that the windward side of a mountain receives more rainfall than the leeward side.
2. Cloud seeding by aircraft
The most common cloud seeding process is by introducing Silver Iodide into the cloud-mass. This is done by firing flares containing the material from the wing tips of aircraft. Silver Iodide is chosen because its crystal structure is very close to that of ice particles and helps water vapor condensation. Other materials that have been used for cloud seeding include ‘dry ice’ ( frozen carbon doixide), common salt and calcium carbonate. In the case of cloud seeding by these materials, instead of being fired as a flare into the cloud-mass, the aircraft flies through the cloud and disperses the material. Cloud seeding has also been attempted by spraying liquid propane into a cloud mass to cause a quick drop in temperature, to start the condensation process.
3. Other methods of cloud seeding
The use of an aircraft for cloud seeding is, of course, expensive. The alternatives tried out include, as in the Beijing Olympics example cited above, to fire rockets of the material into the cloud from the ground. In some parts of Africa, cloud seeding has been tried with burning a charcoal fire on the ground sprinkled with Silver Iodide and letting the smoke carry the seeding particles into the atmosphere. A more advanced technology effort has been in Germany, where laser pulses were fired at the clouds. The scientists postulate that the laser energy causes Sulfur Dioxide and Nitrous Oxide present in the air to combine and seed the cloud.
The benefits of being able to control rainfall are very apparent. The global climate change is already causing major stresses to water availability in many parts of the world. If there is a way to ensure rainfall in the catchment areas of rivers or lakes, the natural water bodies could get replenished. If the same clouds rain over the ocean or where the water will run off to the sea, the precipitation is wasted.
Cloud seeding technology has also been used in applications other than rainfall. It has been used to prevent hailstorms where there is risk of crop or property damage. The cloud seeding in this case causes the water vapor to fall as rain or as smaller hailstones than the size of the hailstones that would have formed without the seeding.
Some ski resorts have tried seeding to increase snowfall on their ski slopes. Another application with some promise is the clearing of fog and smog around airports to increase safety.
The major negative of cloud seeding technology has been the doubt whether it is effective at all. For example, there is no way of knowing if rainfall would have occurred even without the seeding. Confirming these doubts is the fact that seeding a cloud does not always produce rain. There is some effort to study cloud formation and precipitation to better predict the conditions under which cloud seeding would be effective. These are not conclusive, as yet.
A bigger issue is whether causing rain at one place results in preventing rain at another place. For example if the Beijing city reservoirs receive rain through seeding, whether it amounts to ‘stealing rain’ from the rural areas around Beijing. These issues can become even more complex for cloud movements across national boundaries, as for example in Europe or across state boundaries as in the USA.
A smaller concern is whether the prolonged use of Silver Iodide could cause contamination of river and water bodies. Silver Iodide in large concentrations is a pollutant. Already in the US, there is legislation that requires water samples to be tested periodically where cloud seeding is being done. At the present level of use of Silver Iodide, it is perhaps a non-issue.
In one well documented study conducted in early 2009 in six counties in North Dakota, it was reported that a spend of $768,000 on cloud seeding resulted in a 5 to 10% increase in rainfall that led to higher agricultural production of between $8.4 and $16 million in these counties. In addition there was a saving of $3.7 million from crops saved from hail damage. The total savings between $12 and $20 million for a spend of under $800,000 represents fantastic value. The catch, of course, is whether this is representative of cloud seeding technology or whether it is a one-off result.
There is no doubt, however, of the need to develop some measure of control over the environment by applying science. The costs of cloud seeding are much lower than any alternative approaches like diversion of rivers that are being attempted.