Cloud seeding is a controversial, weather modification process, primarily intended to increase precipitation (rainfall) in a targeted area. Some proponents (supporters) of cloud seeding also claim that it may, in some circumstances, prevent or reduce hail, and the resultant crop damage hailstones can cause, by engendering the formation of large water droplets instead of ice in cloud masses that would normally precipitate hail.
It is controversial in two regards: does it really work and, if it does, should it be used?
Before considering these two issues, let’s clarify what cloud seeding is, by discussing the three types of cloud seeding currently used: static, dynamic and hygroscopic; the three substances generally used for seeding clouds: silver iodide, dry ice and salt; and the two methods of application: airborne and ground-based delivery systems.
Static cloud seeding focuses on increasing precipitation by increasing the number of nuclei available for the formation of ice particles within a cloud to the “optimum” number. Clouds are composed of microscopic particles of liquid water; by seeding suitable cloud formations with silver iodide, which has a crystalline structure similar to ice, the microscopic droplets of water within the cloud have more particles to adhere to and freeze upon. This increases both the number and size of ice particles within the cloud, as the natural ice particles are supplemented with those artificially supplied.
When the ice particles grow above a certain size, the atmospheric conditions, heat and pressure, within the cloud can no longer hold them up against the force of gravity. The ice particles fall towards the ground, normally melting in the lower atmospheric regions of warmer temperature and through air friction, before reaching the Earth’s surface, and therefore arriving as rain rather than hail or snow. If the ice accreditation occurs rapidly enough however, and the atmospheric temperatures are low enough, these formations of ice may reach the ground while still in a frozen rather than liquid form.
Dry ice may be used as the seeding substance instead of silver iodide. In these circumstances, the effect is different. Instead of providing additional nuclei for the formation of ice crystals, the dry ice lowers the local temperature as it evaporates. This increases the amount and rapidity of the ice forming around the naturally occurring nuclei in the clouds; speeding up the accreditation process. Large ice crystals form more quickly and then precipitate towards the ground below.
Dynamic cloud seeding uses the same principles but a significantly larger amount of seeding material; as much as 100 times as much in the cloud seeding operation. The intention is not to increase the amount of rain from the cloud, but to vastly increase the size of the cloud. Whether the seeding material is glaciogenic (ice-forming) like silver iodide or temperature lowering like dry ice, the intent is to draw in additional water-bearing air to the cloud formation. The seeded cloud grows in size, both horizontally and vertically. By the time it is no longer capable of retaining its water content against atmospheric conditions, it is capable of producing a veritable deluge.
Hygroscopic seeding uses water absorbing particles, usually salt, rather than glaciogenic particles. Instead of encouraging the formation of ice crystals, these particles attract the microscopic droplets of liquid water from the surrounding air. Clouds are aerial associations of microscopic droplets of water, too small to be effected by gravity. These microscopic droplets vibrate and occasionally combine through collision. By seeding clouds with salt, larger droplets form around the salt crystals, collisions occur more often, and the development of sizable accrued amounts of liquid water droplets results in the formation of raindrops too massive to resist gravity. Water precipitates from the seeded cloud mass. It is this method that is usually advocated as being suitable for preventing hail; its ability to do so is highly subjective, more hope based than experimentally validated.
Cloud seeding is usually performed by aircraft, either airplanes or helicopters capable of providing crop-dusting services. Ground-based generators, in suitable areas, are also able to supply suitable amounts of silver iodide to low level cloud formations. Hygroscopic seeding is normally instigated by salt bearing flares; normally dispersed from aircraft but occasionally, especially in mountainous terrain, fired from ground-based sites.
The first laboratory-based experiments in cloud seeding occurred in the United States of America just after World War II, in 1946. The CSIRO (Commonwealth Science and Industrial Research Organization) of Australia has conducted field trials and experiments since 1947; rather unsurprisingly, considering that country’s climate and environment, and therefore their desire for developing a controllable mechanism for producing rainfall. Experimentation in all parts of the world have, however, made it conclusively clear that such seeding efforts can not produce precipitation in atmospheric conditions divorce of clouds. It is called cloud seeding because clouds are an essential ingredient.
The results of cloud seeding experiments, of necessity, cover extensive areas. Whether or not they are actually effective, therefore, has become a hotly debated topic. There is quite clear experimental evidence that cloud seeding has indeed increased rainfall in targeted areas under certain specific conditions. There are also clear specific results showing that cloud seeding has not only not increased rainfall, but has possibly decreased rainfall in targeted areas. But, predominantly, the majority of results of cloud seeding experiments, over the last 60 plus years and some 40 countries, have been indeterminate.
In Tasmania, the island state of Australia located to the south of the eastern Australian mainland, experiments by CSIRO in the 1960s resulted in an estimated 30 percent increase in rainfall in the mountainous hydro dam catchment areas. The Hydro-Electricity Commision of Tasmania has subsequently commisioned cloud seeding operations in the region ever since.
Experiments carried out in West Texas and Israel have also indicated positive results, but have subsequently been argued against based on inaccurate statistical analysis. There is still ongoing argument between climatoligists on the technical validity of cloud seeding; nevertheless, there are commercial companies existing in the United States of America, surviving in the main, by providing weather modification services in the form of cloud seeding to State government departments.
The other side of the coin is the onflow effect. If you do in fact increase the rainfall from an air mass in one area, does this not logically mean that land areas downwind from your activity will have a reduced rainfall. While this may not seemingly matter in an island state such as Tasmania, since the onflow area is marine rather than land, it can be quite environmentally and politically significant over continental land areas. In the Pacific Basin, cloud seeding experimentation is ongoing currently in Thailand and China; will any potentially positive results from these generate unsuspected and possibly catastrophic environmental impacts in neighbouring regions?