Photo by Michael & Diane Weidner on Unsplash
In response to shortage in water resources, exacerbated by growing populations and a changing climate, an increasing number of countries have invested in weather modification, research and applications. Precipitation enhancement is a subset of weather modification that aims to augment natural rainfall (or snowfall) amounts through airborne or ground-based interventions in the microphysical processes of specific cloud types.
Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation, mitigate hail or disperse fog. The usual objective is to increase rain or snow. Cloud seeding is undertaken by dispersing substances into the air that serve as cloud condensation or ice nuclei. Common agents include silver iodide, potassium iodide, and dry ice, with hygroscopic materials like table salt gaining popularity due to their ability to attract moisture. Techniques vary from static seeding, which encourages ice particle formation in supercooled clouds to increase precipitation, to dynamic seeding designed to enhance convective cloud development through the release of latent heat. Targeting warm clouds, hygroscopic cloud seeding entails introducing large artificial (hygroscopic) aerosol particles into clouds to increase the uptake of available cloud liquid water beyond that expected from the natural background aerosol population with relatively smaller diameter (4-6). The larger seeding particles are expected to trigger a "competition effect" which favors the production of large drops that can activate that collision-coalescence process to enhance rainfall generation.
The basic assumption is that increased cloud buoyancy, achieved through conversion of cloud water content into liquid droplets through condensation, will enhance the release of latent heat, increase cloud depth and result in larger cloud extent and lifetime and also rainfall intensity.
According to the most recent review on global precipitation enhancement activities conducted by the WORLD METEOROLOGICAL ORGANIZATION (WMO) expert team on weather modification, cloud seeding from aircraft platforms is generally considered more effective compared to other techniques such as ground-based generators, customized rockets and artillery shells. Results from operational cloud seeding programs spanning several countries including, Australia, China, India, Israel, South-Africa, Thailand, and the United States record between 10-30% increment in precipitation amounts and cloud lifetime. Alternatively, several studies report on the limited efficacy of seeding experiments for drought relief.
Located on the eastern Arabian Peninsula, the United Arab Emirates (UAE) features a hyper arid climate. Its annual rainfall varies from 30mm in the inland desert to 120mm in the Al Hajar Mountains in the eastern and north-eastern parts of the country. The vast majority of the precipitation falls from November to March with a substantial interannual variability and a long-term decline seen throughout the region. Although less frequent, summertime rainfall can be significant, with localized accumulations in excess of 100 mm.
As an attempt to mitigate the arid conditions and ongoing drying tendency, which may intensify with global warming, precipitation enhancement activities such as cloud seeding, have been increasingly performed in the UAE. Seeding is mostly conducted on the eastern half of the country, where summertime convection is more frequent and cloud seeding aids in the recharge of the under-ground aquifers.
The UAE has an arid climate with less than 100mm per year of rainfall, a high evaporation rate of surface water and a low groundwater recharge rate. Rainfall in UAE has been fluctuating over the last few decades in winter season between December and March. The climate of UAE is a very dry region aside from the coast and the border of the UAE and Oman, where there is high humidity.
Due to industrialization and population growth, the demand for water has rapidly increased. Current resources are being depleted and scarcity issues are arising. Consequently, the UAE is looking to cloud seeding technologies to increase water security as well as renewability to combat water and food scarcity that may arise.
Even if cloud seeding does succeed at increasing precipitation, environmental activities are concerned about its impact. One scientist addressing those concerns is Geochemist, SHAWN BENNER at Boise State University. "The near impossibility of detecting a silver iodide signal in snowpack after seeding attests to its low environmental risk", Benner says.
Nevertheless, if the practice of cloud seeding intensifies at a larger scale, silver toxicity and other environmental issues could become a concern. Aside from silver toxicity, some cloud-seeding critics raise concerns about messing with the balance that Mother Nature holds on to the atmosphere. The amount of moisture in the atmosphere is determined by the balance between evaporation and precipitation. Atmospheric budgets suggests that cloud seeding is unlikely to steal moisture from downwind sites, responds weather modification's Boe. Because clouds represent a modest portion of the moisture in the atmosphere, a cloud-seeding effect of 15% would only remove about 1-2% of the total water vapor in the seeding area.
But at present, making rain is still more of an art than science. With countries increasingly spending hundreds of millions of dollars on weather modification, more research is needed to understand if the practice works and what its environmental, social and governance impacts will be.