Does cloud seeding work? Scientists watch ice crystals grow inside clouds to find out
Posted by admin on 14th March 2018
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Water is a valuable resource that affects nearly all aspects of life on earth. It also is limited, so people use a variety of methods to ensure that supply meets demand.

One such technique is cloud seeding – adding particles to the atmosphere to promote formation of rain or snow. Today many entities across the West – including state and local goverment agencies, utilities and ski areas – seed clouds in an effort to boost winter snowfall in the mountains. More snowpack means more spring and summer runoff, which feeds local water supplies, irrigates crops and fuels dams that generate hydroelectric power.

Cloud seeding has also been used in efforts to disperse fog at airports, boost summer rainfall and reduce hail. In fact, cloud seeding occurs in more than 50 countries worldwide. Yet despite all of this activity, we still don’t know whether it works.

We are atmospheric scientists and recently conducted a field study to evaluate cloud seeding as a means of enhancing mountain snowpack from winter storms. Our results clearly demonstrate that, at least under certain conditions, it is possible to change the evolution and growth of cloud particles, leading to snowfall that otherwise would not have occurred. The next question is whether cloud seeding can be an effective tool for water managers in the western United States.

Snow cover in California’s Sierra Nevada mountains in an average snowfall year versus a drought year. Snowpack is an important water source for western states.
NASA/Modis

Creating crystals inside clouds

Clouds are made up of water droplets that are too small to fall as precipitation. These droplets often supercool to temperatures well below the freezing point – as low as 0 degrees Fahrenheit (minus 18 degrees Celsius) or colder. In many circumstances ice crystals (which can grow rapidly in the presence of supercooled liquid) must be present for a cloud to produce any significant amount of precipitation. For clouds that form as air is lifted over a mountain, if no ice crystals or too few of them are present, many of the water droplets that make up the cloud simply evaporate on the downwind side of the mountain.

Winter cloud seeding is based on a hypothesis that when supercooled water exists in a cloud, it can be modified by introducing particles that act as artificial ice nuclei. This process creates ice crystals that will utilize the supercooled water to grow large enough that they eventually fall to the surface as snow.

Cloud seeding was pioneered by atmospheric scientist Bernard Vonnegut, brother of famed novelist Kurt Vonnegut. In 1947 Vonnegut’s lab showed that silver iodide was an effective ice nucleus that could form ice at temperatures much warmer than naturally occurring ice nuclei.

Over the next 40 years, scientists studying cloud seeding made significant discoveries about nearly all aspects of cloud physics. Despite this, in 2003 the National Research Council concluded that “there still is no convincing scientific proof of the efficacy of intentional weather modification efforts.” Nonetheless, states and communities pressed on with operational cloud seeding, while research on its effectiveness ground to a halt.

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