Behind the Forecast: Can we manipulate the weather

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Science Behind the Forecast: Cloud Seeding? Does it work?

LOUISVILLE, Ky. (WAVE) - Can humans intentionally modify the weather? We’ve certainly tried to do just that for more than 70 years.

One of the most common forms of weather modification is cloud seeding. In the late 1940s, scientists discovered that crystals of silver iodide can form ice crystals in water vapor leading to more research into weather modification.

The goal of cloud seeding is usually to increase precipitation across a region, most often in the hopes of increasing an area’s water supply. Sometimes, weather modification is also used in an attempt to prevent dangerous weather, like hurricanes, fog, or hail, from happening.

For a cloud to be effectively seeded, it must contain supercooled water droplets. Supercooled water is liquid water that is colder than zero Celsius. Substances used for seeding must have a crystalline structure like ice to help initiate freezing. Silver iodide, dry ice, liquid propane, and various salt compounds are used for cloud seeding.

Keep in mind that all clouds can’t be seeded; specific criteria must be met. Summer clouds must be tall enough to reach temperatures cold enough for supercooled water droplets to form, lack natural ice formations, and have a “sustained updraft of moist air,” according to the North American Weather Modification Council. Supercooled winter clouds must stick around long enough for snowflakes to fall.

For mid-latitude clouds, seeding is based on the fact that when ice particles form in supercooled clouds, they grow at the expense of liquid water droplets. Eventually, they fall as raindrops from a cloud that otherwise would not have produced any. The idea of “dynamic seeding” in tropical cumulus clouds or “warm clouds” is based on the idea that additional latent heat adds to the buoyancy within a cloud, strengthening updrafts, causing more low-level convergence, leading to the growth of the cloud.

Does cloud seeding work? There has been no conclusive evidence on the topic as a whole. There has been quite a bit of research on the topic over the past decades. In a statement about weather modification, the American Meteorological Society states that “There remain limits to the certainty with which desired changes in cloud behavior can be brought about using current cloud seeding techniques. Continued effort is needed toward improved understanding of the risks and benefits of planned modification through well-designed and well-supported research programs.”

The National Weather Service along with other government agencies has looked into weather modification, mainly the manipulation of hurricanes. Project STORMFURY ran from 1962 through 1983. Project STORMFURY’s goal was to try to weaken tropical cyclones and flying airplanes into the systems and cloud seeding. The theory was that cloud seeding would help to disrupt the cyclone’s structure, helping to reduce a hurricane’s wind speed by almost 30%. The criteria for hurricanes that could be seeded in this program were very specific. According to NOAA, the storms had to “have less than a 10% chance of making landfall within a day, be in the range of the seeding aircraft, and be an intense storm with a well-formed eye.” Results from seeding Hurricane Beulah in 1963 and Hurricane Debbie in 1969 were promising but inconclusive.

All non-Federal weather modifications must be reported to NOAA’s Office of Weather and Air Quality who then passes the information along to the U.S. Secretary of Commerce. You can find a list, which is updated annually, of them here.

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