Friday, May 17, 2019

Cloud Seeding Creates Potential for Humans to Control the Weather

The general goal of cloud seeding is to find some way of converting the supercooled droplets of liquid water in a cloud to ice crystals to ultimately increase precipitation, disperse clouds and fog as well as suppress hail and lightning.

In 2008, crowds filled Beijing National Stadium, anxiously awaiting their teams to flood out of the gates at precisely 8:00pm. At the Olympic Opening ceremony, there was no room for any error. Looming just above this magnanimous event was the threat of a thunderstorm.  

The Beijing Municipal Meteorological Bureau fired 1,104 rain dispersal rockets from 21 sites in the city between 4:00 pm and 11:39pm. This presumably allowed for the ceremony to proceed without any weather disturbances while heavy rain was reported in areas surrounding Beijing.

How did they manage to save the event? The answer – Cloud seeding.

Cloud seeding is essentially dispersing particles into clouds to alter their physical processes happening on a microscale.

A common physical characteristic that is affected is relative humidity (i.e. the ratio of the partial pressure of water vapor in the cloud to the equilibrium vapor pressure of water at the given temperature - when relative humidity is at 100%, precipitation occurs).

Different seeding agents and means of deployment have to be selected depending on the desired effect, the temperature of the cloud as well as a multitude of other factors.

The seeding agents used alter microphysical processes within the cloud, for example by providing additional condensation / ice nuclei or altering relative humidity by introducing hygroscopic particles.

Solid carbon dioxide (dry ice) and silver iodide have proven most effective but only when used in supercooled clouds (i.e. clouds with water droplets at below freezing temperatures), they form nuclei around which the water droplets evaporate. The resulting water vapour deposits into ice crystals. In clouds at temperatures above freezing, calcium chloride particles provide the condensation nucleii around which raindrops form.

Cloud seeding has been used since the 1950s to increase rain and snowfall in the Rocky Mountains, Sierra Nevada and other mountainous areas of the United States. Although there isn’t concrete data on its effectivity, a 2014 study examining two Wyoming mountain ranges found that cloud seeding could increase snowfall by 5 - 15 percent but only when the right conditions were met. Another cloud seeding project in Nevada claims their project increased snowpack by up to 10 percent which they have translated to 80,000 more acre-feet a year of water - enough to sustain about 150,000 households. However, the proposed number for an increase in snowfall is still within the natural variation of storms, leading back to the deduction that there is no concrete data on its effectivity.

There have also been other concerns regarding the safety of this process; some claim that this weather manipulation can amplify drought conditions in one area or increase the risk of floods in another, but due to inconsistent effectivity data, these claims have yet to be proven. The effects on health of the supplementary exposure to silver iodide is also a posed concern, however, scientists found that seeding did in fact add some silver iodide to the surrounding water/soil but far too little to pose a known threat to human health.

There is way too much that is not known about cloud seeding to definitively conclude whether or not the practice should be continued. All we know that it can possibly stop the weather from raining on your next parade.

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