Ahern.Smith.spring2011

= Cloud Seeding =

__What is it?__ Cloud seeding is a process of weather modification in which substances are disperced into the air to alter the amount or state of precipitation that falls from clouds. Dry ice or more commonly silver iodide aerosols is sprayed into the upper parts of clouds to stimulate the formation of rain clouds and the prcitipation process.

__How it was found__ The idea for cloud seeding can be credited to two Scientists. Schaefer and Vonnegut were the first to discover that cooled water in its liquid state could be converted to ice crystals by using dry ice or silver iodide. These two substances are known as glycogenic materials, meaning they can cause glaciation inside of a cloud. Glaciation is the forming of ice. Silver iodide is the most commonly used in cloud seeding, due to its ability to rapidly increase the ice concentration inside of clouds.

__How does it work?__ When warm air rises from the earth, it cools and eventually forms tiny droplets of water. These droplets of water form around small particlessuch as dust, salt or soil, called cloud condensation nuclei. When gathered together, these droplets form clouds. When millions of droplets merge together in clouds to form bigger droplets, which fall back down to the earth as rain in warm temperatures. In cold weather, ice crystals are formed. Other droplets freeze onto these droplets until they grow heavy enough to fall down to the ground as snow or hail. Basically cloud seeding is a process in which other particles are introduced into a cloud to act as an alternative cloud condensation nuclei. There are three different types of cloud seeding. The process by which cloud seeding occurs, has to due with the ability of the silver iodide, to allow ice crystals to form through a process known as nucleation. In the process of nucleation, the silver iodide provides a particles that ice crystals are able to form around. As molecules of water freeze, they build up around the silver iodide particles acting as a "nucleus," to form either a solid piece of ice know as hail, or a more elaborate form known as a snowflake. The definition of cloud seeding is in fact, "artificial nucleation." The glycogenic materials are key in the process of cloud seeding, with out the artificial nuclei provided by either silver iodide or dry ice, the increase of precipitation through cloud seeding would not be possible.
 * Static mode - adding ice crystals to a cloud to increase rainfall
 * Dynamic mode - essentially the same thing as static mode, but a much larger number of ice crystals are bing added
 * Hygroscopic seeding - salt crystals are shot into a could. The particles then grow until they are large enough to with water droplets to cause precipitation


 * [[image:cloud_seeding.gif]] ||
 * Picture from http://www.edwardsaquifer.net/cloudseeding.html ||

__Methods__ There are two main methods of cloud seeding. There is a static concept which relies on, clouds that are well below freezing temperatures, to increase precipitation, as well as a dynamic seeding concept, which has been found to be more efficient.

The static concept also know as the static mode of cloud seeding. This idea was developed by Cotton and Pielke in 1995. This process relies on the idea that clouds lacked a large enough quantity of concentrated ice crystals. The plan in the static concept was to release seeding material into the base of a cloud and allow ice crystals to form. The main problem with the static mode of cloud seeding was that there was a very specific time window, in which the seeding needed to take place in order to increase the amount of precipitation sufficiently. The window of opportunity to properly seed a cloud using the static concept, relies on three main factors. 1. Clouds needed to be cold based and continental rather than oceanic clouds. 2. The maximum temperature of the clouds could not exceed the range of -10 degrees celsius. 3. There must be a large enough quantity of supercooled water already inside of the cloud before seeding could occur. Due to the limited time window in which seeding cloud be performed, the static mode of cloud seeding is not as efficient as the more widely experimented dynamic method.

The dynamic mode of cloud seeding is know to be the more efficient process for increasing precipitation. The dynamic concept relies on enhancing the vertical air currents within a cloud in order to spread the seeding material over a larger area with in the cloud. This allows more supercooled water already inside of the cloud to be formed into ice crystals uniformly throughout the entire cloud rather than just forming at the base. The main difference between the static mode and the dynamic mode, is the quantity of seeding material used, and how it is introduced into the cloud. In the dynamic concept a much larger quantity of seeding material is used, and is introduced to a cloud by the use of silver iodide flares that are dropped on top of the targeted cloud. This method produces anywhere from one hundred to one thousand ice crystals per liter of water in the cloud. Where as the static method only produces in the range of one to ten ice crystals per liter. This huge difference in production of ice crystals results in a much larger increase in precipitation by using the dynamic method over the static concept.

Usage:


 * [[image:cloud-seeding-5.jpg]] ||
 * http://science.howstuffworks.com/nature/climate-weather/meteorologists/cloud-seeding2.htm ||

Staff members from Beijing's Xiangshan Weather Modification Practice Base stand next to a two-pipe cannon used for rain reduction and cloud dispersion. Bad ass.


 * [[image:slide7.jpg]] ||
 * Photo from http://www.sandylandwater.com/slide7.html ||


 * [[image:slide8.jpg]] ||
 * Photo from http://www.sandylandwater.com/slide8.html ||

Silver Iodine being released into the clouds to serve as the artificial nucleation

__Key Points__

There are still problems that can result from weather manipulation, and specifically cloud seeding. In 2010, the chinese government performed a cloudseeding experiment, that resulted in record snowfall, and over forty weather related deaths. All though there are some instances where cloud seeding hashad negative effects, there have also been several positive benefits. Cloud seeding has been used to increase snow fall in Wyoming for ski parks, help to end droughts in agricultural areas, and relief areas suffering from heat waves where there are dangers of wild fires.