Clouds form when air becomes supersaturated, forcing water vapor in the atmosphere to condense into visible liquid cloud droplets. Ice crystals will also form in regions of the cloud where temperatures are below freezing. However, it is possible for liquid droplets to exist in subfreezing environments. In fact, pure water droplets can remain in liquid form down to temperatures near -40°F. This supercooled liquid water (SLW) is the key to cloud seeding.
For a cloud to produce precipitation (rain, snow, sleet, hail, etc.) particles within the cloud must become large enough to fall through the base of the cloud and reach the surface without evaporating. One way that cloud particles grow is through the collision-coalescence process: as the cloud particles move around they collide with each other to form larger particles which eventually become large enough to fall as precipitation. This process is not very efficient. The other process only occurs in clouds which contain ice and is known as the Bergeron process: it is easier for water vapor to condense onto an ice surface than a water surface, so ice particles within a cloud will grow faster than nearby liquid droplets. Cloud seeding is designed to take advantage of this process.
Some clouds contain an abundance of SLW but a shortage of ice crystals. If the SLW in the cloud could be converted into ice the cloud would be more likely to produce precipitation. SLW is unstable--it will freeze upon contact with ice nuclei or if it is cooled below its critical temperature. One cloud seeding method is to "seed" the cloud with artificial ice nuclei (like silver iodide) to induce contact freezing of the SLW. Another method uses dry ice pellets to cool the SLW below its critical temperature, causing it to freeze. There is also a secondary benefit to the conversion of SLW to ice: the freezing of a substance releases heat to the environment. This heat will add to the energy of cloud and can increase its intensity and/or duration. Seeding for hail suppression is carried out by introducing too many ice crystals for the available water vapor: none of the ice crystals will be able to grow large enough to form large hail stones.
Static Phase Hypothesis
One of the two basic tenets of cloud seeding is the static phase hypothesis, which is based on cloud microphysics. In order for a cloud particle (water droplet or ice particle) to fall as precipitation it must grow to a minimum size. One of the growth mechanisms of water droplets is by the condensation of water vapor onto the surface of the droplet. Ice particles can grow in a similar fashion by the condensation and subsequent freezing of water vapor (riming), or the direct deposition of water vapor onto the particle. For water vapor to condense onto a liquid droplet the air surrounding the droplet must be supersaturated with respect to a water surface. Similarly, for condensation or deposition onto an ice particle the air must be supersaturated with respect to an ice surface. At subfreezing temperatures, supersaturation with respect to ice occurs at lower relative humidities than supersaturation with respect to water. Therefore, in the same environment, ice particles will grow faster than water droplets. As the water vapor is depleted the environment will become subsaturated with respect to water but will still be supersaturated with respect to ice. At this point, the water droplets will evaporate while the ice particles continue to grow. Thus, the ice particles will grow at the expense of the water droplets. An additional factor is the presence of supercooled water droplets within a cloud. Supercooled droplets are unstable and will spontaneously freeze upon contact with ice particles or aerosol particles which act as ice nuclei.
Cloud seeding is designed to take advantage of clouds which contain large amounts of supercooled water relative to the number of available ice particles/nuclei. Precipitation will not occur if there are not enough ice particles/nuclei to act as a focus for the formation of larger particles. Seeding is an attempt to remedy this by the introduction of ice nuclei into the cloud. The nuclei will interact with the supercooled water to produce small ice particles. These particles will immediately begin to grow at the expense of the remaining cloud liquid water. If enough water is present, the ice particles will grow large enough to fall as precipitation. A cloud can be overseeded. If too many ice nuclei are introduced for the amount of liquid water present, none of the ice particles will be able to grow large enough to fall as precipitation. Overseeding is the basis of hail suppression; if there is an abundance of ice nuclei, a single ice particle can not grow large enough to become a hazardous hailstone.
Dynamic Phase Hypothesis
The second basic idea of cloud seeding is the dynamic phase hypothesis, which is based on the dynamics of a cloud. Cumiliform clouds are dependent on a updraft to survive. Air within the updraft experiences adiabatic cooling as it rises, and at some point it will become supersaturated with respect to water. When this happens the water vapor within the updraft will begin to condense into cloud droplets. If the updraft ascends through the freezing level ice particles will begin to form as well. While new cloud particles are forming within the updraft, existing ones are continuously evaporating and sublimating. For the cloud to survive, the updraft must introduce new water vapor in sufficient quantities to counteract the evaporation and sublimation. One of the factors influencing the influx of water vapor is the updraft speed. Updraft speed is proportional to parcel buoyancy, and parcel bouyancy is a funtion of the temperature difference between the parcel and its environment. As water vapor is converted into liquid droplets or ice particles it releases latent heat to the parcel, thus increasing the temperature difference. This increased bouyancy will enhance the updraft and increase the water vapor influx. Positive feedback will occur as the increasing quantity of water vapor condenses/deposits and releases even more latent heat. Particles suspended in the updraft may eventually grow large enough to overcome the upward velocity of the updraft and fall to the ground as precipitation. Precipitation has a very detrimental effect on the cloud. Clouds which develop in weak shear environments will have a vertically oriented updraft. In this situation, precipitation which forms will fall straight down through the updraft. The weight of the precipitation and the drag it creates as it falls will dissipate the updraft. Also, the precipitation is removing large amounts of water from the cloud which the updraft can no longer replenish. Once the updraft has ceased the cloud will quickly evaporate.
In most cloud seeding operations the microphysical enhancement is emphasized over any dynamic enhancement, but both processes are interdependent. The main goal of cloud seeding is to accelerate the conversion of water vapor into cloud particles. Any increase in this process will also increase the amount of latent heat released into the cloud environment. This will enhance the updraft and increase the water vapor influx into the cloud, which will begin the positive feedback cycle again. A stronger updraft will extend the lifetime of the cloud and the duration of any precipitation that cloud produces. Updraft speed also has a more direct effect on precipitation. Precipitation particles are suspended in the updraft until their fall speeds exceed the speed of the updraft. Therefore, a stronger updraft produces larger precipitation particles. Larger particles are more likely to reach the ground as they begin to evaporate in the subsaturated air below the base of the cloud.
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