Precipitation Enhancement Program

Science behind the program

To be successfully seeded, clouds must pass the following three tests:

• Super-cooled: The cloud top must be significantly colder than 32=F. (14=F is ideal).
• Ice Free: If the cloud contains a significant concentration of ice, it may not be seeded, as nature is already acting efficiently.
• Updraft: The cloud must sustain an updraft of at least several hundred feet per minute. This updraft conveys a continuing supply of moisture into the cloud.

Factors that determine cloud top or cloud base seeding are: storm top structure, visibility, cloud base height, or time available to reach seeding altitude. The seeding agent injected in the clouds is silver iodide which has a crystalline structure that resembles natural ice in the atmosphere. This material is released from ejectable flare cartridges at cloud top and burns as it drops through the cloud. At cloud base, the seeding agent is dispersed by the use of acetone generators, releasing the material into the updraft of the cloud.

Cloud top seeding is conducted between +23=F and +5=F. The pencil sized flares fall approximately 1 mile during their 50-second burn time. The seeding aircraft penetrate the edges of single convective cells meeting the seed criteria. Cloud base seeding is conducted by flying at cloud base within the main inflow of single cell storms, or the inflow associated with multi-cell storms.

Seeding effects can range from almost immediately to up to 30 minutes depending on the seeding delivery method (direct injection at the top of the cloud, or base seeding, releasing seeding agent in the updraft at the cloud base). The average time required before a seeded cloud reverts to its natural characteristics is between 15 to 30 minutes.