1998 SEEDING OPERATIONS
EVAC is no longer officially involved with the Oklahoma Weather Modification Program. This page summarizes the 1998 operations as an example of the scope of the program. For current information please go the Weather Modification, Inc. web site. During seeding season (generally late March through October) you can view the latest radar pictures from the Wiley Post and Woodward radar sites. If seeding aircraft are in the air their most recent flight tracks will be displayed as well.
The 1998 Oklahoma Weather Modification Demonstration Program began March 20th and concluded on October 17th. There were three seeding planes in Oklahoma. Two of the planes were based at Wiley Post Airport in northwest Oklahoma City, and the third plane was based at Woodward Airport.
The following table summarizes the flights by month and type. You can also view a similar summary by county. The totals for the two tables will not match because a single flight usually effects more than one county.
Not every cloud makes a good seeding candidate. For seeding to be effective the cloud top must extend above the freezing level and it must contain sufficient supercooled liquid water (see the cloud physics page). It is also important to seed the correct part of the storm complex. Seeding will have little effect on a mature cell, so it is the developing cells which feed into the main cell that are seeded. Seeding can be performed either at cloud base or at cloud top, but top seeding is preferable because the pilot has a much better idea of where to seed and the seeding agent can be targeted more accurately and quickly. Silver idodide (AgI) is used as the seeding agent. Each seeding plane is equipped with burn-in-place flares and ejectable flares. Both types of flare release silver iodide while they burn, but they have different uses. Burn-in-place flares are used for cloud-base seeding in an attempt to draw the silver iodide into a cell updraft. The seeding material will not produce ice crystals until it rises above the freezing level. This causes a considerable time lag and makes it difficult to predict exactly where the material will go. Ejectable flares are dropped directly into the target updraft. They are more effective because they can produce ice crystals as soon as they enter the cloud and are easier to precisely place within the cloud.
A cloud physics research plane was in Oklahoma in May and October of 1998 to conduct research flights. The plane was specially instrumented with several sensors to sample cloud microphysical properties. Sample images of cloud particles are captured using a laser by what is known as a 2DC probe. This information is used to determine the ice concentration. The droplet size spectrum of a cloud is also measured optically using a laser by a forward scattering spectrometer probe. The size spectrum can be integrated to determine the total liquid water content. Liquid water content can also be measured electrically using a King probe. The cloud condensation nuclei (CCN) concentration is measured in a miniature cloud chamber. Along with these parameters, basic information like air temperature, dewpoint temperature, position, altitude, airspeed, and wind speed and direction are continuously recorded.
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