|Frequently Asked Questions
|1. How does cloud seeding work?
2. How successful is cloud seeding?
3. What impact does cloud seeding have on hail?
4. Will the silver iodode or other materials reach the earth?
5. What about the "downwind effect" of cloud seeding?
6. What role does the State of Texas assume in weather modification?
7. Where is cloud seeding being done in Texas?
8. What does one do to get a cloud seeding program underway?
1. How does cloud seeding work?
Cumulus clouds–the billowy, cauliflower-shaped clouds often seen in Texas skies during much of the year–are important rain
producers in every sector of the state. As the predominant cloud type during the warmest eight months of the year (March-October),
cumulus (or convective) clouds are responsible for producing the bulk of rainwater given by the atmosphere in any given year in
Texas. These towering cloud formations form from strong updrafts of warm, moist air into an atmosphere that is unstable. Intense
daytime heating of the near-surface layer of air, or a wedge of cold air moving across the state (as a cold front), usually triggers the
formation of convective clouds.
Not all cumulus clouds become rain producers. In fact, only a small percentage of them ever develop the capability to yield an
appreciable amount of rainfall. Those convective clouds that do produce rainwater are often inefficient: For all the moisture they
incorporate from below, only a tiny fraction of that moisture (as cloud droplets) is ever used to grow large raindrops, which ultimately
fall to the ground as rainfall. This may be due to the fact that an insufficient number of ice particles exists within the cloud, thereby
limiting the amount of cloud droplets that can coalesce to create raindrops. Or the clouds simply do not live long enough, on their
own, to allow those tiny cloud droplets to collide enough times with neighboring droplets to yield larger drops–and eventually
rainwater. Seeding is intended to introduce into the cloud many more of these ice crystals (also called cloud nuclei) to allow much
more of the moisture supply within the cloud to be converted into rainwater.
|If done in a timely way and properly, cloud seeding can assist the natural process in clouds by giving them
enough "seeds" to make a meaningful number of large raindrops. If a lot of the growing convective cloud has
pushed upwards above the freeze level, the bulk of the cloud water above that freeze level becomes supercooled
(which is to say, the cloud droplets remain in liquid form and do not turn into ice). But those supercooled cloud
droplets readily attach to an ice crystal (natural or artificial), converting the ice crystal into a tiny snowflake or
graupel, which can quickly grow into a raindrop before the cloud begins to collapse.
Silver iodide is a favored seeding agent because its crystalline structure is nearly identical to the natural ice
crystal. When placed in the upper portion of the growing convective cloud rich with supercooled droplets, the
silver iodide crystal can grow rapidly by tapping that vast field of available moisture. Indeed, because the vapor
pressure gradient over ice is less than that over water, an ice crystal such as silver iodide will more readily
attract the tiny cloud droplets than those droplets will collide with each other. In a matter of moments, the ice
crystal is transformed into a large raindrop which is heavy enough to fall through the cloud mass as a rain shaft.
The silver iodide particles (nuclei) are sometimes released from below cloud base, using the strong updraft of
the cloud to transport the "seeds" high into the core of the cloud where supercooled cloud droplets are plentiful.
Pyrotechnics, or flares, consisting of silver iodide burn while mounted on the wings of an aircraft that maneuvers
within the updraft field below the bottom of the cloud. At times the seeding material can be dispensed below
|cloud base from an aircraft that is equipped with wing-tipped generators that contain a solution of acetone mixed with seeding material. The seeding of clouds may also be achieved from
above cloud top, using an aircraft equipped with a rack containing ejectable pyrotechnics. These droppable flares are ignited as they fall from the plane’s belly into the upper region of
seedable convective clouds. Either way, from above cloud top or below cloud base, seeding with silver iodide is designed to give an ample number of "seeds" with which to grow rainwater:
one gram of silver iodide can supply as many as ten trillion (10,000,000,000,000) artificial ice crystals!
convective clouds, especially in drought periods, never cultivate
much "supercooled" cloud droplets, so seeding with silver
iodide may be counterproductive. The clouds still have an
How successful is cloud seeding?
Over 50 years of research and actual cloud seeding in more than 40 countries have demonstrated that properly-designed programs operated by competent persons can increase seasonal
rainfall appreciably and beneficially. The American Meteorological Society (AMS) and the World Meteorological Organization (WMO) have issued policy statements on weather modification that
attest to the efficacy of existing technology to enhance precipitation.
It is likely that cloud seeding is more effective in non-drought periods. This is because seeding is predicated upon the availability of clouds—especially the right kinds of convective clouds. It
is presumed that, during severe to extreme droughts, the number of days with treatable convective clouds is reduced. Still, there is ample evidence suggesting that, even when drought is
harsh, there are opportunities to seed clouds.
Consequently, those using weather-modification technology are urged to view cloud seeding as a viable, long-term water management strategy for augmenting fresh-water supplies, not as a
short-term, quick "fix" to the drought problem. Those doing cloud seeding are urged to commit to its use over a period of at least several years, not merely for a few months. If cloud seeding is
done in the midst of a bad drought and results are not satisfactory, the inclination after a few months is often to deduce that cloud seeding, because it does not appear to have been delivered
the desire results, should be discarded. It cannot be overstated that drought is not the optimal time period for cloud seeding.
|Using Federal funds to assess the long-running rain-enhancement program of the CRMWD in the Big Spring area during 1987-
1990, the TNRCC performed a series of cloud-seeding "experiments" which produced evidence that timely seeding, with silver
iodide, enables convective clouds to live longer, process more cloud water, and produce significantly more rainfall (from 50 to 100
percent from individual cells). Moreover, an ongoing statistical evaluation of the CRMWD’s 29-year cloud seeding program has
revealed that rainfall, averaged over the growing season in the area where seeding has been concentrated, has been increased,
during the years of seeding, by an average of 20-30 percent. A similar study of rainfall data from a 5-year cloud-seeding program
conducted for the City of San Angelo (1985-1989) found that rainfall during the months of seeding in the area where seeding was
focused had been increased 25 to 42 percent.
For the past several years, the Texas Department of Agriculture has conducted independent evaluations of ongoing seeding
activities in Texas. A statistical analysis of all seeded thunderstorms in Texas during 2002 (which totaled 897) estimated that an
additional 481,252 acre-feet of water was generated as a consequence of seeding. Comparing the seeded storms with
neighboring untreated storms (designated as “control clouds” for the sake of analysis) revealed that seeded storms lived 39
percent longer, covered 18 percent more territory, and produced a “precipitation mass” that was 89 percent greater than for cloud
towers left unseeded. During the year, a total of 5,090 pyrotechnics (flares) containing seeding material were dispersed at the ten
projects. It was estimated that the seeding activity provided eligible (“seedable”) thunderstorms with 30-40 ice nuclei per liter of air
within the cloud mass.
There is no evidence that the seeding contributes to less rainfall anywhere else. What is more, there is no evidence that seeding causes clouds to grow substantially taller and produce
3. What impact does cloud seeding have on hail?
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