A 10-year Analysis of Operational Cloud Seeding Conducted by the Texas Weather Modification Association

The goal of the Texas Weather Modification Association (TWMA) is to increase water supplies and to promote water conservation by increasing precipitation through the use of weather modification. Current precipitation enhancement programs within the TWMA cover 22.7 million acres, which includes 35 Texas counties. Since 2001, the TWMA has hired an outside contractor to conduct an ongoing analysis of the state weather modification operations. Due to changes in radar use for the 2004 season, analysis from 2001, 2002 and 2003 will not be included in the totals or averages throughout the paper.

Over the currently analyzed eight year period, 1,318 operational seeding days were conducted resulting in 3,106 total clouds being seeded. Clouds were analyzed and sorted into three cloud types; small clouds, large clouds and type-B clouds. Small clouds saw significant increases in precipitation mass and cloud mass, and outside of 2009, each season saw increases in precipitation mass greater than 100 percent. Large clouds had increases in all variables analyzed and saw a notable lifetime increase from 13 to 26 percent over the six year span. Three years of the study showed large clouds to live on average up to 40 min longer than the control sample. Increases were not as notable for type-B clouds since proper timing of seeding cannot be achieved. Cloud lifetime is a critical variable when targeting seedable clouds while performing operations. Type-B clouds are seeded later in their lifetime because they develop outside the target areas where seeding cannot be conducted; they are later seeded when they enter the target area where seeding is permitted and funded. Analysis of type-B clouds indicated increases in precipitation mass of 7 to 16 percent.

Currently, TWMA programs primarily use glaciogenic flares to seed convective clouds. Beginning in 2009, experimental seeding with hygroscopic flares has been conducted. Through the 2009, 2010 and 2011 seasons, analysis of hygroscopic seeding found significant increases in precipitation mass, which led to a more aggressive approach to seeding with hygroscopic flares during the 2012 season. Results are still inconclusive, however, signals found within clouds seeded with both glaciogenic and hygroscopic flares have been evaluated as positive.

Overall, results from the analysis through the 2011 season indicate an increase of 36.89 millimeters of rainfall area wide during the seeding months, March through October. This translates to a total increase of 3,419,506 acre-feet of water per year for those counties who fund weather modification programs. These precipitation increases allow water conservation districts to find economic benefits of funding such programs that aid in water conservation.