Cloud seeding is the process of introducing a substance to a cloud with a small plane to improve the chances of rain from that cloud or increase the production of rain from a cloud. In Texas, cloud seeding takes place during the warmer, wetter months of the year. Cumulus clouds are the predominant cloud type during this time (March-October) and are responsible for producing the bulk of rainwater in Texas. Cumulus clouds are formed in the process of convection, when near-surface air is heated and rises to meet with cold air. Water vapor then condenses into microscopic droplets creating the white puffiness of a cloud. Rain production is dependent on having a high density of moisture in the cloud. Cloud seeders target these cumulous convective clouds. For raindrops to form, the latent moisture in the atmosphere must build up and collect around a central point. This is called an ice crystal or cloud nucleus. Cloud seeding is intended to introduce into the cloud many more ice crystals. It is the process of introducing additional cloud nuclei to convert the moisture supply into water. If done properly and at the right time, cloud seeding can assist the natural process in clouds by giving them enough "seeds" to make a meaningful number of large raindrops.
Materials and Methods of Cloud Seeding
Silver iodide is the primary seeding material used in Texas because its crystalline structure is nearly identical to the natural ice crystal. An ice crystal such as silver iodide will more readily attract the tiny cloud droplets and those droplets will collide with each other. The ice crystal then builds up enough of the droplets that it becomes heavy and drops out of the cloud as rain. Another material commonly used in Texas cloud seeding operations is a hygroscopic substance like common salts (Sodium Chloride or Calcium Chloride). Hygroscopic materials, which attract and absorb water, are used to enhance raindrop formation in the warm sector of a cloud.
Cloud seeding materials are released in the form of flares (See Figures 1 & 2).
Figure 1Figure 2
They are transported on the wings of an aircraft (See Figures 3 & 4).
Figure 3Figure 4
The flares are released into convective clouds under the guidance of licensed meteorologists, who communicate with pilots in real time to ensure optimal placement based on live weather data. The goal of cloud seeding is to increase rainfall in specifically targeted areas. Operators release their flares in a target zone, and the seeded clouds drop any additional rain formed within a short period of time. The released nuclei are swept up into the cloud, where water vapor condenses around them, causing the droplets to grow large and heavy enough to fall through the base of the cloud as precipitation.
TDLR’s Role in Regulating Weather Modification
The Texas Department of Licensing and Regulation (TDLR) is the State agency responsible for administering the Texas Weather Modification Act, enacted in 1967 by the Texas Legislature and now codified as Chapter 301 of the Texas Agriculture Code. The Act requires the agency to regulate the use of cloud seeding through a licensing and permitting procedure. Furthermore, the Act charges the TDLR with promoting the development, and demonstration, of cloud-seeding technology through research.
In 2003, TDLR took over the role of regulating weather modification operations from the Texas Department of Agriculture. In 1997, the Texas Legislature directed a State agency (initially, the Texas Natural Resource Conservation Commission (TNRCC), then in 2001 the Texas Department of Agriculture and, lastly TDLR) to dispense funds to reimburse political subdivisions (such as water districts and county commissions) for costs incurred in conducting cloud seeding operations. Those funds were exhausted in 2005 and have not been renewed.
TDLR can provide technical assistance to individuals and organizations interested in devising and implementing weather modification programs. If assistance is desired, please contact weather.modification@tdlr.texas.gov.
Weather Modification Operations in Texas
Cloud seeding has occurred in several regions of Texas. Currently, there are five weather modification projects underway. They are in South, Central, and West Texas, covering approximately one sixth of the total area of the state. View a Map of Rain-Enhancement Projects in Texas to see operational areas. Operations in Texas take place during the warmer months, from October to November.
Cloud Seeding Results
Research has been ongoing for decades on the efficacy of cloud seeding. It is not a cure all or drought buster in and of itself. However, research supports it to be an effective method for increasing rainfall. It is difficult to show a perfect comparison of a cloud that has been seeded versus the same cloud not seeded because conditions cannot be exactly replicated outside the lab. The results of cloud seeding are difficult to measure and interpret, even for scientists with a strong meteorological background. In Texas, we can, however, say with a high degree of certainty that cloud seeding operations have led to an increase in overall rainfall.
A 10-year study of weather modification operations in west Texas from 2004 to 2013 found that rainfall increased on average by 8-20% (Jennings & Green, 2014). That means that overall rainfall increased by about 2 inches per year. This amount was significant to the region as it increased groundwater recharge in a time of drought. With growing groundwater supply concerns in Texas, cloud seeding provides means of enhancing water supply. Cloud seeding adds to that storage to the benefit of local Texans. The overall cost of weather modification operations during the 10-year Jennings and Green study period in west Texas was $1.5 million. Another study found that the dual seeding method (hygroscopic and glaciogenic materials) produced an average increase of 1.34 inches of rainfall in 2016 (LaRoche, et. Al, 2017).
Results from the individual weather modification operations in Texas may be found on the websites of the organizations. Website links can be found on our Summary of Rain Enhancement Operations in Texas page.
Cloud Seeding During Drought Conditions
Cloud seeding is more effective in non-drought periods. This is because seeding is predicated upon the availability of convective clouds. During severe drought, the number of days with convective clouds is diminished. Consequently, those familiar with weather-modification technology view cloud seeding as a viable, long-term water management strategy for augmenting fresh-water supplies over the long haul, not as a short-term, quick "fix" to the drought problem. Those doing cloud seeding commit to its use over a period of at least several years, not a few months.
Impacts of Cloud Seeding on Health and the Environment
There are two seeding compounds used in Texas. They are silver iodide and calcium chloride. The environmental and health impacts of calcium chloride are considered negligible. It is a common salt that naturally occurs in much higher concentrations than the amount that is released during cloud seeding. Calcium chloride is considered food-safe and is used as a firming agent in food production. Silver iodide is more widely used as a seeding compound. It is formed by a very strongly bonded compound of silver and iodide and is non-toxic in concentrations used for cloud seeding.
The benefit of using silver iodide for cloud seeding is its high insolubility and its structure. Lack of solubility means that it is not easily broken down by water. The second big plus is it has a similar crystal lattice to ice crystals. This is what allows it to work well as a supplemental ice crystal or nuclei for a rain drop. The strong chemical bond between silver and iodide makes it stable and virtually insoluble. This means that silver iodide will not dissolve in the atmosphere or groundwater to potentially cause contamination.
A recent report by the U.S. Government Accountability Office stated that “existing research we reviewed, while limited to a handful of recent studies, suggests silver iodide does not pose an environmental or health concern at current levels. However, it is not known whether more widespread use of silver iodide would have an effect on public health or the environment” (GAO, 2024). The amount of material used in a cloud seeding operation is miniscule compared to the area of effect. The current concentrations of material are highly diluted and pose no risk to human health or the environment. A comprehensive paper on potential effects of silver iodide on humans and the environment from cloud seeding in Utah concluded there was no threat from silver iodide releases from cloud seeding (Cardno ENTRIX, 2011).
Weather Modification for Hail Suppression
Cloud seeding is used to try to reduce both the amount and size of hailstone by increasing rainfall. The additional ice crystals released in the convective cloud compete with the hail seeds (or "embryos") for the available cloud moisture. This reduces the chances of large hailstone formation. Cloud seeding can thus be used as a potential method of reducing hailstone size therefore reducing the damage produced by violent hailstorms. The amount of rainfall produced by the cloud can also be increased when seeded to suppress hail.
Additionally, weather modification law in Texas requires the applicant for a permit to publish a "Notice of Intention to Conduct Weather-Modification Operations" in area newspapers for three consecutive weeks. This Notice apprizes the public of its right to request a public hearing on the proposed cloud-seeding project. In the absence of a call for a hearing, a permit application can be processed by TDLR within 30-60 days.
Comments or questions about weather modification in Texas can be sent to weather.modification@tdlr.texas.gov and the advisory board periodically holds open meetings where public comment can be made.
