11 November 2013 | By Matt Weiser | The Sacramento Bee |

SMUD and PG&E approach cloud seeding differently. PG&E uses ground-based equipment to release silver iodide near Lake Almanor, in the Mokelumne River watershed, and in partnership with the Kings River Irrigation District. Silver iodide in a liquid solution is sprayed into a propane burner, which dissolves the silver iodide and shoots it upward into passing clouds.

All the equipment sits on a trailer that is rolled into position each winter and controlled remotely. The advancement of cellphone networks over the past decade means the equipment requires little tending in the field, which has helped slash costs. The Desert Research Institute pioneered these methods and uses similar equipment on trailers in the Tahoe Basin.

SMUD did the same in the Upper American River watershed until 2008, when it switched to cloud seeding by aircraft. It hires Weather Modification Inc., a contractor based on Fargo, N.D., to manage the program. This company also does the work for TID and MID.

The contractor stations a Cessna 340 twin-turboprop aircraft and two pilots at McClellan Airport in Sacramento for SMUD’s exclusive use during winter. Silver iodide is released by special flares – much like the roadside emergency flares many motorists carry in the trunk – attached to the trailing edge of the plane’s wings and set afire by a switch in the cockpit.

SMUD spent $137,360 on its contract with Weather Modification Inc. last year, McFadden said.

“It’s not that expensive when you consider the value of hydropower,” McFadden said. “Every additional megawatt we can generate from hydropower is one less megawatt we need to purchase. That’s where the cost-benefit pencils out very clearly.”

Weather Modification Inc. also provides the meteorological expertise to decide when to seed clouds. It can be dangerous, because the work calls for flying into storm clouds in conditions that cause the wings to ice over. The planes are equipped with de-icing systems but are often overwhelmed anyway.

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10.1.2 Hazards Summary

The major hazards encountered in the use and handling of silver iodide stem from its toxicologic properties. Toxic by all routes (ie, inhalation, ingestion, and dermal contact), exposure to this odorless, light yellow, crystalline substance may occur from its use in seeding clouds for rain-making, as a photosensitive agent in photography, as a local antiseptic, and as a chemical intermediate. Effects from exposure may include skin rashes, conjunctivitis, argyria (a permanent ashen-gray discoloration of skin, conjunctiva, and internal organs), headache, fever, hypersensitivity, laryngitis, and bronchitis. Exposure should be minimized by engineering controls (eg, local exhaust ventilation, or process enclosure). In activities where over-exposure may occur, workers should wear impervious clothing, gloves, face protection, and a self-contained breathing apparatus. Such clothing and equipment should be removed before leaving the worksite. Skin that becomes contaminated with silver iodide should be promptly washed. Eating and smoking should be prohibited in silver iodide work areas. Silver iodide may form explosive compounds with sodium, potassium, acetylene, ammonia, and hydrogen peroxide. Silver iodide should be stored in cool, dark areas, away from the above materials. Before shipping silver iodide, consult with the regulatory requirements of the US Department of Transportation. For small dry spills of silver iodide, collect the material and deposit in sealed containers for reclamation. Before implementing land disposal of silver iodide waste, consult with environmental regulatory agencies for guidance.     

Link To PubChem Silver Iodide