By Kurtis Alexander, New York Times

MODESTO, Calif. -- Pilots Brandon Thurston and Artie Cifarelli can see their destination even before their twin-engine Cessna leaves its Central Valley hangar: the foothills of the white-topped Sierra Nevada.

Thurston jokes that he's responsible for all the snow clinging thick to the distant peaks. But as an employee of Weather Modification, Inc., which is based in North Dakota, what he says is not entirely untrue.

The two airmen have made about three dozen flights from the Modesto City-County Airport to the high country in recent months, ``seeding'' clouds so they'll produce more snow. The end goal is more water for drought-stricken reservoirs that provide not only for the communities of the valley, but also for the Bay Area.

``For a while, we were flying much of the night,'' said Thurston, 27, who left his home in Trinidad, Texas, late last year for the busy winter cloud-seeding season in the Sierra. ``We were going to bed at 6 in the morning and waking up at 2 in the afternoon.''

Efforts to make snow and rain virtually out of thin air were once the realm of science fiction. Even today, they're dismissed by some as fanciful and hardly worth the time. But after four historically dry years in California, the practice has been on the uptick.

The stormy skies that came with this year's El Nino provided ideal conditions for cloud seeding, which requires enough water vapor in the air so that the introduction of chemicals like silver iodide can coax the clouds to crystallize and send droplets to the ground.

From San Francisco's Hetch Hetchy watershed to the East Bay Municipal Utility District's Mokelumne River to the coastal mountains in Southern California, water managers say cloud seeding is boosting precipitation -- sometimes by 10 percent or more. Many say that alongside this year's slightly wetter winter it's been essential to riding out the drought.

``Some people used to think of it as smoke and mirrors,'' said Jason Carkeet, a utility analyst for the Turlock Irrigation District who hired the cloud-seeding company at the Modesto airport. ``But it seems to have gained more acceptance. There are fewer and fewer districts that don't have a program.''

The Turlock district has partnered with the neighboring Modesto Irrigation District to seed the Sierra's Tuolumne River watershed off and on for the past 25 years. This year has been one of the most extensive efforts, Carkeet said, with the pilots on contract through April, a month longer than usual.

The work benefits not only the two districts' mostly agricultural customers, officials say, but also the San Francisco Public Utilities Commission, which counts on the Tuolumne River to send water to 2.6 million people in the Bay Area.

While Turlock officials estimate conservatively that cloud seeding boosts runoff in the mountains between 2 and 3 percent, San Francisco officials say it's too difficult to pinpoint their gain.

The pilots' work is more precise. At the Modesto airport, their flights begin only after the forecast shows a cold, east-moving front approaching the Sierra.

Thurston and Cifarelli, who is 26, keep their plane loaded with flares that disperse silver iodide from the craft's wings and belly during seeding -- so they're ready to fly at a moment's notice.

``We have to time it so we don't reach the area too early or too late,'' Thurston said.

Naturally, taking off during a storm doesn't make for the smoothest ride, but the pilots insist they've adapted to the modest amount of turbulence.

About 20 minutes into the flight, as the plane hovers above Highway 49 west of Yosemite National Park, they light one flare at a time, from switches inside the cockpit, and the real work begins, Thurston explains.

For up to four hours, the flares emit a visibly orange flame as well as the crucial silver iodide mist, which serves as nuclei around which the water vapor in the clouds bonds and, hopefully, coalesces into precipitation.

``We're trying to put the snow right on the front slopes of the mountains,'' said Thurston. ``The wind is going to do the work and carry the silver iodide in.''

Clouds aren't seeded only from airplanes.

Less than 100 miles to the north, Pacific Gas and Electric Co. is running several ground-based generators to boost snowfall in the Mokelumne River watershed. The cloud-seeding machines, strategically placed on 10-foot stands, essentially burn silver iodide, similar to the airplane flares, so that it vaporizes into chilly winds during storms and helps make snow.

The effort, one of a pair of cloud-seeding programs run by the utility company, seeks to increase runoff into the river for hydro-power plants downstream. Local reservoirs, like the East Bay Municipal Utility District's Pardee Reservoir, also benefit.

For the bump, the East Bay water district is paying 25 percent of PG&E's operating costs.

``We've had quite a few seeding opportunities up here this year,'' said Ken Ericsson, senior meteorologist for PG&E, noting that storms during this year's El Nino enabled them to seed, increasing snowpack an estimated 6 percent.

PG&E's program costs a couple of hundred thousand dollars annually -- the company declined to provide a specific amount -- not much different than the $165,000 tab of the Turlock and Modesto irrigation districts. The sums are relatively small for the big agencies, and well worth the price, they say.

While the science of cloud seeding is real -- silver iodide, in the lab, prompts water vapor to crystallize -- critics of the practice say it's tough to know exactly how effective it is in the field. Measuring how much more rain and snow a cloud produces when chemicals are introduced is virtually impossible.

``Cloud seeding is not going to solve the drought, I don't think ... (but) it can augment cold-water supplies,'' said Maury Roos, chief hydrologist at the California Department of Water Resources. ``It's a matter of how much you're paying for what you get out of it.''

About a dozen cloud-seeding programs are operating in the state, Roos said, with many in high gear this year to capitalize on the wet El Nino weather.

Another concern about the practice is the chemicals it uses. Silver iodide can be toxic to fish and even humans, though experts say not at the relatively small levels used for cloud seeding.

While the technology hasn't changed much since its advent in the 1940s -- when author Kurt Vonnegut's brother, Bernard Vonnegut, reportedly helped discover silver iodide's potential -- its safety has been tested more in recent decades.

Several surveys in the Sierra found no trace metals above background levels in watersheds where cloud seeding had occurred.

Fray Crease, water agency manager for Santa Barbara County, says the criticism she hears most about her county's cloud-seeding program is that it's part of the purported ``chemtrail'' agenda.

The conspiratorial fear is that the planes used to enhance precipitation are actually among a larger government fleet of aircraft that disperse subversive chemicals for dark purposes onto an unsuspecting population.

``Cloud seeding has proven to be safe and effective. ... It's one of our more effective water supplies,'' Crease said, citing studies that suggest the practice has increased rainfall in the mountains above Santa Barbara between 9 and 21 percent.

``Still,'' she said, ``that doesn't always allay people's perceptions.''