Manipulating Clouds from the Ground
Cloud seeding is partly a science, partly a black art.
This segment is part of the Engineers of the New Millennium: The Global Water Challenge Special Report.
Transcript: Nevada's Cloud Wranglers
Tom Swofford: Once the solution flow starts to go, you'll actually be able to see the flame through the bottom of the drum—it'll be a large orange flame comes out. Now we're in normal operation. We're seeding clouds.
Phil Ross: Cloud seeding is partly a science, partly a black art. You have to work through a lot of statistics just to see if anything's happening. Under ideal conditions, seeding may increase precipitation by up to 10 percent. Arlen Huggins, a scientist at the Desert Research Institute, explains.
Arlen Huggins: The ideal cloud is actually relatively shallow. We're looking for clouds that aren't particularly efficient at producing precipitation. Usually the very, very deep clouds are also very, very cold and produce a lot of ice and snow on their own. And as the cloud tops lower, there's a lot less natural ice in them.
Phil Ross: I asked Huggins what he does with all the snow.
Arlen Huggins: I do absolutely nothing with it. Nature just takes over, and as the spring, as it gradually warms up, the snow starts to melt, runs into the streams, and that becomes Nevada's water supply, pretty much.
Phil Ross: We climb into a truck and drive past the flat desert plain to the pine-clad hills half an hour west of Reno—to Troy.
Phil Ross: Arriving at a road underpass at the foot of a hill, we park and follow Tom Swofford, a field technician, past the puddles at the bottom and up a steep hill knee-deep in snow. A chilly breeze rises as we climb, and we all put on our hats and gloves.
Phil Ross: I'm standing beside a sheet-metal box about the size of an office cubicle. On it, there's a 10-foot tower topped by what looks like an oil drum. The fuel consists of acetone, in which a tiny bit of silver iodide has been dissolved. When you burn the acetone, the silver iodide wafts away as microscopic crystals.
Arlen Huggins: The flow rate is set so it burns about a third of a gallon an hour—so 3 hours, you burn a gallon. A typical episode is roughly 8 hours, or 9 hours, make it even, so 3 gallons.
Phil Ross: And in those 3 gallons you'll get a certain amount of powder. If I were to hold all that powder in my hand, it would be just a few handfuls?
Arlen Huggins: It wouldn't even be that much.
Phil Ross: I ask Arlen how much snow it could produce.
Arlen Huggins: You can increase the precipitation rate by about a half to 1 millimeter an hour. So, say you were able to create a half an inch of snow—so, a half an inch of snow times 35 square miles, and then you'd have the volume.
Phil Ross: Let the record show that it comes to 41 million cubic feet of snow, which melts out to 30 million gallons of water—enough to supply Reno's homes for one day.
Tom Swofford: You'll hear a loud click. That'll be the valve opening to allow propane, you'll hear this whoosh, the propane, and the flame will light.
Tom Swofford: Once the solution flow starts to go, you'll actually be able to see the flame through the bottom of the drum—it'll be a large orange flame comes out. The solution will start to flow—there it comes, right now.
Tom Swofford: Now we're in normal operation, we're seeding clouds.
Phil Ross: I'm in a garage almost as big as a hangar, peering at a glass box the size of a refrigerator. It's a cloud chamber, and the scientists are using it to measure some of the effects of their cloud-seeding work.
Arlen Huggins: What you're hearing is the compressor from the refrigeration unit that cools down the cloud chamber. Take the flashlight and shine it in there, and you can see the cloud that's being created. It's just like a fog.
Phil Ross: It contains a cloud of water vapor that's kept way below the freezing point, yet that water can't freeze, because there are no seeds on which ice can form. But when a mote of silver iodide drifts into the chamber, kapow! You get a tiny crystal of ice. To see how well the chamber does its job, we stand 30 feet away and burn a match that's been dipped in silver iodide solution. Then, we wait.
Phil Ross: That was the first one.
Arlen Huggins: See 'em? Did you see the little glints of light?
Phil Ross: Yes.
Phil Ross: Each beep is an ice crystal, each crystal a potential flake of snow, the source of more than 90 percent of the water drunk in Reno.
It all starts with a few handfuls of powder. Mighty storms from tiny seeds do grow. I'm Phil Ross.