When It Rains, It Pours--Cloud Seeders Make Sure
He may not be as successful as Charles (Rainmaker) Hatfield, but, during rainstorms, Dan Keady is in the clouds in a twin-engine plane flying over the mountains near Julian, trying to create more rain, not flood the city.
Keady is a pilot for Atmospherics Inc., a research and cloud-seeding company.
Because of an almost four-year drought, San Diego decided to spend $150,000 to fund a seeding program with the Sweetwater and Helix water districts. The city hired the Fresno-based company for the pilot program last February and is continuing the program this winter.
In 1916, when San Diego was suffering from a drought, the city hired Hatfield and agreed to pay him $10,000 to fill the Lake Morena reservoir. Hatfield set up towers near the lake and on them mixed chemicals that produced periodic explosions of vapor. Seventeen days after he began, 11.4 inches of rain fell.
The lake flooded, dams broke, houses and bridges were destroyed. Residents sued the city for $3.5 million in damages. And the city refused to pay Hatfield for his services.
This time, the city hopes simply to get the maximum local water production and prevent the need for imports.
“Up until the heavy rain in March of last year, we were looking at a three-year drought” and started looking for an alternative water supply, said Frank Maitski, senior civil engineer for the city’s Water Utilities Department.
The areas targeted for seeding are over three watersheds in the mountains near Julian, Maitski said. They cover about 600 square miles and were chosen because the water levels are low, he said. The city hopes the seeding will increase the seasonal rainfall by 10% to 15%.
During the four storms this season, some of the seeding has been successful and some has not, said Rand Allan, a meteorologist for Atmospherics Inc.
How cloud seeding affected this year’s rainfall will not be known until the rainy season ends and an evaluation is made, Allan said. The rainy season is from December to April.
According to the National Weather Service, the total rainfall for the season is 3.94 inches at Lindbergh Field. The mountain areas totaled 8.3 inches, a city flood control spokesman said. The season is measured from July 1 to June 30.
Mountain areas usually get two or three times more rain than the city because, as the storm rises, it is forced to dump more rain, said Wilbur Shigehara, a National Weather Service forecaster.
Allan said the goal last year was to find out if this area was conducive to cloud seeding. He said that Atmospherics wanted to investigate the clouds to see if they met seeding criteria, which they did.
There were only two storms last year in San Diego, which made it difficult to determine if cloud seeding increased rainfall, Allan said. “We only seeded for about one-third of the season because we were hired late,” he said. They started Feb. 15 and finished April 30.
Cloud seeding was discovered in 1946 at the General Electric Laboratory in Schenectady, N.Y., by Dr. Vincent J. Schaefer, the father of modern weather modification.
Scientific experiments are now leading to techniques that can predict the results of cloud seeding, if the seeding is done properly. Comparisons are made between the water levels of areas targeted for seeding and the unseeded areas to determine if cloud seeding was successful. Researchers say there is generally a 5% increase in rainfall when clouds are seeded.
Since 1946, various inorganic and organic materials have been used to create more rain. The most common, silver iodide, can be released from either ground-based generators or from an aircraft.
Cloud seeding during winter storms takes place when clouds are formed as moist air rises and cools, and as they move from west to east across the mountains. Many clouds are inefficient precipitators and retain about 90% of their moisture.
When the radar shows that rain is moving in, Allan said, he monitors the storm to detect its most intense parts and sends Keady up. Keady seeds the clouds 20 minutes before the storm reaches the target area. It takes about 30 minutes for rain to reach the ground.
Keady said that seedable clouds must have a temperature of 23 degrees or lower. They also need supercooled liquid water, which is water below the freezing point. The water is unfrozen because it does not have anything to freeze onto, he said. That is where Keady comes in.
He releases silver iodide pre-mixed with either magnesium or acetone. Silver iodide is used because it is chemically similar to an ice crystal. Twenty flares containing silver iodide and magnesium are positioned on each side of the turbo-charged aircraft, and acetone and silver iodide are poured into a wing-tip generator.
With the flip of a switch, Keady ignites the flares and generator, which burns the chemicals. An exhaust gas containing millions of microscopic particles is released directly into the storm. The process continues for about an hour and a half, Keady said.
The object is to coax the extra liquid out of the clouds to intensify the rainfall, Keady said. The super-cooled water attracts particles to form ice crystals. These crystals grow rapidly until they are heavy enough to fall through the clouds, melting into raindrops on the way down.
“We provide a means for the extra water to get to the ground, when there is a shortage of coalescing material,” Allan said.
“If the clouds are working at 100% efficiency and converting all the supercooled liquid to rain, then we will not seed,” Allan said.
Keady said that, if ice does not form on the plane’s wings, he knows that “it is a good storm,” and that it will rain heavy and long. If ice does form, then he seeds.
On Wednesday afternoon, Allan said that visually the clouds seemed riped for seeding. He said there were some fairly healthy thunderstorms in the northern portion of the target area. Keady went up at about 12:20 p.m. and planned to stay up for about 2 1/2 hours.
Last weekend, Keady seeded twice Saturday, and early Sunday.
“Seeding conditions were good early Saturday evening because we found a steady amount of liquid water in the clouds,” Allan said. “But late Saturday and early Sunday, we did little seeding and stopped when the cold front moved through.”
However, late Sunday morning the seeding conditions improved, and another flight Monday afternoon also proved successful, Allan said. But another storm that hit Tuesday afternoon produced only scattered showers containing little liquid water. The seeding flight proved unsuccessful.
From midnight Tuesday to 3 a.m. Wednesday, seeding conditions were good.
The silver iodide that the plane drops is not hazardous to the environment, Keady said, because the amount released is so small. He said he typically uses about 100 grams of silver iodide per flight.
He has enough chemicals during each flight to seed the clouds for four hours. Twenty grams of silver iodide are in each flare that burns for about six minutes, Keady said. And the generator releases 42 grams of silver iodide per hour.
John Lease, who heads a research group at the Bureau of Reclamation in Denver, said the silver iodide used in cloud seeding is added in such small amounts that it cannot be detected. He is chief of the Water Augmentation Group, which does environmental studies of weather modification.
Data shows that cloud seeding during winter rainstorms increases rainfall by 10% or 15% for the season, Lease said.
Because of the concern that heavy rainfall may cause flooding, Lease said, most cities have a suspension criterion to protect themselves.
Keady said that their contract with San Diego has a suspension criterion that prohibits him from seeding under certain conditions. The city can stop him from seeding at any time, especially if flash floods are expected, or if there are hazardous conditions such as back-to-back storms.
The flooding problems are mainly in the city, where the ground is saturated and there is a lot of pavement, Allan said.
“We let a heavy shower on (Wednesday) morning go by without seeding the clouds, because of the severity of the thunderstorm, in order to avoid concerns of extra-heavy rainfall. We didn’t want it to flood, especially in the section of the foothills area near the target areas.”
“We don’t view ourselves as rainmakers,” said Allan. “We can’t go out, and send (Keady) up to spray silver iodide and expect it to rain. We have to wait for the right conditions and increase the efficiency of a storm.”