Scientists Make Rain In Mexico

A recent three-year randomized experiment in the northern Mexicanstate of Coahuila showed that rainfall from seeded clouds lastedlonger, the rainfall area was larger, and total precipitation washigher (sometimes even doubled) than output from similar nonseededclouds. In many cases the results of the seeding were statisticallysignificant 20 minutes to an hour after seeding.

The Mexico project was designed to repeat the success of agroundbreaking, five-year effort conducted in South Africa in theearly 1990s. The new study, which followed several years of droughtin northern Mexico, was funded by the Mexican state of Coahuila withfinancial support from Altos Hornos de Mexico, a private steelcompany. NCAR's primary sponsor is the National Science Foundation.

NCAR researchers flew into the Mexican rain clouds in a PiperCheyenne twin-engine turboprop airplane, equipped with wing-mountedracks carrying 24 hygroscopic flares and an instrument package tomeasure basic cloud physics indicators. The flares spewed saltedsmoke into the moisture-rich updrafts entering the clouds from below.The tiny particles (a mixture of sodium, magnesium, and calciumchlorides) attracted and absorbed the surrounding water vapor to morereadily create large drops heavy enough to fall out as rain.

"We are very encouraged by the results," says lead scientist RoelofBruintjes of NCAR. However, the number of cases is marginal for anystatistical analysis, he adds. Funding for a planned fourth year ofdata gathering was cut when the Mexican drought ended. This left thetotal number of cases at 94, compared to 127 in the South Africastudy. The team is optimistic that more seasons in the field willextend the results and help establish statistical significance.

Even more problematic is that scientists cannot fully explain how theseeding process works inside the cloud. "We must be able to explainboth microphysical and dynamical responses of the cloud to theseeding procedure," cautions Bruintjes, "before we can claim fullsuccess."

The main tool for "nowcasting" the weather and for evaluating theseeding experiment was a 5-cm wavelength weather radar. Specialsoftware developed at NCAR displayed the radar data and the aircraftposition in real time for directing operations. It also objectivelyidentified storms for evaluating the results.

NCAR is managed by the University Corporation for AtmosphericResearch, a consortium of more than 65 universities offering Ph.D.sin atmospheric and related sciences.