Airborne Germs May Spur Rain, Snow, Hail

Biological particles found at center of inner cores of hailstones, researchers say

TUESDAY, May 24 (HealthDay News) -- Rain-making bacteria may shed some light on the role that biological particles play in the Earth's precipitation cycle, new research shows.

The study, slated for presentation Tuesday at the general meeting of the American Society for Microbiology in New Orleans, revealed high concentrations of bacteria within the center of hailstones, suggesting that airborne microorganisms may trigger precipitation, such as rain, snow and hailstorms.

"In order for precipitation to occur, a nucleating particle must be present to allow for aggregation of water molecules," Alexander Michaud of Montana State University in Bozeman, said in a society news release. "There is growing evidence that these nuclei can be bacteria or other biological particles."

After analyzing four layers of large hailstones collected in June 2010, the highest concentration of bacteria was found in the inner cores -- also known as the embryo, the study authors noted.

"Bacteria have been found within the embryo, the first part of a hailstone to develop. The embryo is a snapshot of what was involved with the event that initiated growth of the hailstone," Michaud explained in the news release.

Snow and other types of precipitation are dependent on the presence of ice nuclei, or particles that ice crystals can grow around. Biological particles, such as the well-studied plant pathogen Pseudomonas syringae, are among the most active, naturally occurring ice nuclei.

"Ice nucleating strains of P. syringae possess a gene that encodes a protein in their outer membrane that binds water molecules in an ordered arrangement, providing a very efficient nucleating template that enhances ice crystal formation," according to Brent Christner, of Louisiana State University, who is also presenting at the meeting.

The researchers concluded that high concentrations of biological ice nuclei may influence a number of factors related to the Earth's hydrological cycle, including: the size and concentration of ice crystals in clouds, horizontal cloud coverage in the free troposphere, precipitation levels at the ground, and insulation of the Earth from solar radiation.

Because this study was presented at a meeting, the findings should be viewed as preliminary until they are published in a peer-reviewed journal.

More information

The U.S. National Weather Service has detailed information on the Earth's hydrologic cycle.

Mary Elizabeth Dallas SOURCE: American Society for Microbiology, news release, May 24, 2011

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