Hail storms seem to arise out of nowhere, leaving those pummelled by the coin- to fist-sized ice balls to wonder how they came to be.

US scientists said plant bac-teria may be to blame.

A Montana scientist collected some large hailstones measuring more than five centimetres in diameter after a June storm last year, froze them, and later analysed the water that melted away in layers.

“Bacteria have been found within the embryo, the first part of a hailstone to develop,” said Alexander Michaud of Montana State University in Bozeman, who presented the research at a meeting of microbiologists in New Orleans.

“The embryo is a snapshot of what was involved with the event that initiated growth of the hailstone,” said Dr Michaud, a lead researcher in the field of bio-precipitation, the study of how bacteria may cause rain, snow and hail.

In order for clouds to make ice, from which snow can fall, a particle must be present for the ice crystals to grow around, known as an ice nucleus.

“In order for precipitation to occur, a nucleating particle must be present to allow for aggre-gation of water molecules,” said Dr Michaud.

“There is growing evidence that these nuclei can be bacteria or other biological particles.”

A plant pathogen known as Psuedomonas syringae is commonly at the root of precipitation events, because its outer surface is so efficient at collecting water molecules around it, said Brent Christner of Louisiana State University.

“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,” said Dr Christner, who also presented research at the American Society for Microbiology in New Orleans.

This provides “a very efficient nucleating template that en-hances ice crystal formation,” said Dr Christner.