High-flying bugs in the atmosphere may largely be to blame for bad weather, scientists believe.
Numerous bacteria have been found concentrated in the centre of hailstones.
The discovery suggests that airborne microbes play a leading role not just in hail storms but in other weather events.
All precipitation -- rain, hail, sleet or snow -- begins with ice crystals forming around cloud particles.
Dust grains and pollution droplets may both serve as "nucleating particles". But the new find has helped confirm suspicions that in many cases living micro-organisms cause the rain to fall.
Lead researcher Dr Alexander Michaud, from Montana State University in the US, said: "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.
"In order for precipitation to occur, a nucleating particle must be present to allow for aggregation of water molecules."