25th May 2011
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 other weather events.
Hailstones vaporise in the sun at Stuttgart Castle in Germany. Scientists have found numerous bacteria in the centre of hailstones, suggesting that airborne microbes play a leading role not just in hail storms, but other weather events
All precipitation – rain, hail, snow or sleet – 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.
HOW RAIN FORMS
Rain formation is part of the hydrological cycle that describes the continuous movement of water.
Rain formation begins with the evaporation of water as vapour after it has been heated by the sun.
Rising air currents take the vapour higher into the atmosphere where it merges with vapour that has come from transpiration in plants to form clouds.
The particles in the clouds collide and grow in size which eventually leads to them falling from sky as rain.
Much of the rain re-enters the sea directly, but some comes from surface runoff, which is water from the land re-entering the oceans or other water sources.
From this stage the water is evaporated again and the whole process repeats.
Lead researcher Dr Alexander Michaud, from Montana State University, 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.
‘There is growing evidence that these nuclei can be bacteria or other biological particles.’
Dr Michaud’s team analysed hailstones over 5cm in diameter that were collected after a storm in June last year.
The large stones were separated into four layers, each of which was analysed in turn.
Living bacteria that could be grown in the laboratory were present in the highest numbers in the inner cores of the hailstones.
A bug that infects plants, Pseudomonas syringae, is the most well-studied biological rain-maker.
Co-researcher Dr Brent Christner, from Louisiana State University in the US, said: ‘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.’
Computer simulations suggest that high concentrations of biological particles may influence precipitation levels at the ground, cloud cover, and even the way the Earth is insulated from solar radiation.
Rain forms when water is evaporated as vapour and merges with other particles to form droplets which then fall as rain
- Germy with a chance of hail (sciencenews.org)
- The role of bacteria in weather events (eurekalert.org)
- Surprising Find: Live Bacteria Help Create Rain, Snow & Hail (livescience.com)
- Bacteria ‘abound in hailstones’ (bbc.co.uk)