Bacteria do many things for us – from making our food and fuels to breaking down waste. But research suggests they could also determine whether or not it rains.


The microorganisms that affect weather are known as ice-nucleation active bacteria. They are often found on crops and can cause plant disease. They have special proteins on their surface that help water in the air turn to ice at slightly warmer temperatures – around -3°C instead of -8°C. On the ground, these proteins can cause frost damage to crops. But when the bacteria are blown into the sky, they can cause ice crystals to form in the atmosphere – a crucial first step in the creation of rain or snow.

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The process, called ‘bioprecipitation’, was once thought to play only a small part in causing snow or rain. But the impact of microbes on rainfall is being reconsidered, according to Dr Cindy Morris, an expert in ice-nucleating bacteria from France’s Institute for Agricultural Research. “It’s much more important than we first thought,” says Morris. “We now know that when ice forms at warmer temperatures, like between -3°C and -8°C, it explodes into lots more ice crystals – so there is a multiplying effect.”

There is even tantalising evidence that bacteria blown off plants could cause rain on the other side of the world, says Morris. “We’ve found bacteria in a creek in the New Zealand wilderness that was genetically identical to bacteria causing cantaloupe blight in France.”

Microbes with rain-causing properties are thought to have evolved long before terrestrial plants existed. “If you are a tiny organism like a bacterium and you get wafted up into the air and into the turbulence, you’re done for – gravity has no effect and you just keep going up,” says Morris. “The only way down is in a raindrop. The chances of being hit by a raindrop are very small, so you need to form one yourself.”

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So given how effective these bacteria are at forming rain, could we use them to seed clouds and create rainfall where it is needed? According to Morris, we are probably already doing it to some degree. She says that large expanses of crops can cause huge numbers of these bacteria to be blown into the air. She believes that by colonising mountain slopes with plants, directly below currents of wet air high in the sky, we could help create rainfall in areas in desperate need of water, such as California. “But it’s not simple,” she explains. “Do the farmers there want yield or rain? You’d need to work out systems to pay people growing those crops. These bacteria also cause disease, so it’s about a balance between driving rain and causing disease.”

What’s more, ‘seeding clouds’ is a fine art – too many ice-nucleating particles can actually ‘constipate’ a cloud, preventing rainfall. And political disputes between nations ‘taking’ others’ rain have stifled previous projects to manipulate weather. So it could be a while before we can truly make it rain – but our understanding of precipitation is changing. Just think, next time you feel a drop of rain on your head, it could be a microbe returning from an epic journey.

This is an extract from issue 330 of BBC Focus magazine.

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Tom Ireland is a freelance science journalist, and editor of The Biologist, the bi-monthly magazine of the Royal Society of Biology.