Page last updated at 08:32 GMT, Monday, 15 June 2009 09:32 UK

Grey sky research

Richard Hollingham
Presenter, Frontiers

Rain clouds
Clouds are not as pure as they look

On the edge of an east London rooftop stands an increasingly bedraggled man. Arms outstretched, the rain lashes at his face and cascades down his beard onto sopping clothes.

In each hand he holds out a dish containing tiny metal crucibles; he grins as the raindrops ping against the containers.

Just another day at the office for University of East London scientist Bruce Moffett and the perfect weather to investigate the biological properties of rain.

The idea that bacteria in the clouds cause rain might, at first, rank as one of the more bizarre scientific theories.

It takes a quarter of a million of these to make one litre

However, over the past 25 years a small group of scientists has been studying the role bacteria in the clouds might play in our weather and, with papers published in leading scientific journals, the evidence that they're right is beginning to stack up.

"It takes something like a quarter of a million rain drops to make one litre," Moffett shouts to me through the wind.

Fortunately we don't need a litre. "I've got to make sure I have a raindrop in each." And then it's back to the lab to see what's in there.

Ice makers

Before rain can fall - at least in temperate climates - the water in clouds has to freeze.

But - and you may not believe this - sometimes, water doesn't freeze at 0C. Pure water will not freeze until -40C, and clouds rarely get that cold.

So to get water to freeze you need some help. A catalyst such as soot or dust will do the trick but if you want water to freeze at relatively warm temperatures, say around -5C or -6C, bacteria turn out to be the best "ice nucleators".

Cindy Morris, a plant pathologist at the French National Research Institute for Agriculture in Avignon, has identified a particular bacterium, Pseudomonas syringae, which is extremely effective at making water freeze.

In her lab she takes a tube of water, cooled to -6C (and not frozen), and puts in a single drop of bacterial culture. Within about two seconds, the water has turned to ice.

Rainclouds and cropfields
Bacteria found in crops may make the best "ice nucleators"

It's an impressive demonstration. "There's nothing magic about it," laughs Morris. "You can't break the laws of physics."

Pseudomonas syringae is found on the leaves of plants. By forming ice, the bacteria damages cell walls, releasing nutrients that it can then feed on.

But these organisms can easily get carried off by the wind and, once airborne in the clouds, pull off the same trick and persuade water droplets to freeze. At least that's the theory.

"It's a big question," admits Morris. "Are they involved in the events that lead to rain formation? Because if they are - these bacteria are the products of agriculture - does agriculture have any consequence for the amount of rain that could be formed?"

Rain makers

This theory of a natural cycle between crops and clouds is one of the most exciting ideas to emerge in this new science of what is sometimes called bioprecipitation.

Drought in Australia
Could planting the right crops be a tool to control rain?

Morris has a series of experiments underway to try to demonstrate, beyond question, that these bacteria can be carried into the air. Proving that would at least complete part of this scientific jigsaw.

So if - and it's a big if - these scientists can prove that biological particles play a significant role in rainfall, might it be possible to manipulate the process?

David Sands, professor of plant pathology at Montana State University, suggests that without the right crops, there won't be the rain.

"In Darfur, or even in Australia, where they haven't had rain, we could do the wrong thing by not controlling our overgrazing, by not growing varieties of wheat or barley that can support these bacteria," says Professor Sands.

Crops to change the weather? Perhaps that researcher standing on the roof in the pouring rain isn't so crazy after all.

Frontiers is on Radio Four at 2100 BST on 15 June.

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