Science Interviews

How Cloud-living Bacteria Could Trigger Rainfall

Bruce Moffett and Tom Hill

Chris - Are there many bacteria in clouds?

Tom - There are between 1500 and 20 thousand per millilitre of cloud droplet. All samples hold a lot of the bacterial species Pseudomonas. These are known to make ice form at higher temps than would occur normally.

Bruce - A lot of Pseudomonads are plant pathogens and cause ice to form on plant surfaces. The ice crystal formation damages the plant, minerals leach out, and that's how the bacteria obtain nutrients. Bacteria cause the same thing to happen in rain in clouds, leading to ice crystal formation, triggering rain, and at the same time use the cloud as a means of transport. One way to get down from the hostile environment in the clouds - it's very cold, it's very dark, there's a lot of UV light - is to get yourself rained back down. We know from plant studies that Pseudomonas forms ice crystals. The ice crystals attract water. They get larger and larger until the convection currents within the clouds can't hold them out. Rain falls and the bacteria fall back down with it.

Chris - How do the bacteria get back in the clouds again?

Bruce - They can be blown up by wind currents. If air is heated it will rise. Because bacteria are so small, they don't fall out quickly. So once they get up there, they will be carried along for quite some time

Chris - When you did your field work, how do you know the bacteria weren't blowing off the grass or plants? That they weren't ground-dwelling bacteria that were contaminants?

Tom - Some of them could have come off the ground, but whatever is on the ground will be up in the air a few kilometres away anyway. We got as close to the ocean as we could and we had westerly winds, so we minimised the effects of the ground. But we are going to have to go up in the air. We have a design for what we call a Cyclomic Air Catcher - a vacuum cleaner that spins small particles into a collector. We have a design but we are after someone to build it. We can then go up in aeroplanes or alternatively in balloons.

November 2004