The same bacteria that cause frost
damage on plants can help clouds to produce rain and
snow. Studies on freshly fallen snow suggest that 'bio-precipitation'
might be much more common than was suspected. Before
a cloud can produce rain or snow, rain drops or ice
particles must form.
Act as nuclei
This requires the presence of aerosols: tiny particles
that serve as the nuclei for condensation. Most such
particles are of mineral origin, but airborne microbes
- bacteria, fungi or tiny algae - can do the job just
as well. Unlike mineral aerosols, living organisms can
catalyze ice formation even at temperatures close to
0 degrees Celsius.
Now a team, led by Brent Christner, a microbiologist
at Louisiana State University in Baton Rouge, has managed
to catalogue these rain-making microbes by looking at
fresh snow collected at various mid-and high-latitude
locations in North America, Europe and Antarctica. They
filtered the snow sample to remove particles, put those
particles into containers of pure water, and slowly
lowered the temperature, watching closely to see when
the water froze.
The higher the freezing temperature
of any given sample, the greater the number of nuclei
and the more likely they are to be biological in nature.
To tease apart these two effects, the team treated the
water samples with heat or chemicals to kill any bacteria
inside, and again checked the freezing temperatures
of the samples.
In this way they found between 4 and 120 ice nucleators
per litre of melted snow. Some 69 per cent to 100 percent
of these particles were probably biological. The results
are published in the journal Science. The researchers
were surprised to find 'rain-making' bacteria in all
samples; the snow from Antarctica had fewer than that
from France and Montana, but it still had some.
"Biological particles do seem to play a very important
part in generating snowfall and rain, especially at relatively
warm cloud temperatures," says Christner.
scientists note that this freezing ability also means
that the bacteria get out of clouds and back to Earth
Changes in land-use, forestry and agriculture, such
as expanding monoculture, change the composition of
microbes in the atmosphere. As biological components
seem to have a large role in how rain forms, such changes
may affect rainfall and climate in many places on Earth.
"It is about time for atmospheric and climate scientists
to start thinking about the implications," says