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.
Mostly
biological
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.
Some scientists note that this
freezing ability also means that the bacteria
get out of clouds and back to Earth more quickly.
Human
factor
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 Christner.
