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 commonthan 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 rainmaking 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
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 were 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
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,”
”The Hindu” Dated: 15 May, 2008)
CENTRE Newsletter Vol.6, No 2 Jue