"The amount of microbial life present in the cloud droplets that
make up a winter storm is amazing."- Gary Franc
, microbiologist and plant pathologist at the University of Wyoming, in
The Clouds Are Alive
Douglas Fox, Discover, April 2012
Here's an easy question. At what temperature does water freeze or
If you answered zero Celsius (32 Fahrenheit) you would be with the
majority. If you are savvy enough to know about salt water, such as in
oceans, you will know that sea water may remain liquid down to -4C. And
salt on icy winter roads will melt down to -4C. That puts you in a
If you answered anywhere between -40 (C or F, they coincide at that
temperature) and +10C you would be technically
correct--"technically" because even though this is true in
nature it's not a fact you would want to argue in court, or even
with your mother. It's not simple.
Water in the stratosphere has been found still liquid at nearly -40 and
ice crystals have been seen forming at +10C in some clouds. No, I am not
lying to you. Keep reading so you will learn how these are possible.
First of all, what do you think clouds are made of
Water droplets, yes. (Not steam, which is water as a gas, and that is
technically invisible.) Water droplets tend to form around dust
particles in the air. Generally speaking, when you get smacked in the
face with rain drops each one has at least one particle of dust in it.
Who cares about the dust? Maybe you if you realize that the dust may
have travelled the world more than you have. A dust particle in a
droplet of water in a cloud in North America could well have come from
Africa's Sahara Desert. Or from China's Gobi Desert. Or, who
The well read among us will know that "foreign" dust could have
brought along with it microbes from its land or origin. These microbes,
blown in the wind, hitchhiking on dust, might deliver infectious disease
right to your nose without your ever owning a passport.
True, the likelihood of your dying of a disease blown from a different
continent than your own, on dust, is small. But microbes in the air are
far more prevalent that you may imagine. As our quote at the beginning
said, the air is full of microbial life.
Most of it will do you no harm. But so little study has been done on
this subject that we have no way to know today what diseases and
afflictions we and those we know may get that may have begun thousands
of kilometres away from our home. We may accept that SARS and swine flu
are delivered peer-to-peer by human travellers, but not that other
diseases might be brought to us in the air. From a distant continent.
Kimberley Prather <http://atofms.ucsd.edu/kprather> , an atmospheric
chemist who heads her own research group at University of California San
Diego, is not shy about getting up into the clouds (even rumbling ones)
to find out what is going on inside. What she has learned is enough to
make your jaw drop.
Think about it: what makes ice form from water, other than the obvious,
ambient temperature? Why do some clouds drop rain while others
don't? The answer in both cases is microbes.
Bacteria, algae and fungi get swept up by wind at ground level and make
their way into the air as high as jetliners. "There's a whole
ecosystem going on in the clouds that's largely undefined," says
Two million tons of bacteria, 55 million tons of fungal spores and an
unguessable (at this time) quantity of algae make their way into the
atmosphere each year. Never mind pollution in the air, this is nature in
action. A great deal of study will be needed to determine what effects
these have on our weather, on next year's harvest, even on our
Ice will form by itself from water (so far as we know today), but this
happens very slowly (like ice cubes in your fridge freezer). How can
this happen so quickly in the atmosphere when ice crystals form and snow
falls to the ground?
Professor Prather and others have shown that the bacterium Pseudomonas
syringae has a gene in its DNA that prompts ice to form from water
droplets. You read that right. At least one variety of bacteria can
cause water to freeze into ice by activating a gene in its body.
Why is this important? Ice crystals are heavier than water droplets. Ice
falls, delivering water (as it melts) to the land below. If cloud
seeding silver iodide were loaded up with P. syringae bacteria when
sprayed into clouds, drought-dried land could be persuaded to become
That means more food to feed the seven billion (and growing) of us on
the planet today.
It also could mean new ways to control the spread of diseases that seem
mysterious and stubborn to us now.
Stay tuned, the most populous life form on the planet, bacteria, have
much to teach us.