New Scientist - 29.02.2020

(Ben Green) #1

10 | New Scientist | 29 February 2020

WE HAVE the strongest
evidence to date that cloud
seeding – spraying clouds with
powder – can cause more snow
to fall. However, the problem is
making it work reliably. Not every
cloud can be seeded and we don’t
know why. It also isn’t clear when
it would be cost effective.
The technique has been used
since the 1940s, says Sarah
Tessendorf of the National Center
for Atmospheric Research in
Boulder, Colorado. In theory, it
should make more rain or snow.
It usually involves spraying a
powder, normally silver iodide,
into clouds. Each tiny particle acts
as a seed for an ice crystal to grow
around it and fall as precipitation.
However, despite decades of
research it has been hard to show
that the method works. Tests have
compared what happens to clouds
that are seeded with those that
aren’t, but it hasn’t been possible

to get a large enough sample size
to control for natural factors. “The
weather’s... variable, it changes all
the time, it’s very complicated,”
says Tessendorf.
That has now changed, thanks
to a project called SNOWIE
(Seeded Natural and Orographic
Wintertime clouds – the Idaho
Experiment). On 20 days in
January 2017, Tessendorf and her
colleagues seeded orographic
clouds, which form when air is
forced up over mountains. They
sprayed silver iodide from an
aeroplane flying in a zigzag to
create a pattern in the sky.
The team used radar to look for
the seeding pattern in the clouds,
with mobile radars on ridges to
scan for subsequent snowfall in
places where normal weather
radar couldn’t reach. On three
days, the team found evidence of
snowfall resulting from seeding.
On the ground, this amounted to

a light dusting of snow between
0.05 and 0.3 millimetres deep.
Crucially, the team has
estimated the total volume of
water produced on those three
occasions. On 31 January, the most
successful day, snow equivalent
to 340,000 cubic metres of water

was released from the clouds.
The least successful was 19 January,
when snow equivalent to
123,000 m3 of water was produced
In total, the three successful
days produced about 282 Olympic-
sized swimming pools worth of
water, the team say.
“We now have scientific
evidence that seeding of
orographic clouds can increase

precipitation,” says Andrea
Flossmann at the University of
Clermont Auvergne in Clermont-
Ferrand, France. “The increase is,
however, below 10 per cent [of
what would occur naturally].”
Tessendorf agrees that there
are still challenges, but they
could only demonstrate the
effect on three days when there
was no natural precipitation. “In
cases where there’s background
precipitation forming, it’s much
more complicated,” she says.
Worse still, clouds vary.
“The same cloud over the same
watershed might have some
areas of it that are seedable and
others that might not be,” says
Tessendorf. In particular, seeding
only works when water droplets
are “supercooled”, meaning they
are still liquid below 0°C. All this
suggests that cloud seeding may
not be a cost-effective way to
increase water supply. ❚


Michael Marshall


Animal navigation

Sun in magnetic
‘eyes’ could lead
whales astray

A STUDY of nearly 200 strandings
of healthy gray whales over the past
30 years has found that the animals
are four times more likely to strand
themselves during solar storms.
Jesse Granger at Duke University
in North Carolina and her colleagues
think that radio frequency noise
made by the storms interferes with
the magnetic compass of whales,
preventing them from sensing
direction. But her team has only
shown a correlation between the
two events, Granger stresses. “This
is not direct evidence,” she says.
We know little about how whales
navigate in featureless oceans
during their long migrations. Like

many other animals, it is likely that
they sense magnetic fields, but this
is difficult to demonstrate.
To investigate, Granger and her
team looked at 186 instances
where individual gray whales with
no signs of any injury or interaction

with people had become stranded,
presumably due to navigational
errors. Strandings were twice as
likely on days with more sunspots
(Current Biology,
Sunspots are associated with
solar magnetic storms that can

affect Earth’s magnetic field and
make magnetic compasses point
in the wrong direction. But the
team found no link between
deviations in Earth’s magnetic
field and strandings.
However, strandings were four
times more likely on days when
high levels of radio frequency
noise due to solar activity had
been measured. This fits in with the
idea that a protein in animals’ eyes
called cryptochrome is involved in
sensing magnetic fields. If so, the
mechanism thought to be involved
would be disrupted by radio
frequency noise, effectively blinding
animals to magnetic fields. ❚
Michael Le Page

We really can control the

weather but it isn’t very useful

Gray whales (Eschrichtius
robustus) strand on days
with more sunspots

Olympic-sized swimming pools of
water produced by cloud seeding

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