New Scientist - 29.02.2020

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

(PNAS, doi.org/dnd5).

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. ❚

Environment

Michael Marshall

News

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, doi.org/dnd4).

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

282

Olympic-sized swimming pools of

water produced by cloud seeding

CLAUDIO CONTRER AS/NATURE PL