Tom Choularton: Can we really control the weather?

The seeding method used is to add tiny particles of silver iodide to the
clouds and there is solid science behind this method. At temperatures a few
degrees below zero degrees centigrade, clouds consist mainly of supercooled
water droplets. These clouds can be quite stable, but silver iodide has an
ice-like structure and it will cause a few of these water droplets to
freeze. Once you have ice particles mixed in with the supercooled droplets,
these crystals grow rapidly to form snowflakes, causing the cloud to
precipitate. The effect is that the water is released from the cloud.

This form of cloud seeding is not new – It has been used since the 1960s in
the western USA to try to make rain, as well as being used in Israel in the
past. The difficulty is it has always been hard to prove whether the cloud
would have rained naturally if it hadn?t been seeded. Even tests where
seeding lots of clouds takes place and un-seeded clouds are used as controls
haven?t produce statistically significant results. Therefore, if it works at
all it can?t be hugely effective.

So although the underlying science behind the technique seems to be sound,
what is presently wrong with the technique?

We at Manchester University have been flying the UK atmospheric research
aircraft in clouds that may or may not produce rain or snow, to investigate
effective conditions for cloud seeding in different environments. Last
winter, we were flying in low clouds with temperatures just below freezing
and we did not manage to seed any of the clouds and they were left entirely
natural. We found that on many occasions these clouds already contained a
mixture of supercooled water droplets and ice crystals, and the ice crystals
were growing and falling out as snow. However this wasn?t on account of our
seeding efforts.

Interestingly, the origin of these ice particles seemed to be due to freezing
caused by particles of dust, ash and organic material carried up into the
cloud (some of which can be attributed to pollution). We found that once
some ice is formed there is a powerful secondary ice particle production
process which occurs at about minus six degrees Celsius. This produces lots
of ice crystals, meaning that even if the clouds had been seeded with silver
iodide, it would have had little effect. Snow was being produced very
effectively and quite naturally anyway.

Interestingly, when we flew higher into clouds, well above the ground and away
from a lot of the particulate material, we did find clouds at colder
temperatures, as cold as -30C that were made up entirely of supercooled
water droplets so maybe seeding these would have had an impact.

So does cloud seeding work? Well our studies indicate that in many clouds that
produce lots of snow it does not seem to, because there is plently of
natural ice already. However, I don?t completely dismiss it as a method ? I
do believe it?s possible it can be effective in some clouds in the right
conditions and at the right temperature. Nevertheless, I feel some of the
stronger claims made recently need further verification, before we herald
this as a breakthrough in scientists? ability to manipulate natural weather

Professor Tom Choularton is Head of the School of Earth, Atmospheric &
Environmental Sciences is supporting the Science:
[So what? So everything]
campaign, which aims to highlight the leading
UK science research that will shape the future of Britain.

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