Air pollution causes bigger, more destructive hail*

TRIESTE, Italy, Sept 12 (AFP) Sep 12, 2007

Air pollution hugely increases the size of hail, and thus the amount of
damage it can cause to crops and property, according to a study
presented Wednesday at the European Conference on Severe Storms.

The interaction of man-made particles with airborne ice could also
interfere with the way heat is distributed in the earth's atmosphere,
with possible consequences for climate change, one of the study's
authors told AFP.

Like tornados, hail is generated by small-scale storms that are
difficult to track, and thus almost impossible to forecast. While they
rarely last more than a few minutes, they can be devastating.

The most destructive European hail storm on record hit Munich, Germany
in July 1984 with egg-sized rocks of ice, injuring hundreds and causing
1.5 billion euros (two billion dollars) in damage, according to German
re-insurer Munich Re.

It lasted all of 20 minutes.

To see how pollution affects hail formation, researchers at The Hebrew
University of Jerusalem ran a computer simulation of another, less
severe storm that pelted Stuttgart, Germany last summer with a volley of
hail the size of golf balls.

Because it is impossible to do controlled experiments in the atmosphere,
explained Daniel Rosenfeld, numerical modelling was the only way to
measure the varying impact of different densities of particle pollution.

"Clean air comes in from the ocean after a week or so with about 100
particles per cubic meter," he said. "Most air over central Europe has
more than 1,000, while over a very polluted city, the concentration is
up to 10,000."

Comparing the impact of 100 and 2,500 particles per cubic metre, he
found that pristine air produced far less hail, especially when humidity
was high.

It had been widely assumed that large hail formed inside updrafts,
columns of air moving vertically through storms at speeds of 30 to 40
meters per second, giving the ice stones time to grow.

But observing an anomaly in the tropics in Brazil led Rosenfeld to
speculate that the cause may lie elsewhere.

"We compared clouds that appeared in the smoke emanating from forest
fires" -- which generate large quantities of particle pollution -- "to
pristine clouds nearby," he said.

The smoke-laden clouds produced hail over the rainforest, an "unheard
of" phenomenon.

"The ice microphysical structure of clean and dirty clouds are
dramatically different, especially when it come to hail," said Rosenfeld.

The hail inside clouds formed in dirty air starts at higher levels and
fall through the whole cloud, collecting droplets along the way.

The result are rock-like ice stones about 10 times the mass of hail
formed in clean, pollution-free clouds. The kinetic energy -- that is,
the force of impact -- of hail falling on the surface from dirty clouds
was 30 to 50 times higher.

The findings could be useful to forecasters, Rosenfeld said, in regions
where air pollution rates vary on a daily basis, especially nearer the
coast.

The area around Trieste -- where a couple hundred meteorologists are
meeting this week to compare notes on severe weather -- is subject to
frequent hail storms, which cause severe damage to vineyards. Several
European countries seed clouds in an attempt to prevent hail from falling.

Whether the impact of pollution on hail formation influences global
warming is unknown, Rosenfeld said.

"But it is definitely relevant to the energetics of the climate system
because the energy that is absorbed by the earth's system from the sun
uses mainly evaporating water," he said.

"Climate change is not only about rising temperatures, it is also about
changing the circulation systems, and the storm tracks and intensity,
and the amount of precipitation."