By Stephanie Nebehay

GENEVA, Aug 13 (Reuters) - Asia's heavy monsoons, a record
heatwave in Russia and severe droughts in Africa show the need
for new yardsticks to rate extreme weather to guide everybody
from road builders to insurance companies, a U.N. expert said on
Friday.

Scales exist to measure the power of hurricanes or air
quality, but there are none to quantify risks from heatwaves,
floods and droughts which are likely to become more extreme and
frequent because of global warming.

A series of disasters, including floods in Pakistan and
mudslides in China, have followed droughts in Australia and a
record number of high-temperature days in the eastern United
States, said Ghassem Asrar of the World Meteorological
Organisation (WMO).

"The general conclusion is the magnitude, the severity and
extent of extreme events will be greater, which means we have to
prepare," Asrar, director of the WMO's World Climate Research
Programme, told Reuters in an interview.

"We need to develop standards, or indices, with a degree of
confidence in our assessment to deal with extreme weather," he
said. "In the case of hurricanes this is very developed and for
heatwaves, drought or flooding there is a need to do the same."

He said the insurance industry was interested in the issue
and Willis Group Holdings, the world's third largest insurance
broker, was co-sponsoring a WMO workshop in Paris from Sept. 27
to 29 attended by climate scientists and statisticians.

The workshop, expected to attract about 100 people, would
try to translate existing scientific models on extreme weather
events into a quantitative scale that the public can easily
understand.

"We will examine how very difficult scientific concepts can
be boiled down into simple measures or yardsticks for the
non-expert to use," Asrar said.

LEVELS OF RISK

The yardsticks should include a description of the level of
probability -- or conversely the uncertainty -- associated with
the predictions, he said.

The standards developed for assessing risks associated with
such climate extremes will be published and could be used by
national weather services worldwide to issue alerts.

"When they see it is based on sound scientific understanding
they will promote it as part of their services. That is how we
can get it into the mainstream and to the public," Asrar said.

A longer time range is required to establish firmly whether
the latest series of disasters matches projections of a U.N.
panel which predicted more frequent and more intense extreme
weather events because of global warming, Asrar said.

"We need to have enough evidence to connect the dots, to say
'yes, this is the cause and the effect'," he said.

A report by the panel in 2007 said it was at least 90
percent likely that most warming in the past 50 years was caused
by mankind, a finding questioned by sceptics pointing to errors
in it such as an exaggeration of the melt of Himalayan glaciers.

Pakistan's floods, the worst in 80 years, have killed more
than 1,600 people, forced 2 million from their homes and
disrupted the lives of about 14 million.
(Editing by Jonathan Lynn and Andrew Dobbie)