Jonker said it was unclear, for example, whether there would be more or fewer low clouds such as cumulus in warmer conditions, which would affect the rate of global warming because of their role in reflecting sunlight away from the earth.

"In a warmer climate, if there is more evaporation, that could lead to more of the lower clouds, which could diminish the effects of climate warming," said Jonker.

He added warm air could hold more water vapor than cold air before it formed clouds, so there might be fewer low clouds as the earth heated up, which would accelerate global warming.

Rising sea levels and increased risk of droughts, flooding and species extinction are some of the likely effects of global warming, caused mainly by emissions of greenhouse gases from burning fossil fuels, the UN Climate Panel has projected.

European and Japanese space scientists have turned their attention to clouds because of the pressing need for research.

A 350-million-euro (US$452 million) satellite project, due for launch in 2014, aims to improve understanding of the role they play in climate regulation.

The project, known as EarthCARE, is being assembled mainly by the Astrium unit of the European aerospace group EADS and combines the technology of existing cloud observation satellites with new instruments for a more accurate picture.

"It's much more complex than anything that's flying at present," said Stephen Briggs, head of the Earth Observation, Science, Applications and Future Technologies Department at the European Space Agency. "The difficulty with clouds is that you can't see into them, so you have to find ways of looking into their three-dimensional structure, such as with radar systems."

Advances in research are followed closely by cloud enthusiasts who spend their leisure time looking out for unusual varieties and learning about their effect on the planet.

"We believe that clouds are unjustly maligned and that life would be immeasurably poorer without them," says the Cloud Appreciation Society, a club for spotters, on its Website, where it regularly posts a "cloud of the month."

The wispy high cirrus, the ominous cumulonimbus and the fluffy cumulus have all held the title and have led to heated debates in Internet chat forums.

Thousands of people capture unusual or striking clouds on camera and share them online.

Enthusiasts are keen to challenge negative attitudes to clouds, which have spawned sayings such as "even the darkest cloud has a silver lining."

"I thought it was about time someone stood up for the clouds because too many people complain about them," said Gavin Pretor Pinney, author of "The Cloudspotter's Guide."

"They are rather chaotic things, difficult to predict, difficult to fully understand, but the facts are emerging that they play a crucial and essential role in regulating and affecting the temperatures on the planet."

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Editor:Yang Jie