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Last Updated: Friday, 16 July, 2004, 10:38 GMT 11:38 UK
Monster raindrops delight experts
By Paul Rincon
BBC News Online science staff

The drops were seen in cumulus congestus clouds

Scientists have observed the biggest raindrops recorded on Earth - which may be a whopping 1cm in size.

The monster water droplets were observed from the air, by atmospheric experts studying clouds.

They were recorded over Brazil and the Marshall Islands, a group of atolls and reefs in the central Pacific Ocean.

US scientists report in Geophysical Research Letters that a large fire may have influenced the formation of the huge raindrops recorded over Brazil.

"They are the biggest raindrops I have seen in 30 years of flying," Professor Peter Hobbs, co-author of the report told BBC News Online.

Professor Hobbs and colleague Arthur Rangno, of the University of Washington, US, recorded the droplets as being about 8.8mm and possibly as large as 1cm. He speculated that some of these giant droplets even reach the ground.

Raindrops in free-fall are often depicted as teardrop-shaped. In fact, raindrops with diameters larger than 2mm are flattened on their undersides and gradually change in shape from spherical to jellyfish-shaped.

Average drops of rain are between 1 and 2mm in diameter. The previous largest raindrops recorded - 8mm wide over Hawaii - were reported by researchers in 1986.

Smoking gun

Images of the raindrops were taken by a laser instrument on a research plane that flew through cumulus congestus clouds spawned by burning forest in Brazil's Amazon and in clean marine air over the Marshall Islands.

Comparison of raindrop with match head, insect and daisy, BBC
The authors propose that in Brazil, the giant raindrops were formed by condensation of droplets on to giant smoke particles.

However, this was clearly not the case for the mammoth raindrops observed in the Marshall Islands. The scientists think that here, the droplets rapidly grew in size by colliding with each other in narrow regions of cloud with an unusually high content of liquid water.

It was previously thought that, in practice at least, droplets would break up before reaching this size.

"It is remarkable that in two quite different environments, albeit both tropical but one extremely polluted and the other very clean, we measured raindrops that must have undergone numerous collisions without breaking up," Hobbs and Rangno write in Geophysical Research Letters.

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15 Jan 04  |  Science/Nature
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