Monday, February 21, 2011

Asperatus Cloud

Photograph courtesy Merrick Davies

An "asperatus" cloud rolls over New Zealand's South Island in an undated picture.

This apparently new class of clouds is still a mystery. But experts suspect asperatus clouds' choppy undersides may be due to strong winds disturbing previously stable layers of warm and cold air.

Asperatus clouds may spur the first new classification in the World Meteorological Organization's International Cloud Atlas since the 1950s, Gavin Pretor-Pinney said.

Since the last addition to the atlas, the emergence of satellite imagery has pushed meteorologists to take a much broader view on weather and focus less on small-scale cloud formations.

But "the tide is turning back again," in part because the humble cloud is seen as a "wild card" in climate-change prediction, Pretor-Pinney said.

LeMone agreed that clouds are a "big unknown" in climate change, mostly because climate-change models do not provide a high-enough resolution to determine what clouds' impacts will be on a changing world.

An undated picture could be examples of the first new type of cloud to be recognized since 1951. Or so hopes Gavin Pretor-Pinney, founder of the Cloud Appreciation Society.

The British cloud enthusiast said he began getting photos of "dramatic" and "weird" clouds (including the above) in 2005 that he didn't know how to define.

A few months ago he began preparing to propose the odd formations as a new cloud variety to the UN's World Meteorological Organization, which classifies cloud types.

Pretor-Pinney jokingly calls it the "Jacques Cousteau cloud," after its resemblance to a roiling ocean surface seen from below. But the cloud fan has proposed a "formal," Latin name: Undulatus asperatus--roughly, "a very turbulent, violent, chaotic form of undulation," explained Pretor-Pinney, author of the new Cloud Collector's Handbook.

Margaret LeMone, a cloud expert with the National Center for Atmospheric Research in Boulder, Colorado, said that she has taken photos of asperatus clouds intermittently over the past 30 years.

It's likely that the cloud will turn out to be a new variety, LeMone said.

"Having a group of people enthusiastic about clouds can only help the field of meteorology," she added.

Asked how has such a striking cloud type could go unrecognized, Pretor-Pinney cites its rarity--and the proliferation and portability of digital cameras. "Technology has allowed us to have this new perspective on the sky."

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