December 06, 2005
Tuesday

Talk about clouding the issue.

Are the skies not cloudy all day? Is it partly cloudy, or overcast or downright dark as night?

In an age of technical precision, definitions of what constitutes a cloud can still vary widely depending on which instruments are used, scientists complain.

While human observers on the ground can still use a fairly simple scale of obscured sky to measure cloudiness, the issue gets stickier when satellites look down on clouds from above.

"The problem is that what we define a cloud as depends on the type of instrument we're using to define it," said Steven Ackerman, director of the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison. He is leading a discussion on the problem later this week during the fall meeting of the American Geophysical Union, in San Francisco.

Getting an accurate measurement of clouds is important, Ackerman notes, because their patterns and formations are used both in short-term weather forecasts and plugged into models used for long-term climate prediction and monitoring.

Not only can't it rain or snow without clouds, but cloud cover also determines how much solar energy reaches Earth.

Many technical issues make good measures of cloud cover a "fuzzy" matter, the researcher said. For instance, some satellites use a technique called "active sensing" to detect the presence of clouds by directing the energy of varying wavelengths at successive sections of the atmosphere.

But different satellites use different scales, Ackerman noted, so that varying the energy threshold by just 3 percent or 4 percent can significantly alter cloud-cover measurements.

Other instruments can produce readings that differ by as much as 5 percent to 10 percent, Ackerman said. "It's like assigning grades. If you score 90 and above, you get an A; but what if someone gets an 89.8 and gets a B? Is that fair? Well, that's the same problem with measuring clouds."

Cirrus clouds, the high, wispy formations that often look like smoke or horsetails, are particularly difficult to define, according to Ackerman. As the clouds fade into nothingness, scientists often struggle to define the point at which they cease to be clouds.

Satellites scan the atmosphere in consecutive blocks of varying size, which can also make getting readings tricky.

"An observation area may contain different types of clouds, or may be half-full of cloud or packed with thick cloud," Ackerman said, "so then we're left asking: Is that a cloud or not a cloud?''

He argues that some international standards for cloud measurement need to be considered to allow research and forecasts to be more easily compared.

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