From: Andy Soos, ENN
Published June 15, 2010 02:47 PM

Strange and Curious Clouds

It is always nice to lie back and look up into the sky and watch the drifting patterns of clouds. However, some clouds are odder or weirder than others. Some were made by nature and some were made accidentally by the actions of man. As turboprop and jet aircraft climb or descend under certain atmospheric conditions, they can inadvertently seed mid-level clouds and cause narrow bands of snow or rain to develop and fall to the ground, new research finds. Through this seeding process, they leave behind odd shaped holes or channels in the clouds, which tend to be fascinating to see.

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Across the world, sightings of strange cloud holes have triggered amusement and speculation. One such was in October 2009 and was called the "halo over Moscow" that some called a alien spacecraft.

"Sky holes", also known more formally as "Hole Punch Clouds" or "Punch Hole Clouds", are not some secret Government experiment, nor Alien cloaked spacecraft. They are instead, holes that form in thin, puffy cloud layers.

There are many hypothesis for what causes them: jet planes passing through the cloud layer, wind, and others. But they remain an enigmatic example of complex weather processes.

One prevailing theory is that Cirrocumulus clouds contain both ice crystals, and supercooled water droplets. The introduction of the extra water from the jet exhaust causes a rapid increase in the rate of crystallization, and all the water turns to ice as the ice crystals get larger. This rapidly desaturates the air, which then pulls in moisture from the air around it, thus spreading the hole.

For decades, scientists have wondered about the causes of these clouds with gaps seemingly made by a giant hole punch. Researchers have proposed a number of possible aviation related causes ranging from jet acoustic shock waves,local warming of the air, and the formation of ice along jet contrails.

Researchers in the 1980s observed that propeller aircraft could transform supercooled droplets into ice crystals, and experiments were launched in the 1990s to characterize the phenomenon.

The key ingredient for developing these holes in the clouds may be water droplets at subfreezing temperatures. As air is cooled behind aircraft propellers or over jet wings, the water droplets freeze/precipitate and drop toward Earth.

“Any time aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby," says Andrew Heymsfield, a scientist with the National Center for Atmospheric Research (NCAR) and lead author of a new study into the phenomenon. “Just by flying an airplane through these clouds, you could produce as much precipitation as with seeding materials along the same path in the cloud."

But scientists had not previously observed snow as it fell to the ground as a result of aircraft until Heymsfield and his colleagues happened to fly through some falling snow west of Denver International Airport with an array of instruments. While the research team did not notice anything unusual at the time of their 2007 flight, a subsequent review of data from a ground based radar in the area revealed an unusual echo, indicating that the band of precipitation had evolved quickly and was unusually shaped.

In looking over the evidence the scientists found a cloud hole in the same direction as the standard flight tracks of commercial aircraft in the region. Heymsfield then surmised that a plane flying through the cloud might have somehow caused ice particles to form and snow along its path, leaving a canal shaped hole punch cloud behind.

A subsequent review of flight track records from the Federal Aviation Administration revealed that turboprop planes operated by two airlines flew close to the hole-punch location, following a standard flight path that produced the subsequent band of snow. Snow crystals began falling about five minutes after the second aircraft flew through the cloud. The snowfall, in a band about 20 miles long and 2.5 miles wide, continued for about 45 minutes, resulting in about two inches of snow on the ground.

The cloud layers outside Denver on that day contained supercooled droplet particles of water that remain liquid even at temperatures as low as -35 degrees Fahrenheit (about -34 degrees C). When a turboprop plane flies through such a cloud layer, the tips of its propellers can cause the air to rapidly expand. As the air expands, it cools and causes the supercooled droplets to freeze into ice particles and fall out of the clouds as snow or rain.

There may be multiple explanations of such odd clouds. In November 2008 over Tampa, a hole punch cloud appeared.

There was a layer of clouds high enough to be mostly ice crystals instead of simply condensed water on that day. The air was highly stable, so the clouds were fairly uniform. A pocket of cold air, which sinks toward the ground, "punched" the hole in the clouds and created the circular opening or so stated meteorologists at local News Channel 8.

Whatever the case may be, these strange cloud formations are the result of natural processes not clearly understood and sometimes are precipitated by the actions of Man.

For further information: https://www2.ucar.edu/news/mysterious-clouds-produced-when-aircraft-inadvertently-cause-rain-or-snow

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