"Hole punch cloud" spotted above Western Washington

A KIRO 7 viewer captured a rare phenomenon in the skies above Enumclaw Thursday afternoon: a fallstreak, or "hole punch cloud."

These "holes" seem to just randomly appear in a relatively stable and consistent cloud deck during fair weather days.  And scientists believe they're almost always caused by human activity.

Specifically, by aircraft!

Aircraft responsible for "hole punch clouds"

Most of our clouds are made up of either ice crystals or water droplets. It all depends on the temperature, of course, of the water vapor in the cloud and whether it is liquid or ice.

Higher clouds are usually made of ice and low clouds are made of tiny water droplets. %

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However, under certain conditions, a cloud in the middle levels of the lower atmosphere (say, about 15,000 or 25,000 feet up) can be made of water droplets in an environment that is actually below freezing!  That is called supercooling. 

Now, if the environment can be further cooled in the cloud (even for a brief time), then the process of water droplets freezing and forming ice crystals can start.

Air above and just behind an aircraft wing rapidly cools as a result of the wing producing lift.

Sometimes, if you have a window seat along or behind the wing, you might see "fog" briefly form above and just behind the wing.  That's water vapor briefly being cooled into a cloud because of the lower air pressure, or vacuum, that forms above an airplane wing in flight.

Higher in the sky in that supercooled cloud, this further cooling can be enough to start the formation of ice crystals from the already-present water droplets.

What happens next?

Then it gets even more complicated!

During the process of freezing into ice crystals, there's also some evaporation going on of nearby water droplets.

I'll spare you the lesson on what is called latent heat... that's certainly a lesson for meteorological textbooks!  

But, bottom line is that the air in the "hole punch" starts to cool a bit.

And as cooler air is more dense, it will sink, taking the ice crystals that have formed along with it. %

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That's why "hole punch clouds" are more typically spoken of by meteorologists as "fallstreaks."  The falling ice crystals appear in a streak or ribbon and can be dazzling as the crystals shimmer in iridescence when sunlight passes through!

Pretty cool, huh?

These "hole punch" or fallstreak clouds are most often seen dozens of miles away from airports, usually along typical approach or departure paths.  Sometimes, given the right circumstances and volume of air traffic, there can be dozens that appear.

I always love seeing your weather photographs!  Send them to me on my Facebook page, via Twitter, or at newstips@kiro7.com

Cox Media Group