Wednesday, February 18, 2015

Rare and Beautiful Clouds Over the Inland Northwest

If you live in Eastern Washington or Northern Idaho and looked up to the skies the morning of February 18, 2015 between 6:30 and 8am, you may have observed a rare and spectacular phenomenon!  This morning, many folks (including us at the NWS Spokane office) couldn't help but be in awe of these amazing clouds.
Photo courtesy of Jon Fox

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But what are they and why do they form?  

This phenomenon is often referred to as fallstreaks or cloud holes.  Much research has been done on the subject dating back to the late 1950s.  Since this phenomenon is relatively rare, it's easy to conclude that the atmospheric conditions in which it forms are also very specific.  Research has shown that the holes or fallstreaks mostly commonly form in altocumulus or stratocumulus clouds.

After several days of mostly clear skies over the Inland Northwest, moisture was approaching from the west bringing the region a layer of altocomulus stratiformis.  As we do every morning at 3am, we launched a radiosonde that sampled the atmospheric conditions just a few hours prior to the sunrise and cloud punch holes.  This image below shows that sounding.
The area on the sounding circled shows increased moisture where the cloud layer most likely formed a few hours later.  The approximate height of this cloud is at about 400 mb, 7 kilometers, or 23,000 feet.   According to the sounding, the temperature at that layer is approximately -30° Celsius.

On visible satellite imagery, we captured a few images as the sun came up and illuminated the feature from above.
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So what's the explanation??  I'm going to quote one of many articles and papers that have been written on the subject.  This excerpt comes from an article that appeared in Weather - December 2008, Vol. 63, No.12 by David E. Pedgely.

"These circular (or sometimes cylindrical) holes are formed when aircraft penetrate supercooled liquid cloud layers such as altocumulus.  The passage of the aircraft is believed to produce large numbers of ice crystals, which then grow quickly by the Bergeron-Findeison mechanism in the vapor-rich conditions (water-saturated air is supersaturated relative to ice).  This growth depletes the cloud of water vapor, causing the liquid droplets to evaporate and the formation of a hole in the liquid cloud layer with a visible trail of ice crystals falling out beneath."

"It seems that their most likely explanation is the penetration of shallow, highly supercooled altocumulus clouds by aircraft whose wakes freeze some droplets over a radius of about 100 m.  These ice particles are then spread by wake turbulence over a radius of 1 km or so in about 10 minutes within the cloud, where they grow at the expense of the droplets.  The resulting crystals fall into, and continue to grow in, clear but ice-suspersaturated air, and gravity sorts them into a fallstreak, where a weak downdraft develops through drag and evaporation."

In short, an aircraft traveled directly above or through the cloud layer.  It has been proven that it is not a result of anything the aircraft is emitting, instead it's the sudden and drastic decrease in pressure over the wing or propeller tip that initiates the process of  depleting the cloud and causing the liquid droplets to evaporate.  The result is the appearance of a hole that has been created with trailing ice crystals falling out beneath.

A quick search of aircraft routes this morning show how many flights follow a similar path departing out of Seattle and head east-southeast near the origin of the fallstreaks.  

Many folks also noticed a sun dog (or parhelion) as shown in the picture below to the right of the sun.  This also has to do with atmospheric optics.  The plate-like hexagonal ice crystals falling through the hole refract the light from the sun.  This halo is fairly common with cirrostratus clouds present.


While the cloud hole punch or fallstreak phonemena are rare because of the very specific atmospheric conditions necessary, there are numerous examples available online from around the world.  Many of the meteorologists (including myself) here at the National Weather Service have never seen this in person so we certainly understand the curiosity that this ignited.

The combination of cloud physics and optics can result in stunning scenery.  Here are a few more photos from NWS employees.




2 comments:

  1. I've heard them called "Hole Punch Clouds, I believe

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  2. One time I saw a sky full of lined up pink puffy powder puff clouds...what a breathtaking wonder!

    ReplyDelete