|Photo courtesy of Jon Fox|
But what are they and why do they form?
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.
On visible satellite imagery, we captured a few images as the sun came up and illuminated the feature from above.
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.
The combination of cloud physics and optics can result in stunning scenery. Here are a few more photos from NWS employees.