Courtesy of Lawrence Wallman
Courtesy of Lawrence Wallman
Courtesy of Justin Green
These strange features, known as hole-punch clouds...among other names, are caused by the penetration of aircraft through a cloud deck made of supercooled liquid water.
Believe it or not, many middle-level clouds are below freezing but are still made of liquid water. Turns out that clean water can take its time to freeze when temperatures are below 0C.
We had a cloud deck over Seattle around 3 PM Wednesday that was made up of such supercooled water. A vertical sounding (vertical ascent from a weather balloon) at Quillayute on the WA coast at 4 PM suggested that the cloud was at roughly 450 hPa pressure or around 6600 m (22,000 ft) and a temperature around -22C (-8F).
We were dealing with an altocumulus or cirrocumulus clouds of liquid water. A past colleague of mine, Peter Hobbs, and one of his past associates (Art Rangno) studied this situation and called the holes APIPS (Aircraft Produced Ice Particles). Let me tell you why.
Look closely at the picture shown above (repeated below). Do you see the fibrous looking material in the middle of the hole. Those are ice crystals! You can tell because they have a wispy, less distinct look to them.
But what caused the transition between liquid water clouds (with much sharper edge to ice clouds? The passage of an aircraft!
When air goes across the wings of an aircraft the air is speed up and that causes pressure to fall on the upper side of the wing (that is what keeps a plane in the air!). When air parcels experience lower pressure they expand and cool (this is called adiabatic cooling).
Now clouds can be comprised of liquid water at -10C to around -22C, but if temperatures drop further, it is harder to remain liquid. Also air can become saturated when it cools on the air frame and go directly to ice if the air is cold enough (-30C to -40C).
It turns out that ice and liquid water find it difficult to be in the same environment, water vapor leaves the liquid water and heads over to the ice crystals (this is often called the Bergeron process). The ice crystals grow rapidly, become heavy, and fall out of the cloud, producing the hole. You can see that happening in the cloud picture above.
Mystery solved....
