A pretty rare phenomenon was seen over Western Washington on Wednesday and several KIRO 7 viewers sent us photos.
Fallstreaks, often called “hole punch clouds,” occur when conditions in a mid-or-high level cloud deck consist of supercooled water droplets — or water droplets that can actually exist as a liquid even though the air temperature is below freezing!
This happens when the air is very clean aloft and the air doesn’t have particles of soot, smoke, salt, or any other substance for the water molecules to condense upon — which would allow for ice to form.
When an aircraft taking off or landing moves through the cloud deck, the exhaust from the aircraft combined with the disruptive airflow is enough for the supercooled water droplets to form into ice crystals instantly.
The freezing process causes the ice crystals to fall out, creating wisps of ice descending from the location of the “hole punch” — creating the “fallstreak”! Meanwhile, the hole in the cloud slowly expands and more and more of the supercooled water droplets turn to ice.
This is very rare as our clouds usually exist as ice crystals or liquid water droplets, not in this more unusual “supercooled” state.
Here are some more examples of this phenomenon from the National Weather Service.
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