Shades of shiny red, orange, yellow and blue flooded the sky in Athens and Attica at dusk on Tuesday, February 2020. I was speaking on the phone with a friend and we were both watching from our windows how the threads of the shiny colors intermingled with the layers of thin clouds. “Aaaawesome! Look, it’s like embroidery!” I told my friend and she answered “God is embroidering the sky and is sending us a gift.” We kept chatting and stunning, not knowing that we were witnesses of a rare phenomenon called Fallstreak Hole.
The Fallstreak Hole with its impressive colorful cloud formation is a rare one.
According to the meteo.gr website of the National Observatory of Athens, this particular weather phenomenon requires some special conditions to happen.
A Fallstreak Hole (also known as hole punch cloud) is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds.
Clouds above the 5-6 km height in the troposphere, such as the uppers and turrets in the video, are mainly composed of water even if the temperature is very low, below -20 ° C.
The water in this state is supercooled and this is because there are no condensation cores (salt, carbon, silver iodide or other metals) to form regular drops or ice crystals (to form a raindrop or an ice crystal there is a need for a crystal surface ).
Such holes are formed when the water temperature in the clouds is below freezing, but the water, in a supercooled state, has not frozen yet due to the lack of ice nucleation.
The ice crystals find themselves surrounded by droplets, and grow quickly by the Bergeron process, causing the droplets to evaporate and creating a hole with brush-like streaks of ice crystals below it
Drops evaporate and the water vapor increases in size of the ice crystals until they grow large enough and begin to fall, as shown in the center of the hole.
The most common way for condensation nuclei to appear at these heights is by passing aircraft that releases some, but additionally creates a low pressure area, so the air cools rapidly and the first ice crystals are created.
It is very likely that the formation of the “Fallstreak Hole”, as it was called, was formed due to the above mechanisms.
PS And the colors? ah, well, there were because the beams of the sun going to bed were waving us “good night.”
