Featured above is a colorful circumzenithal arc and a fall-streak observed from Civitavecchia (Rome), Italy, below the approach path of the Fiumicino Airport. Fall-streak holes occur in cloud decks (altostratus clouds, for example) where the droplets of liquid water that compose them are in a supercooled state; that is they remain in liquid even if the air temperature is well below the freezing point of water (0 ° C). However, an aircraft passing through the cloud layer can cause the supercooled droplets to suddenly freeze, resulting in a cascade of ice crystals. When these crystals become sufficiently heavy, they’ll detach from the cloud and fall, leaving a void in the cloud layer – a hole in the sky. In this case, the falling crystals were oriented so that sunlight entered their uppermost horizontal faces and exited through one of their vertical side faces, forming this charming circumzenithal arc. Note also the cloud iridescence near the bottom of the photo.
Both climbing and descending aircraft will trigger fall-streaks in shallow cloud decks, which explains why such voids are often seen in the vicinity of busy airport runways. Photo taken on April 12, 2021.
(Tratto dall'EPOD del 14 Giugno 2021)
