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Skywarden,
Ursa Astronomical Association
Kopernikuksentie 1
00130 Helsinki
taivaanvahti(at)ursa.fi

Ursa Astronomical Association

Other special clouds - 9.8.2017 at 08.00 - 9.8.2017 at 08.30 Espoo Observation number 66015

Visibility IV / V


At eight o'clock an almost perfect hole cloud and wavy clouds were visible. The hole gradually closed over the next half hour.


Additional information
  • Havainto
    • Other special clouds
  • Cloud coverage in the sky
    • Covered 1/8 of the sky
  • Other special clouds
    • Fallstreak hole info

      Fallstreak hole (Hole punch cloud) is a rare phenomenon observed in cloud cover.

      Altocumulus clouds consist mainly of subcooled cloud droplets, which can be well below freezing, but which have not yet frozen in the absence of suitable freezing cores.

      When freezing nuclei, such as small ice crystals, come into contact with the cloud, the droplets of clouds may begin to freeze outward from this point so that an annular opening appears to form in the cloud. Such an opening is called a hole cloud.

      One source of freezing cores may be airplanes that create a cloud of holes as they pass through the altocumulus layer. Hole clouds can also be oblong in shape when the aircraft has flown a little longer at cloud height. In this case, ice crystals floating down from the opening remains in its path. In the middle of these openings or canals, a thin line of ice crystals, or Virga, visible below the cloud layer, can typically be seen. If the sun happens to be at a suitable distance from this rain line, a halo phenomenon, such as a side sun, can be detected in the wake.

      A hole cloud formed in the Altocumulus layer with a rain line of ice crystals, or Virga, in the middle. Image by Jari Luomanen.

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