A hole within clouds appeared over Hong Kong on 19 January 2015 (Figure 1). This phenomenon is known as "Cavum" (also known as "fallstreak hole") in meteorology.

In some high clouds such as cirrocumulus or altocumulus, supercooled water sometimes exists due to a lack of condensation surfaces. As supercooled water is unstable, it may form ice crystals quickly when surrounding air cools down rapidly due to disturbances such as expansion of air caused by a passing aircraft. These ice crystals will grow larger as neighboring droplets freeze, and eventually gain enough weight to fall out of the clouds. The ice crystals melt and evaporate while falling, creating a hole with brush-like streaks, hence named "fallstreak hole".

Multi-functional Transport Satellite (MTSAT) of the Japan Meteorological Agency (Figure 2) and the all-sky camera of the Hong Kong Observatory (Figure 3) also captured the images of fallstreak hole over Hong Kong that day.