Observations of a glaciating hole‐punch cloud

Abstract

In his fascinating review of fallstreak holes or ‘hole-punch ’ clouds, David Pedgley (2008) remarked that there were no measurements into the microphysics and dynamics of hole-punch clouds from ground-based remote sensors because the probability of a hole drifting into the instrument sample volume is so small. Here we present some remarkable measurements of exactly that, which we were lucky enough to make at the Chilbolton Observatory in Hampshire. What are they? These circular (or sometimes cylindrical) holes are formed when aircraft penetrate supercooled liquid cloud layers such as altocumulus. The passage of the aircraft is believed to produce large numbers of ice crystals, which then grow quickly by the Bergeron-Findeison mechanism in the vapour-rich conditions (water-saturated air is supersaturated relative to ice). This growth depletes the cloud of water vapour, causing the liquid droplets to evaporate, and the formation of a hole in the liquid cloud layer with a visible trail of ice crystals falling out beneath. Photographs of these spectacular phenomena are a regular feature in Weather (see for example the front cover of March 2009’s issue); more detail of the physical mechanisms