02-07-11 Planes punch holes in clouds,making it snow

Aeroplanes passing through the atmosphere are creating holes in the clouds which can cause disruptive weather, say researchers.In a study published in the current issue of Science,researchers explain how cloud holes are created when part of the cloud freezes and falls to the ground as snow a process that can be triggered by an aircraft.The result is otherworldly:a spectacular hole-punch cloud above Moscow in 2009 prompted murmurings of alien invasions and Russian weather weapons."We examined how frequently cloud holes occur near several major airports", said lead author Andrew Heymsfield from the National Centre for Atmospheric Research in Colorado, U.S."We show an example where this process produced a snowfall rate of 2 inches (5 cm) per hour.Snow near and around airports has effects on airport traffic and de-icing operations." There are two key ingredients required for the creation of a holey cloud:supercooled water and a sudden temperature drop.Supercooled water seems to defy the laws of physics by remaining liquid at sub-zero temperatures.This phenomenon occurs in clouds at temperatures between 0 and -40 Degrees when there are no ice nuclei (e.g. dust particles) present to allow the water droplets to crystallise.However,a sudden drop in cloud temperature may help to initiate ice crystal formation.An aircraft can provide this cooling via an expansion of air behind the propeller tips or by airflow over the wings.The introduction of ice into a supercooled cloud triggers a domino effect of rapid ice formation.When the ice becomes too heavy to remain in the cloud,it falls to the ground as snow,leaving behind a void.The method by which ice crystals grow in a supercooled cloud is known as the Bergeron-Findeisen (B-F) process,in which water droplets evaporate,condense and freeze onto the ice crystals,leading to further crystal growth.However,the B-F process is unlikely to provide the whole story.Andrew Heymsfield and colleagues studied satellite images of a holey cloud layer over Texas,taken by the Geostationary Operational Environmental Satellite (GOES) on 29 January 2007.In total,92 holes were spotted,produced by a wide variety of aircraft,including large passenger jets, military jets and turboprop aeroplanes.A typical hole grew for an hour after it was first detected,up to a maximum length of over 100 km in some cases.Several holes remained in the sky for more than four hours.