The rain in Spain falls mainly on the plane. Or so the saying goes, but new research has confirmed that airplanes do cause clouds to dump their contents prematurely, often around airports, and in this week's show we explore this weather-altering effect of aviation. 
A fallstreak hole (also known as a hole punch cloud, punch hole cloud, skypunch, cloud canal or cloud hole) is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water, in a supercooled state, has not frozen yet due to the lack of ice nucleation. When ice crystals do form, a domino effect is set off due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud. 
An obvious hole in a stratus deck due to cloud seeding with aircraft, using dry ice as a seeding agent. This is an example of cold cloud seeding, where supercooled cloud droplets are converted into ice crystals, which then precipitate out of the cloud deck. (USAF photo; boxed caption in the lower right reads "Effects of seeding Altostratus Clouds over Green Bay, Labrador: 45 minutes after seeding with dry ice". Photo and boxed caption obtained from Sewell, W.R.D., et. al., 1973: Modifying the Weather; Western Geographical Series, Vol. 9, University of Victoria, Victoria, British Columbia, Canada). 
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