Location: Oneonta, NY
Date: October 2011
The sky over Oneonta on this autumn evening was covered with an altocumulus stratiformis cloud layer. These mid-level clouds (typically located between 6,500 and 20,000 feet or 2,000 to 6,000 meters above the ground), are primarily made up of water droplets, but at these levels in the atmosphere, where temperatures are below freezing, some of this water remains in liquid form: this is called supercooled water. In order for these supercooled droplets to freeze, the temperature has to either be below -40 degrees (Celsius or Fahrenheit) or there needs to be something other than water (a tiny particle) to serve as a nucleus to freeze upon. Once this freezing gets started, a process that gives off heat, nearby supercooled droplets evaporate at the expense of the growing ice crystals creating a hole in the cloud layer. As these ice crystals grow then begin to fall, you can see them in the center of the hole, giving this phenomenon the name Fallstreak Hole (also referred to as a “Hole punch cloud”).
But what kicks off the freezing process? While this could happen naturally, research has found that aircraft flying through this supercooled cloud layer can set off freezing. As air flows around airplane propellor tips and over jet wings, it can cool in a localized area, at times to temperatures colder than the -40-degree threshold required for spontaneous freezing of drops. This sets off the freezing process, which, as previously mentioned, gives off heat. Computer simulations (described in a research article in Science) indicated that this process can induce vertical motions in the atmosphere than can least more than an hour, leading to the expansion of the hole.
Notice how the hole in this particular picture looks more like a line. This is due to the lower angle through which the aircraft flew through the clouds.
Reference: Heymsfield et al., 2011: Formation and Spread of Aircraft-Induced Holes in Clouds. Science, Vol. 333 no. 6038, pages 77-81.