About two weeks ago people noticed strange cloud formations in the skies above San Antonio, TX. A deck of altostratus clouds were moving across the area, and people noticed wide, winding openings in the cloud deck and in some cases large circular openings. Photos began being posted to the NWS San Antonio Facebook page with inquiries about what caused these unusual formations.
|Cloud formation seen above San Antonio on March 13, 2015|
The phenomenon is known as "hole punch" clouds, or fallstreak holes. They are not that uncommon, especially near major airports and along air traffic routes, but the conditions have to be just right for them to occur. What happens, in simple terms, is that aircraft cause precipitation to occur as they fly through these clouds of supercooled droplets. The precipitation falls out of the cloud, but evaporates in the typically dry air underneath. When the cloud layer is thin and there is no supply of moisture to replace that which precipitated out, a hole remains. The wispy streaks you often see beneath the hole are ice crystals precipitating out of the cloud. The difference between a "canal" in the clouds or a circular hole depends on whether the aircraft was cruising through the cloud layer or descending or ascending through the layer.
|This is a photo of a "hole punch" cloud I took over western Indiana|
on October 17, 2014
What is really neat about the San Antonio occurrence is that the forecasters at the National Weather Service, through a little "detective" work, were able to determine which individual airplane caused these hole punch clouds using sounding, satellite, and air traffic data! You can read the details about "The Mystery of the 'Cloud Hole Punch'" at the NWS San Antonio web site.
There is also more information about hole punch clouds at from NASA at this link. Some striking photographs and more explanation of hole punch clouds can be found at EarthSky.
There are more spectacular photos and explanation on the Cloud Appreciation Society web site. Keep your eyes to the sky - you never know what you may see.