No, it’s not a UFO or the onset of the apocalypse…it’s a hole punch cloud, an usual and mysterious sight to witness. Hole punch clouds, which are also called canal clouds, were spotted in the skies over the Twin Cities on Sunday and again Tuesday. WCCO-TV viewers in Minneapolis, Blaine, Chaska, and Mahtomedi captured photos of this rare and intriguing phenomenon.
The first ingredient in the formation of a hole punch cloud is a deck of altoculumus clouds, which exists at mid to high levels in the atmosphere. Altocumulus clouds are mainly comprised of water droplets that are ‘super-cooled’; meaning the droplets remain in liquid form at air temperatures below-freezing.
The presence of super-cooled water droplets at atmospheric temperatures of 0°C to -20°C is fairly common, in fact these droplets can maintain liquid status to temperatures of -37°C! If the droplets do cool to -37°C or colder, they lose the phase change fight and spontaneously freeze into ice crystals, a process known as homogeneous nucleation…fancy, eh?
Now, to the ‘altocu’ clouds, we introduce an aircraft. No, not flown by extra-terrestrials, by humans. As a jet plane ascends into the altocumulus cloud layer, the surrounding air is lifted by passage over the plane’s wings. This lift results in rapid cooling of the air, by as much as 20°C, thereby potentially chilling said air to that -37°C threshold at which super-cooled water droplets transform into ice crystals.
An interesting related factoid: the amount of air cooling is proportional to the wing loading (the loaded weight of the airplane divided by the area of the wing). Thus a larger aircraft can induce a higher rate of cooling.
Back to the ice crystals…They are real divas and want to command the scene. They do so by stealing water vapor molecules present in the air away from the super-cooled water droplets, in order to grow in size. The ice crystals can do this because vapor pressure over water is greater than over ice, and just like many humans, the water vapor molecules want to hang in an environment of low pressure instead of high pressure. This behavior is termed as the Bergeron process. The ice crystals’ grow at the expense of the super-cooled water droplets and thus the droplets are depleted in size and forced into evaporation.
The demise of the super-cooled water droplets though is our gain, as the depletion of the droplets creates the hole punch in the cloud deck. The hole expands with further water droplet depletion via the Bergeron process.
The ice crystals add to the exciting display by falling from the hole punch cloud as virga, creating the wispy appearing mass directly beneath the hole. Pretty cool.
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