The Fallstreak Hole
Whilst out on a walk in December 2016, I noticed a rather unusual atmospheric phenomena. It was a mild day for December, and much of the UK was within the warm sector of a Atlantic cyclone out to the north-west. There was a dense layer of altocumulus at around 3-4km, but within this were vast holes, which exposed the clear blue sky above and contained fine streaks of ice crystals that were falling from the centre. It looked like this: 

So what is this phenomena, and what causes it?

It's called a fallstreak hole.

Fallstreak holes are rather rare, as they are limited to very specific atmospheric conditions and geographical locations. However, they are not naturally occurring - they are man-made! (There is a clue to this in the above picture.) Although they are anthropogenic, fallstreak holes actually display one of the most beautiful processes in the atmosphere - the one that produces almost all precipitation in the mid-latitudes. 

For these giant holes to appear in the cloud, we need to have a layer of cloud that contains supercooled liquid water droplets. Without ice condensation nuclei to form on (which are relatively rare), cloud droplets tend to remain liquid until about -40˚C, at which point they freeze spontaneously. In the intermediate temperatures, they remain in a supercooled liquid state: in other words, the droplets are liquid water, but below freezing. They will freeze instantly on contact with any sub-zero surface. In the case above, the layer of altocumulus was at just the right altitude to remain supercooled. 

The gaping holes are caused by the sudden freezing of large portions of the cloud. Airplanes are the culprit for their formation. When a plane flies through the cloud, ice condensation nuclei may be present in the exhaust fumes of the aircraft, which can aid the sudden formation of ice crystals. In addition, the sudden reduction in atmospheric pressure behind the propellor blades and wingtips of the plane can lead to rapid drops in the temperature, allowing spontaneous freezing of the liquid droplets to occur. Basically, planes provide an artificial method for ice crystals to grow in the cloud. 

But this doesn't explain how large holes can appear in the cloud.

When ice crystals start to grow in an environment dominated by supercooled liquid water droplets, a chain reaction occurs where the ice crystals continuously grow at the expense of the liquid droplets, which evaporate. The process by which rain generally forms in the mid-latitudes is called the Wegener–Bergeron–Findeisen process. (See my section on "How Rain forms" for a better explanation.) Once ice crystals form in the supersaturated environment of a cloud, it's easier for them to grow than the liquid water droplets. This is because they have a lower saturation vapour pressure than the liquid droplets, and so will  grow at the expense of the liquid droplets. The small number of ice crystals that form initially continually grow by vapour deposition, and eventually get so large that they can be seen. Meanwhile, the evaporation of the liquid droplets will leave a large hole in the cloud, and the ice crystals will ultimately begin to fall to the surface under their own weight, leaving a "streak" of ice crystals in the sky. These streaks can be seen from the centre to the left of the hole in my photo above - hence the name "fallstreak hole". If they don't evaporate, the ice crystals will reach the ground as snow or rain. 

All of the bright areas in the cloud in this image are single fallstreak holes - apologies for the poor image quality! 

Living in south London, I happen to be in a prime location to view fallstreak holes, due to my close proximity to Gatwick airport. Planes are constantly circling overhead, and so when the atmospheric conditions are in place, you can expect to see them fairly often! Generally, fallstreak holes tend to just be a single hole punched through the cloud, as a plane ascends/ descends through it on its way to/ from cruising altitude. However, with planes constantly circling overhead at roughly the same altitude of the cloud, large tears or scars in the cloud can be seen, like the one I captured in the first image. 

A perfect example of a fallstreak hole, taken in Oklahoma City, USA.