Fallstreak holes are a cloud formation that occurs as gaps in mid or high level cloud layers. Below them, trails of ice crystals dangle. In order for a fallstreak hole to form, the cloud layer must be composed of supercooled droplets (liquid water), despite the temperatures at cloud level being well below 0°C. When one region of the cloud begins to freeze, the fallstreak hole forms. This begins a chain reaction whereby all the moisture from the supercooled droplets in the nearby area is drawn in and joins the ice crystals. These then grow big enough to fall beneath the hole; fallstreak holes have been known to reach 50 kilometres across just an hour after the hole began to form.
Though these cloud formations have been known about for some time, scientists were unclear as to why the freezing began in one area of a cloud layer and how it grew in size. Research published in Science in July 2011 sought to answer these questions.
The research was performed by Andy Heymsfield and his colleagues at the
To test this theory, Heymsfield ran a detailed computer model of the internal workings of a cloud, introducing a line of ice crystals like those produced by an aircraft. The simulation showed a hole that grew to a diameter of 4.4 kilometres in 90 minutes. If the heating effect of creating ice in the simulation was turned off, the hole grew slowly; if the effects of evaporation were also removed, the hole didn't grow at all.
The research confirmed that aircraft flying through the cloud could be what is setting off the freezing process. The air expands as it passes over the aircraft wings and over the blades of its propellers. This causes the air to cool; this cooling can be long enough to decrease the temperature enough for the droplets to start to freeze. When these droplets change into solid particles, they emit a small amount of heat, which causes the air around them to expand and rise somewhat. This rising current causes the surrounding air to sink slightly.
When the air around the frozen fallstreak sinks and begins to warm up, the droplets within begin to evaporate. This appears to be why the fallstreak of ice crystals produced by the plane flying through the cloud layer forms a circular hole that looks like it has been ‘punched’ out of the cloud layer. It does not seem to matter whether the plane is ascending or descending through the cloud layer.