Even when temperatures are below freezing point, clouds often contain supercooled water droplets. When such clouds are suddenly disturbed, for example by the passage of an aeroplane, the expansion of air that results can be enough to cool the air enough for the droplets to freeze. This is especially likely when such aircraft inject pollutant particles into the air, around which the water droplets can freeze. The resultant ice crystals can then fall out of the sky, leaving a hole in the cloud that’s left behind. Fallstreaks re most noticeable within relatively thin cloud layers, such as stratocumulus.