What's that cloud? Holes in the sky

This unusual sight, known as a fallstreak hole, or holepunch cloud, was snapped in East Sussex, but what is actually happening here?

This impressive and unusual formation appears as a gap, or hole in the surrounding cirrocumulus or altocumulus clouds. In this shot from Eastbourne, they are cirrocumulus clouds.

Cirrocumulus are recognisable by their bright white colour and tiny puff-ball appearance, whereas altocumulus clouds are darker in shade and larger.

These clouds are high enough, and consequently cold enough, to contain supercooled water droplets. These water droplets within the cloud are sub-zero, but not actually frozen.

Regular water like our drinking water contains small particles in the form of dust or other impurities. These allow for ice crystals to form at 0°C.

But for the pure water droplets found within clouds, no other particles are present, and so the water can remain as a liquid all the way down to around -40°C.

So when aircraft pass through this cloud layer, it causes the surrounding air to expand and cool. The change in temperature can then cause the water droplets to suddenly freeze into solid ice crystals.

These ice crystals then fall from the sky as virga, or fallstreaks, which is precipitation that doesn’t reach the ground.

It might sound surprising, but the process of freezing actually gives off a tiny bit of heat. This heat is enough to evaporate the surrounding water droplets, leaving the sharply defined hole in the clouds that you can see in the image.