Aeroplanes cause rare 'hole-punch clouds' that resemble strange purple UFOs at sunrise

This incredible photograph shows a supernatural-looking sunrise with clusters of purple clouds resembling UFOs floating in the sky.

The stunning purple and orange display was captured by gardener Adrian Sparrowhawk, 50, who said the sky was a 'spooky' sight.

The golden morning light streaked through the dark purple clouds which dappled the sky above Tetbury, Gloucestershire, with six clusters of thicker clouds floating underneath, like flying objects from a sci-fi movie.

Adrian said he stopped to photograph the sunset as he set to work at 6.40am on Monday morning, as he had 'never seen anything like it before'.

He said: 'I just thought I had to capture it, it was absolutely wonderful. I've never seen anything like it. It looked really spooky at that time in the morning.'

He added that he has since been told the cloud formations are known as 'hole-punch clouds'. Also known as a 'fallstreak hole', hole-punch clouds are formed when the water temperature in the clouds is below freezing, but the water, in a supercooled state, has not yet frozen. 

Experts say that the bizarre shapes are caused by aircraft. They form when part of the cloud layer forms ice crystals which are large enough to fall into water droplets - water below 0°C but not yet frozen.

These water droplets need a tiny particle to freeze or to be cooled below -40 °C. Aircraft punching through this cloud layer can cause air to expand and cool as it passes over the aircraft wings or propeller.

This change in temperature can be enough to encourage the supercooled droplets to freeze and fall from the cloud layer in this distinctive pattern.

Sometimes the holes in fallstreak formations can expand up to 30 miles (50 km) across within an hour of appearing. When ice particles form quickly, it causes a domino effect as the water droplets connect with the crystals.

These get heavier and then start to fall all of a sudden - leaving a large hole in the cloud. The wispy clouds that form in the centre of the hole are the water particles falling.

Passing aircraft are thought to be the reason behind the formation of these clouds, since a drop in pressure caused by their wings or propeller tips cools the air quickly. 

Rainbows are sometimes seen with fallstreak clouds, thanks to light refracting off the water droplets.