lenticular clouds
Rare spectacle: These lenticular clouds, also known as altocumulus standing lenticularis, are hardly ever seen in British skies
Swirling through the air in mesmerising saucer-like shapes, these exceptionally rare lenticular clouds are the most common explanation for UFO sightings. The remarkable-looking disc shapes were spotted by amateur photographer Glenn Spencer, 42, in the sky above Chester in Cheshire.

The formations, also known as wave clouds, were scattered over miles, producing a stunning spectacle. Rarely seen in the UK, lenticular clouds are formed when a tall geographic feature, such as the the top of a mountain, obstructs a strong wind.

The interruption in airflow creates a wind wave pattern in the atmosphere on one side of the mountain and at the top of these waves, moisture in the air condenses and forms a cloud. As air moves down into the trough of these waves the water evaporates again, leaving behind clouds in a characteristic lenticular shape.

Mr Spencer noticed them while driving to the shops and was so amazed that he stopped the car to retrieve his camera from the boot. He said: 'I tried to follow them to higher ground to get a good view. They were just so mesmerising and I thought they looked amazing. It is easy to see why people mistake them for UFOs. They are brilliant. I've never seen anything like it.'

disc shaped clouds
Stunning: They form when wind blows over high ground and then undulates down in a stream of waves creating a rippled effect as the air cools
lenticular clouds
Pilots of powered aircraft tend to avoid flying near lenticular clouds because of the turbulence of the rotor systems that accompany them