Don't panic - it's not an alien invasion! These cool "flying saucers" are actually lenticular clouds which have been spotted over the UK at sunset in the past week.
The lenticular clouds looked like red UFOs under the setting sun. Photo taken in Drighlington, Leeds, by Sue
They look fantastic in the sky during the day but take on a special reddish glow when illuminated by the rising or setting sun.
This stack of lenticular cloud was seen over Biggar, South Lanarkshire, by BroughtonWatch
From the Latin word lenticularis meaning "lens-shaped", lenticular clouds are quite rare, and are often mistaken for UFOs.
A solitary lenticular cloud over the Pairc hills, Isle of Lewis. Photo by aimsir eilidh
They are formed near mountains when the air is both stable and humid. As air is blown over the mountain, it produces a standing-wave pattern and these clouds form in the crest of the waves.
Garry Hornet photographed this stunning array of lenticular clouds in Sheriff Hutton, North Yorkshire
Lenticular clouds are unusual because they appear stationary even when it's windy. In fact the water droplets are racing through the cloud, carried by the wind. It's just the points where they condense and evaporate that are fixed, creating a well-defined cloud that hangs motionless in the sky.
Seeing red in Silsden, Bradford. Photo by Daleswalker
Another fantastic stack of UFO clouds - this time above Heworth, North Yorkshire. Photo by Gary Gimmick
