A creepy shelf cloud loomed over a car park in Arlington Heights as a storm approached the Chicago area.

Dramatic footage captured by Kelly Cassady shows the dark formation rolling southeast on October 6.

Kelly said: 'Gorgeous little thunderstorm passing by the office in Arlington Heights this afternoon.'

Shelf clouds are large, horizontal cloud formations that develop along the leading edge of a storm. They occur when cold, dense air from the storm pushes outwards and undercuts warm air, forcing it to rise and condense into a wide arc of cloud.

They differ from wall clouds, which are smaller, more compact, and form when the base of a supercell or rotating storm lowers closer to the ground.