Annlee Landman might have been forgiven for fearing an imminent alien invasion of Boulder, Colorado, US as this ominous dark red form loomed overhead as the evening approached.

Altocumulus lenticularis form in stable, moist airflows passing over mountainous terrain, like the Front Range of the Rocky Mountains here to the west of Boulder. When standing waves develop as the air rises and dips in the lee of the range, clouds can form at the invisible crests of air where it cools enough to condense into smooth, curvilinear forms that often look like flying saucers. Annlee’s cloud was so extensive it almost filled the sky, catching the last light of the Sun as it set behind the mountains.

Annlee knew right away who to send her extended Altocumulus lenticularis formation to: her cloud-loving friend Linda Seward (Member 48,760), who shared it with us. So it turned out to be a cloud encounter of the nerd kind.