Lenticularis is a species of cloud that can be found at low, medium and high levels, although the most striking and dramatic examples tend to be the mid-level Altocumulus lenticularis, like the one shown here. These clouds often have the appearance of UFOs, and are caused by a moist airstream flowing over raised ground, such as a hill or mountain peak. When the airstream consists of alternating moister and drier layers of air, it can lead to this specific form of lenticularis, which is known as pile d’assiettes (French for ‘a stack of plates’).