Clouds may form in clear air and their characteristic forms can be broadly grouped in a classification in terms of "genera", "species" and "varieties", eventually accompanied by "supplementary features or accessory clouds". They may also form or grow from other clouds, called "mother-clouds". One of the most beautiful species is lenticularis which may form on genera Cirrocumulus, Altocumulus and Stratocumulus.  The definition of lenticularis: clouds having the shape of lenses or almonds, often very elongated and usually with well defined outlines, sometimes showing irisation. They are common sight in mountainous regions, but may appear in rare cases also over flat territory. 

The picture was taken in the early afternoon and depicts a rather large solitary Stratocumulus lenticularis which was stationary for hours near a mountain of about 2500 m height.  In a rare case these smooth features on Stratocumulus clouds are also visible over flat regions, but usually they travel fast according the upper winds. A photo of latter case, taken at Kampenhout on May 24 2005, can be found here.