A cloud species the elements of which have the form of more or less isolated, generally smooth lenses or almonds; the outlines are sharp and sometimes show irisation. These clouds appear most often in formations of orographic origin, the result of lee waves, in which cases they remain nearly stationary with respect to the terrain (standing cloud), but they also occur in regions without marked orography. This species is found mainly in the genera cirrocumulus, altocumulus, and (rarely) stratocumulus. Altocumulus lenticularis differs from cirrocumulus lenticularis in that, when smooth and without elements, it has shadowed parts while the latter is very white throughout. When undulated or subdivided, the altocumulus species differs from stratocumulus lenticularis in that its elements subtend an angle of less than 5° when viewed at an angle of more than 30° above the horizon. See cloud classification.