M4 Clouds: Altocumulus (Ac) continually changing shape
Notes: These clouds often appear to be dissolving in some places and forming in others. These clouds are usually thin and do not progressively invade the sky. They often resemble a net or honeycomb.
They also often form in patches in the shape of almonds or lenses called Altocumulus Lenticularis (ACSL) or wave clouds. These formations are caused by wave motions in the atmosphere and are frequently seen in mountainous or hilly areas. They may be triggered off by hills only a few thousand feet high and may extend down wind for over 60 miles (100 km).
The cloud elements form at the windward edge of the cloud and are carried to the downwind edge where they evaporate. The cloud as a whole is usually stationary or slow moving. These clouds often have very smooth outlines and show definite shading.
