C_M4 Ac continually changing shape

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Gallery
© R.K. Pilsbury	© R.K. Pilsbury
© R.K. Pilsbury	© P.H. Jeffries
© Mr Sharrocks	© D. Philips

The irregularly arranged elements of altocumulus of specification C_M=4 are continuously changing in shape. They often appear to be dissolving in some places and forming in others. This can be seen by comparing the top two photographs which were taken within five minutes of each other. These clouds are usually thin and do not progressively invade the sky. They often resemble a net or honeycomb.

The altocumulus of C_M4 often forms in patches in the shape of almonds or lenses and is then called altocumulus lenticularis. These formations are caused by wave motions in the atmosphere and are frequently seen in mountainous or hilly areas. They are often called wave clouds. They may be triggered off by hills only a few hundred metres high and may extend down wind for over 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. At sea, they are likely to be seen only towards land. They may appear well distributed over the sky (middle left - stratocumulus also present), or as a single element (middle right). At bottom left the elements appear one on top of the other resembling a pile of plates. At bottom right the elements are parallel to each other and occur at two levels. Cirrostratus and ragged looking stratocumulus also appear in this photograph.

Classification
Description: Patches of generally semitransparent Altocumulus, often in the shape of almonds or fish, at one or more levels, the elements continually changing in appearance.	Designation: CM=4 (C3)
	Latin name: Altocumulus lenticularis (Ac len) or Altocumulus translucidus (Ac tr).

Related pages: Cloud classification | C_L guide | C_M guide | C_H guide | Cloud index