In Cirrocumulus undulatus the heaps or patches of cloud may be arranged in rows or billows. The clouds may have sharp or poorly defined edges, but always show clear parallel gaps between them. The individuel cloudlets may be circular or elongated parallel to the rows. Billows and gaps can be very narrow, but also widely spaced. They form at the boundary of two different air-masses under the influence of wind shear.
If they appear from the westerm portion of the sky, they are indicating a weather deterioration within the coming 15 to 36 hours.