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From Glossary of Meteorology
(Often called roll or shelf cloud.) A dense and horizontal roll cloud or wedge-shaped accessory cloud, sometimes appearing ragged and turbulent and other times smooth, occasionally with multiple layers.
It is seen most often in association with cumulonimbus clouds on the leading edge of a gust front. When extensive and approaching, it gives the appearance of a menacing arch. It may also occur with fronts in the absence of a cumulonimbus or in the absence of any thermal boundary.
See morning glory.
