Mean green Texas shelf cloud thunderstorm.
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The terms Arcus and Shelf (clouds) can be used interchangeably. Arcus clouds most frequently form along the leading edge or "gust fronts" of thunderstorm outflow; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. Order print above and watch movie below.