Shelf or Arcus clouds appear near a thunderstorm and can warn of their proximity.
Sometimes a cloud rides ahead and heralds the imminent arrival of a storm cloud. It is called a shelf or arcus cloud because it may appear as a flat shelf or wedge spreading out from the base of the cumulonimbus.
The shelf cloud forms when downdrafts spread after they hit the ground, then lift the warmer surrounding air. If you turn on the kitchen tap you will see the water flowing downward, then spreading horizontally over the flat surface of the sink. The main storm turns on the wind tap, and the shelf cloud is carried out by the spreading winds at ground level. Unfortunately, it doesn’t give you much warning, as it is attached to the main cumulonimbus.
If a shelf cloud passes over you, the chances are that you will be more focused on the gusting wind than on the clouds above.
Text from ‘Storms’ chapter of the book, The Secret World of Weather.
With thanks to Liam McAlpine for all photos and video and permission to share.
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