A coriolis storm was any major sandstorm on Arrakis where the winds across the open deserts were amplified by the planet's own revolutionary motion. This caused them to reach speeds of up to 700 kilometers per hour.
The storms represent one of the principle dangers to settlements on Arrakis. The Fremen guard against them by building sietches deep into the rock, Arakeen is protected by a natural formation of sheer rock called the Shield Wall.
Coriolis storms are a common reference or feature throughout the setting. In Dune, Coriolis storms enter the story twice. The first storm is used by Paul to escape from Harkonnen Ornithopters, the second overloads the shields of forces and ships station in Arakeen during Paul's attack on the Emperor, following the destruction of the shield wall.
Considering that all cyclones, hurricanes, etc. on Earth are caused by the Coriolis effect, it can be accepted that the idea of "Coriolis storm" was created by Frank Herbert as a generic term to describe all tornado-like phenomena on Arrakis; amplified in violence by the presence of airborne sand.
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