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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 dust whirls and storms generated considerable dust charging through friction making electrical phenomena an integral part of the environment: Lightning discharges occurred within the clouds with bolts struck the surface. It was also a minor contributor to total atmospheric ozone.
Tiny grains (about sand sized) remained mostly in the lower levels of the Coriolis storms which caused severe erosion that in theory would erode all protrusions of the planet (if the planet were not so geologically active; see Geology) consisting it totally flat.
When the storms decay, much of the dust settled back onto the surface. Water vapor absorbed on the grains was carried downward also (a phenomenon locally called El-Sayal). Ionization by ultraviolet rays gradually released the water back to the atmosphere and return was complete within a few days.
Wind speeds as high as 800 km/hour were recorded within these storms by certified instruments (which also managed to survive the storms' fury).
Dorit Pachtra (10002 AG) reported a single measurement of wind speed in excess of 1000 kilometers per hour. This is generally discredited since his instrument was out-of-certification at the time. Such a speed would exceed the speed of sound (ground level) on Arrakis.
Supersonic winds have never been confirmed on any planetary surface. According to Th. Zed Ghralic "The residents of Arrakis reported fearfully loud noises during the greatest-storms. Could it be that the winds exceeded Mach -1 at times? The entire subject merits an attempt at laboratory duplication under controlled conditions."
L.L. Nefad (9156 AG) simplistically proposed that the Coriolis storms were but a manifestation of the accumulation of multiple vortices common to the planet associated with the turning of the wind vector by planetary rotation; Joon F. Hohshas (11301 AG) provided a better basis of understanding.
Coriolis forces did indeed play a major role, but the severity of the storm involves several other factors, which combined in harmony to produce the greatest of these storms.
An almost constant temperature difference (annual mean) of 27o C existed between the equator and the poles; this difference caused atmospheric flow between equator and poles. The cooler and denser air from the poles flowed along the surface, and was warmed quickly by surface radiation. In the northern hemisphere this flow was deflected westward, in the southern hemisphere eastward (Coriolis effect, planetary designation 3).
Planetary rotation is in the B class, meaning that the general circulation was broken up into eddies (cyclones and anticyclones).
On all Neta-type planets with a 3 classification the cyclones produce storms, violent on Arrakis, primarily because of excess ozone. Temperature was high at the surface, low with altitude, but then considerably increased around the ozone layer level. Convection and advection associated with the cyclones was thus normally confined within 2 km of the surface. The strength of the ozone heat barrier depended upon the ultraviolet flux from Canopus, which varies. This confinement greatly intensified storm severity.
The greatest Coriolis storms on Arrakis were produced differently: a truly great storm occurred only when the ozone heat barrier was broken something which Nefad almost recognized. The strength of the cyclones varied also, but most important were the convective vortices, most prevalent during a daytime frontal passage. The stronger the cyclone the more vortices. Each vortex carried surface heat upward to the ozone barrier. On those occasions when conditions were right the heat transported upward could produce temperatures immediately below the ozone layer which were greater than that within the layer itself. The barrier was destroyed when this occurred. The reaction was self-perpetuating and explosive in its impact, and a truly great dust/sand storm evolved as the atmosphere in a real sense was overturned.