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On most habitable planets storms are preceded by a large, readily noticeable decrease in wind speed. Arrakian storms were preceded by an extremely small decrease in integrated wind speed. The stronger the storm, the bigger the interval between the decrease and the storm: one day before in the case of minor storms, or 2 days for a major storm, or 5 days for the greatest Coriolis storm/DE.
all weather phenomena of the planet (at least in its inhabited regions) were associated with dust storms.
The method involved a line of poles (or double line), without any sophisticated monitoring equipment, just coated with amborite, the most easily abradable material the Fremen could find. These were emplaced on the lower slopes of garres and ridges. Each line ran along the direction of a slope.
The slope had to comform to several specifications: it had to face a large open area as well as the direction from which storm winds blew; it had to be devoid of large boulders (which could produce eddies); it required an angle of 15 ± 5 degrees; and had to be backed by a steep walled topographic feature rising at least 500 meters above the surroundings.
The upslope winds, which always carried dust, when impacting the topographic feature, were deflected upward suddenly and also crested a wind shadow zone on the upwind side of the obstacle. In that specific geometry, the winds within the shadow zone are slightly less with the degree of difference increasing as wind speed increased.
According to the above, the poles within the shadow zone were slightly less exposed to wind action than those down the slope. The coating one the poles would be slowly worn away by even the lightest of dust winds.
The mass coating loss was measured each day giving the integrated product of wind speed and dust content. By taking the ratio of this product for the down-slope poles to the shadow-zone poles, one obtains a wind-speed ratio (in that case directly proportional to the wind speed), which was all that was needed for weather prediction on Arrakis.
In theory, wind speed could be calculated from the wind-speed ratio and knowledge of the surface geometry, but this was not needed and there is no record that the Fremen did so (if they had the mathematical knowledge).
The square of the magnitude of the decrease was proportional to the size of the storm, and of course, the day when it would occur.
The accuracy was greater than could be determined with instruments in existence at the time and was better than some of the instruments of 5000 years later. It is still a source of wonder to many scientists how the Fremen were able to discern the relationships between the speed decrease and the size of the storm.
References and notesEdit
- ↑ Arrakis - Weather prediction