Winds and Atmospheres - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 31

Fig. 3.9 Weibull fits to the average windspeed distributions at two
desert sites on Earth. At left is the differential frequency (i.e., number of
occurrences in a range of wind speed, here 1 m/s wide) which shows
clearest the most common speed. Note the influence of the Weibull shape
parameter k—a higher value gives a more 'normal' distribution with a

distinct and symmetric peak, while a lower value gives a more skewed
fall-off. Often more useful for aeolian studies than the differential
frequency is the cumulative frequency (the fraction of time that wind is
above a given windspeed). Note that these hourly average winds are
considerably lower than the peak gusts indicated in the figure above

Fig. 3.10 A wind rose diagram for the Bruneau dunes, generated
by the Western Regional Climate Center website for two
different seasons. It shows that strong winds (6-10 m/s) blow

consistently from the SE in winter, but the spring wind regime is
rather more bidirectional. See also Chap. 8 . Image generated by
J. Zimbelman

feature of the inertia of the atmosphere and the planetary
size and rotation, and develop without specific forcing. A
casual inspection of midlatitude weather records on Earth
will typically show a *7-day periodicity in pressure and
wind. Other planets have different characteristic periods.
Similarly, coherent weather structures can form (notably
dust storms on Mars, and hurricanes and related systems on
Earth).

Predicting the winds on other planets, then, is not a trivial
business. Given some broad parameters like the rotation rate
of a planet, the density of its atmosphere and so on, one might
predict the typical amplitude of surface winds (although
efforts to do so at Titan met with only limited success). But
dune formation is concerned with the winds at a given
location, and quite possibly just the exceptional winds at that
location,

and thus prediction requires sophisticated