Ripples in tapped or blown powder

We observe ripples forming on the surface of a granular powder in a contain er submitted from below to a series of brief and distinct shocks. After a f ew taps, the pattern turns out to bt stable against any further shock of th e same amplitude. We find that the wavelength of the pattern is proportiona l to the amplitude of the shocks. Starting from considerations involving ai l flow through the porous granulate and avalanche properties, we build up a semiquantitative model which satisfactorily fits the set of experimental o bservations of either tapped or blown powder.