Viking orbiter images of Mars show several instances of long continuous cloud formations on the slopes of Arsia and Pavonis Mons. We have searched all the images of the planet for occurrences of such formations. Only in the Tharsis region were long clouds unambiguously identified. We have measured the times and locations of occurrence, the wavelengths and when possible, the apparent velocity of motion of these clouds. We have also tabulated the wavelengths of patches of ripple clouds that are often found with the long formations. The long clouds are observed only in the early morning hours, suggesting that they are associated with drainage winds due to a cold planteary boundary layer. We develop simple mathematical models to examine various aspects of such boundary layer winds; these allow us to construct a complete and self-consistent explanation of all the observed features of the cloud formations. We use the results to characterize some physical properties of the Mars boundary layer. There is strong downslope flow in the boundary layer on the high slopes of both volcanos. In the saddle region between the peaks the flow slows and undergoes a hydraulic jump, producing the long clouds. Downstream of the jump, standing internal gravity wave can exist and are excited by flow over surface irregularities. Small variations in their observed wavelengths behind the jump, may be attributed to variation in the flow speed and depth. Finally, we can account for the location of the jumps by the variation of relative strength of the boundary layer flows on the two volcanos.