Images of the limb of Mars reveal discrete cloud layers between 20 and 80 km above the surface. They appear to be composed of water ice and have a number of characteristics similar to hazes that produce ''diamond dust'' precipitation in the continental Antarctic of Earth. Temperatures from 170 K to 190 K are deduced at the condensation levels. Eddy diffusion coefficients around 103 cm2 s-1, typical of a nonconvecting atmosphere, are also derived in the haze regions at times when the atmosphere is relatively clear of dust. This parameter apparently changes by more than 3 orders of magnitude with season and local conditions, with important implications for vertical transport of water and dust and for models of photochemistry and middle atmosphere dynamics. For the cases studied, particle sizes vary systematically by more than an order of magnitude with condensation level, in such a way that the characteristic fall time for particles is always about 1 Mars day, which is the dominant thermal forcing time. The hazes may play a key role in the seasonal water cycle on Mars. They provide a mechanism for growing particles large enough to move atmospheric water closer to the surface, thereby improving the efficiency of adsorption and ice deposit formation in the regolith. This is particularly likely in late northern summer, when the rapid hemispheric decrease in atmospheric water vapor may reflect the precipitation of snow. This rapid decrease in late summer involves atmospheric water vapor in about the quantities needed to supply the mid-latitude regolith with the water that appears in the atmosphere early in the following spring. ¿American Geophysical Union 1990