International audienceThis study reports on one year (May 1, 2007 - April 30, 2008) of dew, fog and rain measurements carried out in the dryland area of Mirleft, Morocco in order to be used as alternative or supplemental sources of water. Four standard dew condensers and a passive fog net collector of 1 m² surfaces were used. Meteorological data were collected. 178 dew events (18.85 mm), 31 rain events (48.65 mm) and 7 significant fog episodes (1.41 mm) occurred, corresponding to almost 40% of the yearly rain contribution (48.65 mm, 31 events). Chemical and biological analyses were carried out. Dew and rain pH were neutral (close to 7) and the total mineralization was considerable (dew: 560 mg/L; rain: 230 mg/L). Ca²+, K+, SO42- and NO3- are found of continental origin; Cl-, Na+ and Mg2+ are of sea origin. The ions concentration agrees with the World Health Organization recommendations for potable water. The biological analysis shows harmless vegetal spores and little contamination by animal/human bacteria. A cost analysis shows that, with little investment, the population of the arid and semi-arid coastal areas of south-western north Africa could make dew water an interesting supplementary alternative water resource

The mechanisms of heat and mass transport in a side-heated square cavity filled with a near-critical fluid are explored, with special emphasis on the interplay between buoyancy-driven convection and the Piston Effect. The Navier–Stokes equations for a near-critical van der Waals gas are solved numerically by means of an acoustically filtered, finite-volume method. The results have revealed some striking behaviour compared with that obtained for normally compressible gases: (i) heat equilibration is still achieved rapidly, as under zero-g conditions, by the Piston Effect before convection has time to enhance heat transport; (ii) mass equilibration is achieved on a much longer time scale by quasi-isothermal buoyant convection; (iii) due to the very high compressibility, a stagnation-point effect similar to that encountered in high-speed flows provokes an overheating of the upper wall; and (iv) a significant difference to the convective single-roll pattern generated under the same conditions in normal CO2 is found, in the form of a double-roll convective structure.

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