Dew in a desert

The topic of dewfall in deserts was reviewed by Linacre (1). In the Negev desert of Israel (31ºN), dew falls on about 200 days each year in the most suitable locations (2). Detailed dewfall measurements were taken there in September 1997 to study the microclimatic effect of sand dunes (3). The daily variations of air temperature at 2.5 m above the ground, the net radiation and the flow of heat at the ground surface all were in accordance (Fig 1). Typically dewfall was twice as large and 50% more frequent in the hollow between dunes, compared to the dune tops. The dewfall was slightly less on the slope facing north than on the opposite slope, but so was the subsequent rate of evaporation, which may explain the thicker salt crust on north-facing slopes. All the dew evaporated away between 6-10 am, at a rate which was proportional to the difference between the respective vapour pressures of air and ground surface. The diffusion resistance of this flux was of the order of 100 s/m, which is a high value.

Fig 1. Daily variations of air temperature T_a at 2.5 m above the ground, the net radiation Q* at the ground, and the conduction of heat into the ground G, on September 28-29, 1997.

(1) Linacre, E.T. 1992. Climate Data and Resources. (Routledge, London) 366pp.

(2) Zangvill, A. 1996. Six years of dew observations in the Negev Desert, Israel. J. Arid Environments, 32, 361-71.

(3) Jacobs, A.F.G., B.G. Heusinkveld and S. Berkowicz 1998. Dew deposition and early morning evaporation in a desert environment. 23rd Conf. on Agricultural and Forest Meteorology, Albuquerque, New Mexico (Amer. Meteor. Soc.), preprints, 349-52.