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Photosynthetic activity of soil crust lichens was thoroughly investigated. Its interrelations with microclimatic boundary conditions was measured during two field experiments in the central part of the sand dune field in the north-western Negev Desert. After nocturnal rainfall the lichens were active well until noon when they dried out finally. However, over most of the year dewfall seems to be the primary controlling factor for activation as in other lichen communities. The microclimatic conditions for activity were determined in detail. It was found that after sunset terrestrial radiation leads to a progressive development of a stable air layer above ground accompanied by decreasing temperatures and wind speed. Well before midnight dewpoint temperature differences drop below 1.0 K and leaf wetness sensors indicate the formation of dew. It is exactly in this situation when lichen activity starts. Maximum activity, however, is reached a few hours later when cumulative dewfall exceeds 0.1 mm at dewpoint temperature differences around 0 K. In nights with advective labilization and subsequent dewfall evaporation, no lichen activity was observed. Even a heavy foggy night did not lead to any activity at the soil surface.