Angle and aspect dependent dew and fog precipitation in the Negev desert

Knowledge regarding dew and fog precipitation in relation to angle and aspect may be of great importance to plant photosynthesis and to rock dwelling microorganisms, especially in arid land where water is highly limited. Measurements of dew and fog were carried out periodically during the fall of two years (1989, 1992) at a hilltop in the Negev Desert Highlands, Israel. Measurements were carried out using velvet-like cloths (6×6×0.15 cm) attached to the center of different angle sides of 5 pairs of wooden boxes (50×50×10 cm) and to the top of an additional pair (10×10×5 cm) that served as a control (horizontal surface). The sides of each pair of boxes had angles of 30, 45, 60, 75 and 90° facing north, east, south and west. Whereas fog showed an advective pattern of precipitation, with windward aspects receiving higher amounts, that was not the case with dew. When the dew values received in the different aspects within each angle were grouped together, surfaces at 30, 45, 60, 75 and 90° received 99.8 (SE±11.9), 78.2 (SE±17.9), 42.0 (SE±18.3), 33.8 (SE±10.8), and 25.6 (SE±9.7) percent of the amounts condensed on the horizontal surface, respectively. A positive linear relationship characterized the dew amounts with cos (θ). The data also showed an increase in dew duration with west>north>south>east. The data thus imply that aspect and angle may play an important role in controlling dew and fog availability.