Abstract
The relative contributions of dewfall (a flux of water vapour from air to surface) and distillation (a flux of water vapour from soil to canopy) to dew formation on closed canopy and bare soil surfaces are assessed, and the dependence of dew amount upon wind speed, absolute temperature, atmospheric stability, relative humidity, soil characteristics and cloudiness, all of which are significant factors, is evaluated. Some of these evaluations provide refinements to similar ones given in Monteith (1961). High dewfall rates are usually ≲0.06 mm hr−1 over canopy or bare soil, though upon a canopy under soil-saturated and air-saturated conditions, rates of dew formation may reach 0.07–0.09 mm hr−1 with contributions from distillation. Various sets of observations are reanalyzed to illustrate the importance of the horizontal advection of moisture in the nocturnal boundary layer (NBL) to observed high rates of dew formation arising from the atmospheric contribution of water vapour (dewfall). These locally observed high dewfall rates must be the result of small-scale or mesoscale horizontal advection of moisture in the NBL, since the humidity changes within the typically shallow NBL required to balance the loss of water at the surface are not observed. Over extensive areas of uniform surface (horizontal scales ≫10 km), such continuously high dewfall rates could only be balanced by a local supply of atmospheric moisture since advection of moisture would necessarily be small.
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Garratt, J.R., Segal, M. On the contribution of atmospheric moisture to dew formation. Boundary-Layer Meteorol 45, 209–236 (1988). https://doi.org/10.1007/BF01066671
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DOI: https://doi.org/10.1007/BF01066671