42 J.L.MONTEITH 



Still higher values came from the lysimeters at Coshocton, Ohio (Harrold 

 aiid Dreibelbis, 195 1). From 1944 to 1949, annual condensation recorded on 

 various crops was 230 to 270 mm, and daily records showed that the 

 maximum condensation rate was about 0-5 mm/hr (in the early evening) 

 exceeding the theoretical maximum by an order of magnitude. These 

 figures reveal a curious failure in lysimeter performance and it is surprising 

 to fmd them seriously quoted as estimates of dew. 



3. DEW GAUGE MEASUREMENTS 



Many workers have attempted to 'measure' dew from the increase in 

 weight of a 'dew gauge', often a plate of metal, glass or wood of arbitrary 

 dimensions exposed during the night at an arbitrary height above the 

 ground. Such measurements are open to dangerous misinterpretation, but 

 useful observations are possible when the design and siting of a gauge are 

 guided by physical principles rather than by whim or expediency. Similar 

 condensation will form on a dew gauge and on adjacent (horizontal) leaves 

 provided their thermal and aerodynamic properties are similar, and the 

 following conditions should therefore be satisfied : 



(a) the gauge should be a thin plate so that the area of vertical faces is 

 much less than the area of horizontal faces. Equations 5, 7^ and jb 

 are then valid ; 



(b) emissivity should be close to unity. Pohshed metal surfaces are 

 unsuitable except when their low emissivity is exploited dehberately 

 (as in Hofmann's instrument) ; 



(c) the heat capacity per unit surface area should be small enough to 

 make M<^R over the shortest period of observation; 



(d) the gauge should have approximately the same size and shape as 

 adjacent leaves. 



In saturated air, condition (d) can be relaxed because potential condensation 

 is independent of wind speed and surface shape. Theoretically, potential 

 condensation on a gauge satisfying conditions (a), (b) and (c) will be the 

 same as potential condensation on a crop (per unit ground area) if the gauge 

 is exposed close to the crop canopy but is unshaded by surrounding leaves. 

 The gauge designed by Duvdevani (1947) was designed primarily for 

 optical estimates of dew from the pattern of condensation, but it has also 

 been used gravimetrically. (The optical method can be used by relatively 

 unskilled observers with httle auxiUiary equipment, but has the serious 

 disadvantage that the surface properties determining the dew pattern 

 change after a few months' exposure.) This gauge is a block of painted wood 



