44 J.L.MONTEITH 



Empirical formulae for the dependence of gauge condensation on 

 physical properties and exposure were presented by Suzuki (1944)- 



4. DEW ON NON-HORIZONTAL SURFACES 



By definition, potential condensation on a surface is independent of 

 aerodynamic properties and is proportional to radiative loss (eq. 4^). When 

 the surface is horizontal this loss can be calculated from a simple empirical 

 formula (eq. 6) and, in practice, the same formula is vahd for a crop with 

 horizontal dimensions much greater than the vertical irregularities of the 

 canopy. The crop may then be treated as an infinite surface with total 

 emission and absorption independent of surface contours. Isolated plants 

 and trees have surfaces which are not horizontal and their radiative loss is 

 found by integrating R{9) cos {e-ijj)^e AS where R{e) is the net loss of 

 radiation from a horizontal surface at zenith angle 6, and d5 is an element 

 of surface with zenith angle ifj. If this integral exceeds the radiative loss 

 from a horizontal surface with the same projected area by a factor p,(>i) 

 potential condensation per unit projected area will be 



W=pAR^l'\{A^-y) (9) 



where R^ is evaluated for a horizontal surface (jp=i)- Theoretical values 

 of p will be given for a cylinder, cone, and hemisphere. 



(i) Vertical cylinder. 



According to Linkc (quoted by Hofmann), R{e) is proportional to 

 cos"0 and n:=o-4 at e^ = 9 mb. For a vertical cylinder with height x and 

 diameter d, evaluation of the integral gives 



p=i+i-6xjd (10) 



The approximation that R is independent of d{n = 0) gives 



p=i + 2xld (11) 



The data in Table 4 arc from Hofmann's review of observations by 

 Hiltner. Agreement is less close for other species. 



(2) Cone with base on ground 



A simple analytical solution for non-integral values of n is unobtainable. 



With » = o, base diameter d and slant height / 



;, = 0-5 + lid (12) 



and comparison of eqs. 10 and 1 1 shows that this expression will over- 

 estimate potential condensation. 



