DEW: FACTS AND FALLACIES 



J. L. MONTEITH 

 Rothamsted Experimental Station, Harpenden, Herts. 



I. INTRODUCTION 



When plant leaves cool at night below the dew-point of the surrounding 

 air they become covered with moisture, either in a surface film, or more 

 spectacularly in visible droplets of dew.* 'Every poet who has sung the 

 beauties of Nature has added his tribute to the sparkling dew drop, and 

 Ballantine in his widely known song has drawn a comforting lesson from 

 the thought that 'ilka blade o' grass keps its ain drap o' dew' (Aitken, 1885). 

 Lacking a sound physical basis, many ecological investigations of dew have 

 strayed into descriptions which belong to poetry rather than to science, and 

 in an extensive hterature, fact and fallacy are often confused. Their distinc- 

 tion is attempted here by appeal to physical theories for the dependence of 

 condensation on the properties of the atmosphere and of the soil. 



2. CONDENSATION RATE AND WEATHER 



Fluxes of water vapour at the earth's surface are governed by radiation, 

 temperature, humidity and wind speed ; and from analysis of the surface 

 heat budget, aU four variables may be combined to give a formula such as 

 Penman's (1948) for evaporation and Hofmann's (1955) for dew. The 

 Penman and Hofmann equations are formally similar, and the derivation 

 which follows contains elements from both. 



Condensation on an isolated horizontal surface such as a leaf or a dew 

 gauge cannot begin until the surface is colder than the ambient air. If the 

 condensation rate is W (g cm ~ ^hr "i) the release of latent heat A W (cal cm "^ 

 hr~^) will be accompanied by a transfer of sensible heat C from relatively 

 warm air to cooler surface. With a surface at temperature Tj (°C) surrounded 

 by air at temperature Ta, this transfer rate can be written 



C=h{TA-Ts) (i) 



* Dewdrops have often been confused with guttation, water exuded from within 

 the plant through leaf hydathodes. Distinguishing features were described by Long 

 (1958) with photographic illustration. 



