50 J.L.MONTEITH 



night ; and the potential dewfall rate is never reached. In practice, the dew- 

 fall/distillation ratio does not increase systematically with increasing wind 

 speed but reaches a maximum at some optimum value, about 2 m/sec at 

 3 m over short grass. Gravimetric measurements of dew in south-east 

 England and in Victoria, Austraha (Mcllroy, private communication) show 

 that distillation is normally the dominant component of dew on agricultural 

 crops. 



Some workers expose dew gauges at different heights above the ground 

 and infer the source of dew from its variation with height. This interpreta- 

 tion is invahd. The so-called 'dew gradient' depends on the vertical 

 distribution of temperature, humidity, and wind speed, and is not uniquely 

 related to the direction of vapour transfer. 



The distinction between dewfall and distillation is relevant to studies of 

 plant water balance. Dewfall on plant leaves is a real accretion of water in 

 rainless weather, it is the only significant mechanism for transferring 

 moisture from the atmosphere to the soil/plant system. Distillation is a 

 redistribution of water within this sytem. Transpiration losses arc neghgible 

 v/hile dew is evaporating from plants, but the evaporation rate from wet 

 leaves is normally somewhat greater than the corresponding rate of 

 potential transpiration from dry leaves. Dewfall decreases the extraction of 

 soil moisture by plant roots, whereas distillation increases soil moisture 

 depletion by providing a short-cut for the transfer of water from the soil 

 to the atmosphere. 



6. ABSORPTIONOF WATER VAPOUR 



The uptake of water vapour by plants and by dry soil without the formation 

 of visible water has been called 'liidden dew' and 'occult condensation', 

 names which obscure an important thermodynamic difference between 

 condensation on a plane surface and absorption by a hygroscopic medium. 

 When water vapour condenses, latent heat is released and the condensation 

 rate is Umited by the rate at which this heat can be removed to prevent an 

 increase of surface temperature. When water vapour is absorbed both latent 

 heat and 'free energy' are released but the direction and rate of vapour 

 transfer are determined by the potential gradient of free energy rather than 

 by the availabihty of a heat sink. Absorption may be accompanied by a 

 shght rise in temperature from the 'heat of wetting'. The fundamental 

 unit for free energy is ergs/g but it is more convenient to use the equivalent 

 gravitational potential H expressed in metres. For water in the atmosphere, 

 plant, or soil H<o. 



