40 J.L.MONTEITH 



Case 2 : leaf forming part of crop canopy with underlying leaves at 

 temperature Ta 



R= 2aTs^-uTA^-Ld 

 ^{i-<f>)aTA'-SaTA^{TA-Ts) {jb) 



Writing formally 



Rn=R+uf{TA- Ts): f= 4ctT^3 



eq. 4 becomes 



A + Y{i + iiflh) 



CONDENSATION RATE 

 mm/hr (T= 15 'O 



90 85 



RELATIVE HUMIDITY 1%) 



SO 



Fig. I. Condensation on horizontal leaf for given wind speed and relative humidity, 



calculated from eq. 4. 



The difference in W calculated from eqs. 4 and 8 is approximately 0-002 

 n/F mm/hr for Ta between 10 and 20°C and F between 0-5 and 4 m/sec. 

 With these two sources of error, 'potential' condensation determined from 

 the simple radiation balance of eq. 5 may exceed real condensation from a 

 saturated air-stream by 10 to 20%. 



Anticipating discussion in section 4, eq. 4 gives potential condensation 

 on a closed crop canopy per unit area of underlying ground and can be used to 

 distinguish field measurements which are physically reasonable from those 

 which violate the conservation of energy. Monteith (1956) found that the 

 rate of vapour transfer from the atmosphere to a short grass surface 



