E. J. Russell and E. H. Richards 329 



clouds in summer may account for the smaller oxygen content of rain, 

 as condensation would occur at pressures much below normal. The 

 velocity of the rain drops falling from greater heights may not allow time 

 for equilibrium to be established with conditions at ground level. 



The amounts of dissolved oxygen found were 9-0 parts per million in 

 the summer, corresponding to 20-8 lb. per acre during the period May to 

 August inclusive, and 11 -2 parts per million in winter, corresponding 

 to 26 lb. per acre during the four months November to February. Over 

 the whole year the amount brought down was 66-4 lb. per acre. The 

 results are shown in the table on p. 328. 



The chemical characterisation of rainwater. 



The above discussion shows that the rain falling at Rothamsted 

 in winter differs chemically from that falling in summer: the values 

 being 



Parts per million Lbs. per acre 



Four-month periods Fom'-month periods 



Winter Summer Winter Summer 



(Nov.-Feb.) (May-Aug.) (Nov.-Feb.) (May-Aug.) 



Ammoniacal nitrogen ... 0-35 0-45 0-78 100 



Nitric nitrogen 0-18 0-21 0-40 0-47 



Chlorine 3-38 1-38 7-50 3-08 



Dissolved oxygen ... 11-2 90 26-0 20-8* 



* In the whole year it is estimated that (30-4 D). of dissolved oxygen is brought down. 



The winter rainfall is richer in chlorine and oxj^gen but poorer in 

 ammoniacal and nitric nitrogen than the summer rainfall. 



The marked differences in the amounts of chlorine and ammonia 

 suggest that winter rain may differ in origin from summer rain. The 

 winter rain resembles Atlantic rain in its high chlorine and low ammonia 

 content, suggesting that it is derived from the Atlantic. The summer 

 rain, on the other hand, is characterised by a lower content of chlorine 

 and higher proportion of ammonia which suggests that it arises from 

 the soil by evaporation of water and condensation at higher altitudes 

 than in the case of winter rain; this would also account for the difference 

 in amount of dissolved oxygen. 



SUMMARY. 



The ammoniacal nitrogen in the Rothamsted rainwater amounts 

 on an average to 0-405 part per million, corresponding to 2-64 lb. per 

 acre per annum. The yearly fluctuations in lbs. per acre follow the 

 rainfall fairly closely. The monthly fluctuations also move in the same 

 direction as the rain, but the general level is highest during May, June, 



