DYNAMIC METEOROLOGY. 



409 



(Y) The disturbing force. — The disturbiug force is recognized as the 

 solar beat, either directly absorbed by the air or evolved by the cou- 

 densation of aqueous vapor in the atmosphere; the former has already 

 been considered and the latter must uow be studied. This involves 

 (1) the amount of aqueous vapor and (2; its condensation. 



(1) Amount of vapor. — The normal distribution of vapor in the at- 

 mosphere at sealevel is suggested by the hygrometric table (xvii), 

 which m its three sections, a, h, and c, gives the monthly normal values 

 of the mean dew-point, the mean weight, and tension of vapor for a few 

 stations in Xorth America. 



Knowing the temperature of the dew-point, one can determine the 

 weight of the vapor contained in any volume of air, and also the ten- 

 sion of the vapor by the use of the accompanying table, xviii : 



Table XVIII. — Tension and weight of aqueous vapor. 



The geographical distribution of moisture at the earth's surface is 

 best shown by charts of lines of equal tension or dew-point. A com- 

 parison of such hygrometric charts for each of the daily reports shows 

 the presence of regions where moisture is in excess or is deflcient, and 

 where a given cooling will produce precipitation. 



The normal distribution of moisture in successive strata of the at- 

 mosphere is shown approximately in the second column of the follow- 

 ing table (XIX), computed by the formula of Hann, but which is based 

 on observations in continental areas, and may not so closely represent 

 the couditious over the occaus : 



