April, 1903.] Some Climatic Conditions of Ohio. 



405 



FIRST AND LAST KILLING FROST. 



vST.^TIONS. 



Akron 



Ashland 



Ashtabula 



Bowling Green 



Canton 



Cincinnati. 



Clarksville 



Cleveland .... 



Columbus 



Dayton 



Defiance 



Elyria 



Findlay 



Granville 



Station.s. 



Greenville 



Hillhouse 



Marietta 



Milligan 



Montpelier 



New Alexandria 



Norwalk 



Pomeroy 



Portsmouth 



Sandusky 



Tiffin 



Toledo 



Van Wert 



Wooster 



" be 



>..5 a 

 ca > o 



. 2 rt 



00^ 



167 



131 

 1 85 

 144 

 151 

 165 

 146 

 169 

 178 

 201 



143 

 178 



158 

 147 



RKLATIVE HUMIDITY. 

 Plate 12. Maps X, XI and XII. 



By the term relative humidity, is meant the relation (ratio ) of 

 the amount of moisture actually present in the air — at anj^ given 

 conditions of temperature and pressure— to the amount of moist- 

 ure necessary to produce saturation under the given conditions. 

 This ratio is usually expressed as a percentage. This is not to 

 be confused with the absolute humidity, which is simplj^ a state- 

 ment of the amount of water present without reference to the 

 saturation point, and is usuallj^ expressed as so much weight or 

 volume per unit of air. 



The observations for relative humiditj^ are taken at a few scat- 

 tered stations only as compared to many other phases of the 

 weather, and so the stations are farther apart than desirable for 

 close work. The daily averages are derived from observations 

 taken at 8 a. m. and 8 p. m. seventy-fifth meridian time, and 

 inasmuch as the relative humidity depends to a large extent upon 

 the temperature of the air, it is evident that the relativ^e humiditv 

 during the warmer hours of midday may be an altogether differ- 

 ent thing than the averages given would indicate. The averages 

 as derived from Ohio conditions probably fall short of giving the 

 actual contrast between the Lake Erie stations and those in more 

 central or southern portions of the State, so that it perhaps 

 devolves upon us to make the most possible of any differences 

 apparent. 



Just to what extent we must reckon with relative humidity as 

 an ecological factor is pretty difficult to determine. In the case 

 of plants it must be of some ecological importance even in winter. 

 If the soil about the roots of some of the trees exposing much 

 surface to the air — as the evergreens — be frozen or so cold that 



