April, 1903.] Some Climatic Conditions of Ohio. 
405 
FIRST AND LAST KILLING FROST. 
Stations. 
Last 
Killing 
Frost in 
Spring. 
First 
Killing 
Frost in 
Fall. 
No. Days in 
Growing 
Season. 
Stations. 
Last 
Killing 
Frost in 
Spring. 
First 
Killing 
Frost in 
Fall. 
No. Days in 
Growing 
Season. 
Akron 
Ashland 
Ashtabula 
Apr. 23 
Apr. 27 
Apr. 23 
Oct. 7 
Oct. 9 
Oct. 10 
167 
165 
17U 
Greenville 
Hillliouse 
Marietta 
Apr. 24 
May 18 
Apr. 16 
Oct. 8 
Sept. 26 
Oct. 19 
167 
13 1 
186 
Bowling Green .... 
Canton 
May 7 
Apr. 25 
Sept. 25 
Sept. 30 
141 
vs 
Milligan 
Montpelier 
May 5 
Apr. 28 
Sept. 25 
Sept. 25 
144 
151 
Cincinnati 
Clarksville 
Cleveland 
Columbus 
Apr. 14 
Apr. 20 
Apr. 21 
Apr. 18 
Oct. iS 
Oct. 2 
Oct. 21 
Sept. 21 
187 
165 
1S3 
156 
New Alexandria 
Norwalk 
Pomeroy 
Portsmouth 
Apr. 27 
May 9 
Apr. 19 
Apr, 16 
Oct. 9 
Oct. 2 
Oct. 5 
Oct. 11 
165 
146 
I69 
178 
Dayton 
Defiance 
Elyria 
Apr. 21 
May 5 
Apr. 24 
Oct. 8 
Sept. 23 
Oct. 8 
17° 
141 
167 
Sandusky 
Tiffin 
Toledo 
Apr. 11 
Apr. 28 
Apr. 18 
Oct. 29 
Sept. 18 
Oct. 13 
201 
143 
178 
Findlav 
Granville 
Apr. 30 
Apr. 21 
Sept. 30 
Oct. 1 
153 
163 
Van Wert 
Wooster 
Apr. 27 
May 5 
Oct. 2 
Sept. 29 
158 
147 
RELATIVE HUMIDITY. 
Plate 12. Maps X. XI and XU, 
By the term relative humidity, is meant the relation (ratio) of 
the amount of moisture actually present in the air — at any 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 simply a state- 
ment of the amount of water present without reference to the 
saturation point, and is usually expressed as so much weight or 
volume per unit of air. 
The observations for relative humidity 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 relative humidity 
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 
