Geography of Ohio 
193 
increment of tomorrow and the next day, so that by the middle of 
August the land of Ohio is quite warm; for several weeks after 
the sun’s rays become more oblique the atmosphere keeps its 
summer heat, simply because it is being added to by heat radi- 
ated from the land. 
As influenced hy altitude. Another factor in temperature is 
altitude. All places on a given parallel do not have the same 
temperature. A plateau two thousand feet high, adjacent to a 
lowla-nd belt, will give different readings of a thermometer than 
will the lowland at the same time of the day. In Ohio the vari- 
ation in altitude is not sufficiently great to show very marked 
temperature differences through this factor. , 
As influenced hy bodies of water. A third influence, bearing 
on temperature, is found in the relationship of land and water 
areas. This follows from these two forms of matter having dif- 
ferent specific heats. Lake Erie and an equal area of the state 
of Ohio exposed to the same insolation during the same length of 
time will show different heat results. It takes longer to warm or 
to cool a body of water, but when its temperature is once changed 
it remains constant much longer than does an equal area of land. 
During the winter a water body stores up a great deal of low tem- 
perature, which it gives off slowly, and as a consequence keeps 
the nearby atmosphere cold in the spring, whereas the atmos- 
phere over the neighboring land has already become balmy. 
Similarly, in the fall the water bodies, having stored up a great 
amount of summer heat, give it off slowly and keep the air over 
the adjacent land still moderately warm, while in areas farther 
away the temperature has already grown cold. A practical 
result of this last condition is shown in the land areas adjacent 
to water bodies being devoted to fruit growing, because in the 
spring the cold of the lake keeps the buds from starting, and the 
time of late spring frosts has passed before they bloom; in the 
fall the warmth of the lake retards early frosts, and the fruit has 
ample time to mature. 
Isothermal maps. Men have devised a method of expressing 
some of the facts of temperature. A temperature map of Ohio 
during any particular year consists of isotherms, which connect 
places having the same mean annual temperature. An isother- 
mal map of Ohio, therefore, shows variation, because Ohio stretches 
through a few degrees of latitude. The range in mean annual 
temperature varies from six to seven degrees. 
