LOCAL CLIMATOLOGY. 73 



It will be observed, also, in the above Table, column fourth, that the 

 hottest day at the equator is March 20th (the same is true also of Sep- 

 tember 22d), and at latitude 5 the hottest day is April 2d; and the 

 farther north the later, until we reach the Tropic of Cancer and June 

 21st. But in fact the greatest heat is not reached until some days after 

 that which is thus indicated as the hottest. Within the troj)ics this 

 difference is slight, perhaps nothing ; but as we pass towards the pole it 

 becomes perceptible. In our latitude the heat does not reach its maxi- 

 mum, as appears on an average of fifteen years, until the first day of 

 August, or about six weeks after the summer solstice. This is owing 

 to the fact that while the sun is decreasing in altitude and the days are 

 decreasing in length, the earth is receiving more heat during the day 

 than it radiates during the night, and it is thus accumulating and hoard- 

 ing up heat, if we may use such an expression. 



Hence, for this and other reasons, to be discussed below, we have in 

 all high latitudes days that are colder, and days that are much warmer 

 than the extremes above indicated. Within the tropics, however, 

 and near them, it is not likely that a degree of cold so great as is 

 indicated in the Table is ever anywhere experienced at the level of the 

 sea coast. 



II. Elevation above the Sea level. 



The next cause influencing climate, in the order of our enumeration, 

 is elevation above the sea level. 



It is commonly held, that as we ascend from the sea level, the air 

 grows colder at the rate of about one degree for every three hundred 

 feet of ascent. This is owing to the fact that the air receives but very 

 little warmth from the direct rays of the sun as they pass through it ; 

 consequently, the air depends for its temperatiu-e chiefly upon the heat 

 that it receives both by radiation and conduction from the earth. That 

 the temperature grows colder as we ascend, all persons know; and even 

 under the equator, snow becomes perpetual at an elevation of about 

 sixteen thousand feet. 



By referring to the preceding Table, it will be seen that between 

 latitudes 35 and 60, an elevation of about three hundred feet, diminish- 

 ing the average temperature one degree according to the above rule, is 

 equal to one degree of latitude in its effects on the climate of any place. 

 Thus, to elevate it three hundred feet, would j)roduce the same effect on 

 its climate as the placing it sixty miles farther north. 

 Cab. Nat. 10 



