170 THE CLIMATE OF CALVERT COUNTY 



degree on its topography, the slope of the surface, whether valley or 

 mountain top, the nature of the soil and soil covering, and lastly, on the 

 position of the region Avitli reference to the prevailing path of storms. 



The sun's power is greatest when the rays strilce the earth's surface 

 vertically, and the highest temperatures might be expected to occur in 

 regions where the sun is overhead at noon, which can take place only 

 within the tropics. The inclination of the earth's axis 23^ degrees from 

 the perpendicular to the plane of its orbit profoundly modifies this 

 simple deduction by causing a variation in the length of the day as the 

 pole is approached. During the summer of the northern hemisphere 

 the length of the day increases rapidly from the equator toward the 

 pole, and the increased duration of sunshine compensates largely for 

 the greater inclination of the sun's rays. Maryland, lying between the 

 parallels of 38° and 40° north Latitude, at the time of the summer 

 solstice, June 21, has a day of nearly 15 hours' duration, and the soil 

 and air are able to accumulate a large store of heat during the long- 

 summer day. The long winter nights which favor tlie loss of heat by 

 outward radiation give a sharp contrast to the different seasons which 

 is quite absent in polar or tropical latitudes. The factors which con- 

 trol climate act together in so intricate a -manner that it is difficult to 

 ascertain precisely what effect latitude itself to the exclusion of other 

 causes may have upon the climate of a region, but a rough way of esti- 

 mating this effect will be found in the discussion of the climate of 

 St. Mary's County. 



The position of a country with reference to mountain chains or to 

 large bodies of water has a profound effect on climate. Over any level 

 plain, even in tropical regions, the temperature decreases in free air about 

 1° Fahrenheit for every 300 feet increase of elevation. Mountains 

 thrust themselves up into this region of colder air and thus lower the 

 temperature of their surroundings. Again, mountains have a strong 

 influence on rainfall by facilitating the ascent of moist air currents 

 flowing up their slopes, and so causing condensation and precipitation by 

 dynamic cooling. On the other hand large masses of water have a con- 

 serving influence, lessening extremes of temperature, and their action 



