MARYLAND GEOLOGICAL SURVEY 187 



The valleys in a mountain region have greater extremes of temper- 

 ature than the mountain tops, being usually warmer during the day 

 and in summer, and colder at night and during winter, because the 

 cold air flows do^m the slopes and accumulates in the depressions. 

 The efl^ect of the nature of the soil and soil covering is also 

 important. The mean temperature of the soil is always higher than 

 that of the air above it. There are great differences, however, in 

 the amount of heat which different soils return to the air. In rocks 

 the temperature is higher at all depths and at all times of the year 

 than in the overlying air, consequently rocky soils give up more heat 

 to the air than other kinds. In sandy land the upper layers only 

 are warmer than air, while moist lands or bogs are colder because 

 much of their heat is lost in causing evaporation. A covering of 

 vegetation lowers the temperature of the soil, and changes in temper- 

 ature over grass and forests are less than over bare soils. Incidentally 

 forests conserve the rainfall, returning it slowly to the streams and 

 diminishing the evil effects of drought. 



The position of a place wath reference to the prevailing path of 

 storms determines the frequency of rainy days, the cloudiness, the 

 winds, and all the variable phenomena called weather, which are 

 non-periodic in occurrence. 



THE PHYSIOGRAPHIC FEATURES OF PRINCE GEORGE^S COUNTY. 



In order to correctly interpret the climate of Prince George's 

 County it is essential to have some knowledge of its physiographic 

 features, but as complete details will be found in other portions of 

 this volume, it will only be necessary to give here a brief recapitu- 

 lation of the main facts. 



Geologists divide the region east of the Appalachian chain into 

 two well-known physiographic provinces : the Piedmont Plateau, and 

 Coastal Plain. In Maryland the Coastal Plain includes all that 

 portion of the State lying east of a line extending from southwest 

 to northeast through Washington, Baltimore, and Wilmington, Del., 

 or about one-half the area of the State. The Coastal Plain is divided 

 into two portions by Chesapeake Bay, the higher western division 



