MARYLAND GEOLOGICAL SURVEY 173 



THE INFLUENCE OP WATER ON THE DISTRIBUTION OF TEMPERATURE. 



Over all land surfaces the temperature of the air is determined by 

 the temperature of the soil, and the modifying effects of large bodies 

 of water on temperature depends on the differences in the effect of solar 

 radiation on land and water. These differences are very striking. Since 

 the specific heat of soil is low (about 0.6 calories) it is readily warmed, 

 but as it is opaque and a relatively poor conductor of heat the insolation 

 acts on a relatively thin surface layer. Soil is a solid substance and 

 therefore cannot equalize temperature differences by movements within 

 itself ; it is a poor reflector and does not turn away much of the radia- 

 tion incident upon it; there is no loss of heat in causing evaporation. 

 These conditions all combine to produce great heat in a relatively thin 

 top layer of soil under direct sunshine, and a rapid loss of heat by radia- 

 tion at night, making for great contrasts in the temperature of the soil 

 and of the air above it. 



Consider the case with reference to water. The specific heat of water 

 is very high; there is no other natural substance known' which 

 requires so much heat to raise its temperature, and therefore a given 

 mass of water is warmed with much greater difficulty than an equal mass 

 of soil. Water is cooled with equal difficulty. Much of the insolation 

 on a water surface is reflected away, and that which does enter the water 

 penetrates to great depths so that the heat is more uniformly diffused. 

 The water, being mobile, changes in temperature produce convectional 

 currents and the winds produce surface currents, both of which tend 

 constantly to mix the warmed water with cooler portions and thus to 

 moderate the rise in temperature. Lastly much of the heat incident 

 upon a water surface is expended in changing some of the water from 

 the liquid state to vapor, an operation which causes a large amount of 

 heat to become latent. The consequences of these conditions are that 

 water becomes very slowly warmed and throughout a considerable mass, 

 and that it cools only slowly when insolation is withdrawn, making for 

 slight contrasts in the temperature of the water and of the air above it. 

 The proximity of large bodies of water must modify the temperature of 



'Liquid liydrogen has a specific heat 6.0 calories. (Dewar.) 



