CONDUCTION AND. EADIATION 85 



its color has an important bearing upon the rapidity with 

 which it absorbs or radiates heat. A black object usually 

 warms more readily than a lighter one because the latter re- 

 flects many of both the light and heat rays, while black objects 

 absorb them and so increase in temperature. It is for this 

 reason that light-colored clothing is worn in summer and dark- 

 colored clothing in winter. Dark soils are usually early soils 

 because they absorb heat so readily in early spring. As might 

 be inferred from the fact that heat, like light, can be reflected, 

 smooth surfaces reflect much heat and warm more slowly than 

 rough surfaces. When a body with a smooth surface is once 

 warmed, however, it cannot radiate its heat as rapidly as a 

 rough one. In general, then, good reflectors are poor radiators 

 and poor reflectors are good radiators. 



80. Distribution of Heat. All parts of the earth receive 

 the same number of hours of sunlight annually, but this by 

 no means indicates that they are all equally warmed. One 

 reason for the difference in the temperature is the unequal 

 periods of time during which different regions are heated or 

 cooled. In the tropics, there are twelve hours of sunlight and 

 twelve hours of darkness throughout the year. In the Arctic 

 or Antarctic regions, there is a six month's period of daylight 

 and an equal period of darkness annually. In the temperate 

 zone, the length of the day and night varies between these 

 extremes. At mid-summer in the Northern States, the day 

 is about fifteen hours long and the night correspondingly 

 shorter, and in winter these conditions are reversed. This 

 difference in the length of daylight over different parts of the 

 earth is well known to be due to the angle at which the earth's 

 axis is maintained with reference to the sun. In summer it is 

 inclined toward the sun and thus the season of daylight is 

 lengthened. In winter it is turned away and the season of 

 darkness is increased. It is not the length of the period of 

 daylight alone, however, which determines the temperature of 



