CONDUCTION AND RADIATION 87 



of the day, and yet this period usually comes one or two hours 

 later. Morning also is likely to be cooler than midnight. The 

 reason for such lags in temperature may be explained by what 

 we know of the phenomena of radiation. When the days are 

 long and the nights are short, as in summer, the period of daily 

 heating is much greater than the period in which the heat can 

 be radiated. Consequently, the earth accumulates heat. In 

 winter we daily radiate more heat than we receive and continue 

 to lose heat until the lengthening days of late winter bring us 

 enough extra heat to balance the daily outgo. The increased 

 temperature after mid-day may be explained in the same way. 

 It is now easy to understand why regions near the equator 

 have a less variable temperature than those near the poles. 

 Their periods of heating and cooling are almost exactly 

 balanced throughout the year. 



82. The Effects of Gas and Dust on Radiation. Dust and 

 smoke in the air intercept the radiation from the earth and act 

 like a blanket to keep in the heat. This explains why the city 

 is warmer than the country during our heated season. The 

 carbon dioxide and water vapor in the air, however, are fully as 

 important in this connection. Though they allow the heat 

 energy from the sun to pass to the earth, they readily absorb 

 the long heat rays and become warm themselves. In fact 

 if it were not for the presence of these two gases in the atmos- 

 phere, the earth would lose heat so rapidly at night, even in 

 summer, as to approach dangerously near the freezing point. 

 The variations of temperature in deserts, where the air is dry, 

 are frequently much greater than in places where there is more 

 moisture in the air. Many orchards are now equipped with 

 means for making a smudge to protect the trees from unseason- 

 able frosts. A fog over the orchard may have the same effect. 



83. Heat and Living Things. A certain amount of heat is 

 necessary to the existence of all living things. Since most of 

 their life processes are due to chemical reactions, much of this 



