TEMPERATURE OF THE AIR 227 



the tube may be be made for each degree, or for each two, five, or 

 ten degrees. The space below 32 is divided into degrees, each 

 degree below 32 having the same length on the tube as each degree 

 above. The of this scale is 32 below freezing-point. The scale 

 below is called "below zero." Thus 20 below zero means 52 

 below the freezing-point, and is written -20 Fahr., or -20 F. 



The Centigrade scale is much simpler. The height of the 

 mercury at the boiling temperature of water at sea-level is marked 

 100 (100 C.), and the freezing temperature under normal atmos- 

 pheric pressure is marked (0 C.). The space between is divided 

 into 100 parts, each of which is a degree. It will be seen that 1 C. 

 is equal to lf Fahr. The Fahrenheit thermometer is most 

 widely used among English-speaking people, but the Centigrade 

 thermometer is generally used in other countries, and in scientific 

 work, and is in every way better. 



The Heating of the Atmosphere 



Sources of heat. The atmosphere gets heat from several 

 sources, but that received from the sun (insolation) is far greater 

 than that from all other sources. That much heat is received 

 from the sun is shown by the fact that the temperature generally 

 rises when the sun rises, and falls when the sun goes down. It is 

 also shown by the fact that the temperature is generally higher 

 on a sunny day than on a cloudy one. 



Insolation. The temperature of space outside the earth's at- 

 mosphere is supposed to be about -273 C. (-459 F.), and the 

 warmer temperature which we enjoy at the bottom of the atmos- 

 phere is due chiefly to the heat received from the sun. The amount 

 received each year is enough to melt a layer of ice 141 feet thick 

 over the entire earth, or to evaporate a layer of water 18 feet deep. 



Each hemisphere receives the same amount of heat each year, 

 but, because of the inclination of the earth's axis, the heat is dif- 

 ferently distributed in different latitudes. 



1. Other things being equal, the earth gets most heat where 

 the sun shines the greatest number of hours per day. In sum- 

 mer, the days are longest in the highest latitudes. So far as length 

 of day is concerned, therefore, the highest latitudes, namely, the 



