The Weather 5 



of the sudden development of a storm and its approach to 

 localities where it may cause damage, warnings are immedi- 

 ately telegraphed so that signals may be shown. Such warn- 

 ings are especially important to sailors and fishermen and to 

 fruit-growers in Florida and California. 



The weather map which gives the information concerning 

 weather conditions and probable changes may present diffi- 

 culties to those who have not studied its main features and 

 their significance. The following study will help the student 

 to recognize the essential features promptly and to interpret 

 them accurately. It will also aid in understanding the general 

 principles involved in .weather observation and prediction. 



The weather maps, Figs. 1, 2, 3, copies of government 

 maps, give graphical representations of the atmospheric con- 

 ditions of the country for December 24, and several days follow- 

 ing. The dotted lines, which represent red lines on the original 

 maps, give the temperatures, with the numbers printed near 

 the ends of the lines or on circles, expressed in degrees Fahren- 

 heit. Only temperatures divisible by ten are given, as the 

 map would be crowded with lines if all variations were shown. 

 All places, for example, reporting a temperature of 20 F. are 

 connected by a dotted line ; similarly all reporting 30 F., and 

 so on from minimum to maximum. It may be assumed that 

 the temperature at any point between two lines, as those 

 marked 20 F. and 30 F., is roughly indicated by the distance 

 between the lines. 



7. Isotherms. The lines running through places having 

 the same temperatures and showing the temperature condi- 

 tions of the country for a given date, are called isothermal 

 lines, or simply isotherms. The term is made up of two Greek 

 words, isos meaning equal, and thermos meaning heat, or equal- 

 heat. Note that the word thermos supplies the first syllable of 

 thermometer, which means heat-measure. 



The isotherms are very irregular in comparison with the 

 parallels of latitude. They curve northward toward a region 



