76 STORMS AND WEATHER FORECASTS 



atmosphere, and depend wholly on the mechanics of the latter. 

 The problem, however, is so complex that it would be hazardous 

 to undertake to explain the great difierences in temi)erature 

 shown on this map of departures for July and August, 1896. 



Think of the atmosphere as a mass of air about 50 miles deep, 

 whose upper surface maintains nearly the same configuration 

 and temperature and is almost entirely without motion relative 

 to the earth's surface. The solar radiation and the terrestrial 

 radiation penetrate this upper region without appreciable ab- 

 sorption, and the ascending and descending currents of air rarel}"" 

 or never disturb this region, but cease before they reach it. Our 

 weather and climate depend on the changes going on in the 

 middle and lower atmospheres, and among these changes that 

 which affects our surface temperature most is the motion of the 

 atmosphere. The great contrast in temperature between two 

 regions l3dng close together, as shown b}' Chart V, is therefore 

 probabh' not due to any special cosmic influence, but to the flow 

 of air as determined by the distribution of air pressure day by day. 



Chart VI shows the beginning of a cold wave in the north- 

 west on the morning of January 7, 1886. Observe that the 

 heavy, black isobar passing through Montana is marked 30.8, 

 while the isobar curving through southern Texas is marked 

 29.8, a diff'erence of one inch in the air-pressure between Mon- 

 tana and Texas. The dotted isothermal line in Montana is 

 marked 30 degrees below zero, while the isotherm on the Texas 

 coast indicates a temperature of 50 degrees. 



Chart VII is auxiliary to Chart VI, and by varying degrees of 

 shading shows the fall of temperature during the preceding 24 

 hours attendant on the high-pressure area of the northwest. A 

 considerable area covered by the darkest shade indicates a fall 

 of 40 degrees in temperature during the past 24 hours. 



The people of the Gulf states, with a morning temperature of 

 40 to 50 degrees, knew nothing of the great volume of extremeh^ 

 cold air to the northwest of them ; but from the distribution of 

 air pressure shown b}'- Chart VI, the forecaster anticipated that 

 the very cold air of the northwestern states would, on account of 

 its great weight, be forced southward to the Gulf and eastward 

 to the Atlantic ocean ; or, more accurately speaking, that the 

 conditions causing the cold in the northwest would drift south- 

 ward and eastward. He therefore issued the proper warning to 

 the threatened districts. 



