National Geographic : 1897 Mar
STORMS AND WEA OTHER 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 differences in temperature 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 rarely 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 lying close together, as shown by Chart V, is therefore probably 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 difference 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 extremely cold air to the northwest of them ; but from the distribution of air pressure shown by 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.