502 POPULAR SCIENCE MONTHLY. 



tinctly seen, and in the cumulus it predominates. The length of the 

 arrows shows the relative velocity, which is seventy miles per hour in 

 the cirrus, on the average, forty-five miles in the alto- and strato-cumu- 

 lus, fifteen in the cumulus and six on the ground. Compare Chart 1. 

 The total motion of the upper air is made up of two parts, a 

 rapid and quite steady eastward drift, and a local or secondary circular 

 movement; these two combine and make up the observed circulation 

 that actually exists. They are due to different causes, the first to the 

 fact that the tropics are warmer than the polar regions, and that the 

 earth is rotating on its axis; and the second to the fact that these two 

 different temperatures seek to combine into an equilibrium by means of 

 an interchange of heat through the air currents, especially in the 

 strata from one to three miles above the ground. These counter cur- 

 rents flow against each other, and at or along their junction they pro- 

 duce rotations and whirl up cyclones or local storms. This is the law 

 in the temperate latitudes from 30° to 50° ; another law prevails in the 

 tropics and possibly in the polar zones, though these belts have not yet 

 been suitably studied. These counter currents can be readily seen on 

 Chart 4 in the arrows drawn with a broken line, where a great stream 

 flows from the northwest and recurves along the Mississippi Valley, 

 while a second stream arising in the Gulf of Mexico flows northward, 

 and recurves to the westward over the Lake Region. Along the edges 

 of these currents is the place for rainfall, for the formation of cold 

 waves, the production of tornadoes and thunderstorms. One can readily 

 judge from relative lengths of the arrows on Chart 5 to what a dis- 

 advantage the forecaster is put in attempting to anticipate the prob- 

 able storm action by using only the small arrows on the surface. He 

 ought to have access to those of the higher strata as well, and this is 

 the reason for the special efforts which have been made by the Weather 

 Bureau through the cloud observations and the kite ascensions to in 

 some measure overcome this defect. It is hoped that by means of 

 nephoscopes and the necessary study of the facts already obtained to 

 be able to make at least three simultaneous daily weather maps, one on 

 the sea level, one on the 3,500-foot plane, and one on the 10,000-foot 

 plane. This will give us three sections through each storm instead of 

 one as at present. The forthcoming report of the Weather Bureau on 

 the Barometry of the United States and Canada contains a discussion 

 of these data and the necessary reduction tables for all the stations of 

 the service. The sea level reduction tables were put in operation on 

 January 1, 1903, and the tables for the other planes are nearly ready 

 for use. There is reason for expecting that a good measure of success 

 will attend these efforts to obtain practical results of considerable 

 advantage in the art of forecasting. 



