BLASIUS'S THEORY OF ST OEMS. 299 



cal current over the polar, varies with the seasons and local circum- 

 stances. In winter the slope between the two currents is very gradual, 

 as there is less difference of temperature, and less power of resistance 

 between them. The warm current passes over the cold at a gentle in- 

 clination (as represented by the line B J in Fig. 1) ; and thus the 

 horizontal or geographical extent of the storm beneath it from B to 

 J) which is the region of low barometer, is much enlarged, and 

 sometimes its oscillations extend or move over several hundred miles. 



In summer, however, the difference of temperature between the 

 two currents, and their power of resistance, are greater, and when 

 they meet they bank up against each other with more momentum and 

 force, and the plane of meeting or conflict is often very steep and 

 sometimes almost vertical (as indicated by B H in Fig. 2). Hence, 

 the geographical extent of a storm in summer is much less than in 

 winter, and the region of low barometer which moves with it is cor- 

 respondingly small. 



Clouds the Precursors of Storms. Whenever a warm cur- 

 rent of air, saturated with moisture, meets or mingles with a cold 

 current, the invisible moisture of the warm air is condensed into visi- 

 ble vapor or clouds. As storms are produced by the movements and 

 conflicts of warm and cold currents of air, the formation of clouds 

 always indicates to the observer the region in the atmosphere where 

 such movements are taking place, which would otherwise be invisible. 

 Clouds, therefore, are the invariable precursors of storms, and the 

 kind of clouds formed will indicate the kind of storm or atmospheric 

 movements which produce them. 



This general fact, however, does not apply to deserts, where the 

 moisture of the warm air is condensed and precipitated before it 

 meets the cooler air, and hence rain-clouds are seldom or never formed 

 by the sand-storms of deserts. 



Classification of Clouds. Whenever, on account of some topo- 

 graphic circumstances, the sun heats any locality on the surface of 

 the earth more than the surrounding region, a gentle current or col- 

 umn of heated air rises, and its invisible moisture is condensed into 

 small masses of clouds called cumuli, which spread and produce the 

 mottled appearance commonly known as " mackerel sky," as indi- 

 cated at 1 in the accompanying illustration (Fig. 2). 



But when, as is frequently the case in summer, a valley or plain, or 

 island, or any other place, is much more highly heated by the sun than 

 the surrounding region, the heated air over such locality rises more 

 rapidly and with more ascensional momentum ; and, as it reaches the 

 higher and cooler regions of the atmosphere, its moisture is condensed 

 into large rounded volumes, or mountain-like masses oi cumulus clouds, 

 as indicated at 2 in the illustration. Such cumulus clouds always pre- 

 cede and characterize a local summer storm or shower. 



When the warm horizontal current from the south, as in winter, 



