BLASIUS'S THEORY OF STORMS. 303 



barometer now rises again, but is not as high as before the storm, 

 because it is in the tropical current which has reached the locality. 

 If, now, the wind from the south, which has prevailed and driven 

 back the northern current, continues in the same direction until the 

 entire atmospheric area of the storm passes over the zenith north- 

 ward, and the sky clears up from the south or southwest, as is gen- 

 erally the case in early autumn or late spring, then the next storm or 

 change of weather will come from the north. But if the wind changes 

 its direction again before the storm is over, as is mostly the case in 

 mid-winter, and blows from the north, as it did at the beginning, 

 until the entire atmospheric area of the storm is carried backward 

 over the zenith, and the sky clears from the north, then the next 

 storm or change of weather will come from the south, as described 

 above. In this case the polar current has prevailed, the air is colder, 

 the thermometer falls, the barometer rises higher than in the other 

 case, and the atmospheric conditions existing before the storm are 

 gradually reestablished. 



Summer Storms. Before a progressive summer storm, the air is 

 usually warm and sultry, the sky cloudless but somewhat dim, and a 

 light southerly breeze is blowing. Suddenly the sound of distant rum- 

 bling thunder is heard, and large masses of dark cumxdus clouds rise 

 and arrange themselves on a long bank of stratus clouds in the north- 

 ern or northwestern horizon. This is the cumulo-stratus combination 

 of clouds which is the herald of a polar or progressive summer storm. 

 Soon the south wind increases in violence, and drives clouds of dust 

 before it. The thunder, rolls, and lightning flashes more frequently. 

 The clouds bank up higher and higher, and advance more slowly, until 

 at last they become stationary. These are the ordinary indications of a 

 violent progressive summer storm, which sometimes ends in a tornado. 



Like a winter storm, it is produced by the meeting and conflict of 

 the polar and tropical currents under greater differences of temperature 

 and other conditions, and is therefore attended with more violent and 

 complex phenomena than those of a winter storm. The changes of 

 wind, and of the barometer and thermometer, during its development 

 at any locality, are similar to those of a winter storm in its return, 

 oscillation southward ; that is, these changes occur in a reverse order 

 to those of a winter storm during the regular progress of the tropical 

 current northward, in the same order as during its oscillation south- 

 ward. 



In most cases of this kind of summer storms, after the clouds have 

 remained stationary for some time, discharged their rain and restored 

 the disturbed equilibrium of the atmosphere, the polar current which 

 produced it by moving southward oscillates back to the north again, 

 and the storm at this locality is over although similar phenomena 

 and changes will be occasioned by it later at other localities over 

 which it sweeps in its oscillation northward. 



