PUlLOSorUY OF STORMS- 193 



rounding air. Clouds may form even under such circumslances, but 

 they are the result of rising masses of air having become detached from 

 up-moving columns underneath. If these clouds be closely observed 

 they will be seen to dissolve soon after they form. 2. When the ground 

 is colder during the day than the air in contact with it, ascending col-^ 

 umns cannot exist, and of course cumuli cannot be formed. This we 

 sometimes see after a period of cold weather, when a warm breeze sets 

 in from the south, saturated with moisture to such a degree, that a por- 

 tion of it is condensed upon the cold bodies, with which it comes in 

 contact. 3. When there has been a great rain just before, up-moving 

 columns will fail in producing the rain cloud. The upper air has still 

 within it a large quantity of caloric, resulting from the previous conden- 

 sation of the vapor, and therefore, the ascending columns, for want of 

 buoyancy, will not continue their motion in it far enough to produce 

 rain. 4, When, as is sometimes the case, the air at some distance above 

 the surface of the earth, and below the base of the cloud, is very dry, 

 rain will not be produced ; because much of this air goes in below the 

 base of the cloud and up with the ascending column, and consequently 

 large portions of the air in the cloud may thus not be saturated suffi- 

 ciently with vapor to produce rain. 5. When there are cross cur- 

 rents of air strong enough to break into an ascending column, clouds 

 cannot form of any very great size, and rain cannot occur. 



The upper current of the atmosphere, although it does not contri- 

 bute to the formation of cloud, has much to do with its integrity and 

 continuance, after it has been formed. In order to insure the formation 

 of rain, it will be necessary for the ascending column of air to go up 

 sufficiently high, not only to deposit its vapor, but also high enough for 

 a large quantity of this condensed vapor to accumulate. For this pur-^ 

 pose a favorable condition of the upper current is necessary. If it 

 should be too strong, or its direction contrary to that of the current be-, 

 low, it will cut off the tops of these column clouds, no matter how rap- 

 idly they may be generated, Whenever the tops of these clouds are 

 swept off by the upper current of air, the heavens become studded with 

 them in the form oi cirro-cumuli, which are a sure indication, that it will 

 not rain on that day. The reason is, that although enough vapor may 

 be condensed, which if accumulated in the ascending column, would 

 ultimately descend in rain, yet as fast as a cloud is generated, its top is 

 broken offand carried, beyond the storm-power, into regions, where it is 

 reconverted into gaseous vapor, instead of falling in the form of rain. — 

 If, however, the upper current is in the same direction, and of similar 

 velocity with the current below, then the up-moving column of air is 



