MlVrEOROLOGY. 315 



nbove tlum below. On the other han;l, it will occasionally- lia]>pen that 

 the upper currents an- slowrr cr reversed, in w^hich cane the suuiinit of 

 the cumulus follows instead of preceding the main cloud. (/>. M. Z., i, 

 p. 29.) 



203. [Two facts not mentioned by Koppen should be remembered in 

 order to achieve a fuller explanation of cloud phenomena, namely, that 

 the upper part of the cumulus cloud is surrounded at a little distance 

 by the drier air into which it has risen, which, therefore, tends to evap- 

 orate and dissipate the surface of the cloud; and, again, the powerful 

 effect of the solar rays (which increases slightly with altitude) is tO' 

 evaporate the moisture at the surface of the cloud and to surround the 

 whole sunny side of the cloud with a thin layer of hot moist air, whichi 

 must have a special ascensional tendency. This action of the sun is. 

 undoubtedly also very eflicacious on the upper surface of extensive lay- 

 ers of stratus clouds from which vai)or currents rise, which in turn form 

 a second layer at some distance above, while the density of the whole 

 mass of warmed cloud and air is diminished, and an additional ascensive 

 tendency is felt at the earth's suif;u'e over a htrge and extended terri- 

 tory.] 



204. The increasing interest in Ihe observations of clouds has led 

 van Bebber and Ko]»pen to attemjit a system of corresponding obser- 

 vations, which were planned after consultation with Dr. Klein and 

 ]\I oiler and Assmann. The same methods of description are used at all 

 stations, and the comi)lete record of clouds is made a part of the regu- 

 lar entries in the daily journal as well as in the s[>ecial records. (D. 

 M. Z., I., p. 30.) 



205. Eev. Clement Ley, in an essay on the structure of thecirrofilum, 

 or thread-like clouds, presumably formed of ice spicuhT, states that this, 

 is one of the most important forms in the prediction of the weather ; he- 

 has observed it for twenty-five years, and offers a classification of thei 

 more reliable observations and an explanation of some of the more prom- 

 inent i)henomena. His classification of the clouds relates to their form,, 

 their structure, and their relations to each other. The following three, 

 generalizations seem to him of importance: 



1. If we turn the face toward the wind as we stand on the earth's sur- 

 face, then in the majority of cases tlie ui)i)er currents of air mov^e from 

 the right to the left. 



2. Winds from the west lie above those from the east in the upper re- 

 gions of the atmosphere much more frequently than is the case at the 

 earth's surface. 



3. The upper strata of the air as a rule move much more rapidly than 

 the lower, lie gives a table showing the number of cases in wliich the 

 cirro-filum cloud moves from the res[)ective i>oints of the compass. He 

 restricts the isame cirrus to the ui)per clouds formed of bent or twisted 

 bundles of threads, whereas those composed of long lines or filaments 

 have a much greater velocity, and are characterized by him as linear 



