PAPER BY PROF. BEZOLD. 283 



although such mixtures cau not produce heavy rain or snow yet they 

 can be of great importance in the formation of fog and cloud. 



In accordance with this there are three processes that can, either by 

 themselves alone or in conjunction, cause a condensation of the aqueous 

 vapor in the atmosphere: 



(a) Direct cooling, whether by contact with cold bodies or by radia- 

 tion. 



(/>) Adiabatic expansion, or at least expansion with insufficient addi- 

 tion of heat. 



(c) Mixture of masses of air of different temperatures. 



In a corresponding manner the dissolution of fog and cloud already 

 present may take place through the following processes: 



(a) Direct warming, either by radiation or by contact with warmer 

 bodies. 



(b) Compression, whether adiabatic or at least with an insufficient 

 abstraction of heat. 



(c) Mixture with other masses of air having sufficient temperature 

 and moisture. 



Of these three different processes the one first mentioned is always 

 the most effective. 



In order to condense or dissolve a given quantity of water there need 

 be only a relatively slight direct cooling or warming. When the con- 

 densation or dissolution of a certain quantity is to be accomplished by 

 adiabatic expansion or compression the cooling or warming must be 

 greater, that is to say, must cover a wider range of temperature than 

 for direct cooling or warmiug. 



Still larger temperature differences must come into play when the 

 same quantity is to be condensed or evaporated by the process of mix- 

 ture, in so far as this is any way possible. 



The first pair of these processes, namely, the direct cooling or direct 

 warmiug, comes especially into consideration in the formation of fog 

 proper, which beginning at the earth's surface, extends upwards to 

 greater or less altitudes. At times of excessive radiation the earth's 

 surface first cools. When the cooling has reached the dew point there 

 occurs condensation in the very lowest layer. Hereby the emissivity 

 of this layer itself is increased. It then cools in its upper portion also 

 by radiation, and thus the layer of fog grows upwards more aud more 

 until subsequently, at the time of increased inflow of heat, it dissolves 

 itself in a precisely inverse manner. 



No other considerable precipitation is formed by this method of con- 

 densation except the so-called drizzle. The reason of this undoubtedly 

 is that the growth of the layer of fog upwards removes the possibility of 

 further more intense radiation by the lower stratum. In the higher 

 strata of the atmosphere such condensation by direct radiation can 

 certainly only occur when cloudiness has already been produced in some 

 other way, whether by mixture or by expansion or possibly by smoke. 



