392 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



or a rainfall of 11.9 millimeters; but since at least 4.05 kilograms 

 of air at 10 0. must be mixed with each kilogram at 25° in order to lower 

 the temperature to nearly 10°, and to absorb the latent beat of the 

 13.47 grams of vapor that will thereby be condensed, therefore it is im- 

 probable that this maximum will ever be attained. This quantity of 

 water will [most probably] be distributed over a largeextent of surface. 



By the flowing of warm and cold currents over each other, notwith- 

 standing the slight depth of the stratum in which the mixture of the 

 air and condensation take place, there can fall, in the course of time, a 

 considerable amount of water upon a unit of surface, but the density 

 of the precipitation in this case will be less than in the foregoing. 



From this exposition it appears to me to follow that the mixture of 

 masses of moist air of different temperatures cannot produce any very- 

 intense or heavy precipitation, and that the ordinary presentation of 

 Button's rain theory leads to an overestimate of the quantity of pre- 

 cipitation due to this source. The frequent occurrence of heavy precip- 

 itation on the occasions of inflow of colder currents of air we explain to 

 ourselves by the co-operation of an ascending current in connection 

 therewith. 



The cooling of the air at the earth's surface [itself cooled by radia- 

 tion] can cause only local precipitation confined to a thin stratum of 

 air. In this manner originates the ground fog, which is with us most 

 frequent in the early spring, and also the ground fog of the Polar Ee- 

 gions, often extraordinarily dense, but reaching only to a slight altitude, 

 of which Middendorf, in his volume of journeys in Siberia, has given 

 graphic descriptions. The radiation of heat from the moist air itself, 

 even if we, with Tyndall, ascribe to it a great power of radiation, can 

 certainly only produce a relatively thin covering of clouds, whose for- 

 mation must in great part or entirely hinder a further loss of beat from 

 the lower strata. 



The ascending movement of moist air must therefore be considered 

 as the most fruitful source of precipitation, a result which was long since 

 ascertained by observations, but is in the preceding now also deductively 

 established. We will in our next article turn to the consideration of the 

 causes of the ascending movement of the air, concerning which subject 

 in recent times exhaustive works have been published, which have not 

 yet received corresponding notice in this journal. 



NOTES. 



No. 1. Zeitscbiift, vol. ix, page 24'). 



No. 2. Zeitsclirift, vol. viii, pages 102 and 177. 



No. 3. Zeitschrift, vol. viii, page 177. 



No. 4. We take this occasion to emphasize the importance of the publication of 

 hourly observations of all the meteorological elements for at least one normal station 

 in each country of average magnitude. That which at the present seems superfluous 



