As is evident from the above analysis, the labiUty due to heating alone 

 is not sufficient to cause thunderstorms, but the co-operation of heating 

 and cold air advection is most important for the formation of heat 

 thunderstorms. Roughly speaking, the labile energy due to surface 

 heating is about 1,500 joules at most and that due to cold air advection 

 is 1,500 joules or more. The co-operation of both agencies is most 

 effective at the time of maximum heating energy ; therefore the present 

 result does not contradict the statistical result of maximum occurrence 

 of heat thunderstorms at about 14h. 



It must be noted here that in the above discussion frontal or cyclonic 

 thunderstorm is excluded. 



Part II — Some Properties of Cumulo-nimbus 



The following conclusions are drawn on the properties of thermic 

 cumulo-nimbus from the data of dense network of temporary stations 

 equipped with self-recording instruments (about thirty stations within 

 the area of 100 km. square), which were specially distributed to investigate 

 the minute structure of thunderstorms : — 



(1) Growth and Decay. — The development of cumulo-nimbus is well 

 explained by moist-labile energy alone from energetical point of view. 

 The cold front accompanying the cloud has only secondary meaning for 

 its development (see (3) ). The amalgamation of cumulo-nimbus masses 

 and the rejuvenation by amalgamation are also explained from the above 

 point of view. 



The moist -labile energy is rapidly consumed, so the cumulo-nimbus 

 generally decays rapidly in energetically isolated environment. _ The 

 air mass in old cumulo-nimbus is cold and stable, and of entirely different 

 thermodynamical properties from the same cloud in developing stage, 

 which shows the prevalence of radiational cooling. 



(2) Movement. — The motion of developing cumulo-nimbus is governed 

 by the law of amalgamation ; in other words, the cumulo-nimbus rhoves 

 towards the region of more labile energy. The rejuvenation of cloud mass 

 by amalgamation may be considered as the natural consequence of the 

 above principle, while the old cumulo-nimbus is drifted by the upper 

 current of 2-3 km. level. 



(3) Cold Front. — The cold front is formed within the cloud mass, as 

 the result of decending cold air (this front is local and must be distin- 

 guished from that in synoptic weather chart). The front generally 

 moves faster than the cloud mass and runs outside the cloud, then it 

 soon disappears. Sometimes front and cloud show different direction of 

 motion. This is because they obey the different laws of motion. The 

 above facts show that the cold front has no primary effect energetically 

 upon the development of cumulo-nimbus. With the separation of 

 front from cloud, the cloud decays, but, in developing cloud mass a new 

 front is generated after the old front has gone away. 



INVESTIGATION ON FOGS 



By U. Nakaya, Hokkaido University 



This report contains the results obtained by investigation on fogs 

 occurring along the Pacific coasts of Hokkaido in the summers of 1944 

 and 1945. In the first year the investigation of the property of fog, 

 the meteorological investigation on fog, and the preliminary investi- 

 gation on the artificial dissipation of fog were mainly carried out by 



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