24 



On the Distribution of Cyclonic Precipitation in Japan. 



Wa\er 



we can expect any precipitation at all. In winter, the relation will 

 be different ; we may generally expect heavier rain or snow on that 

 side of the depression which viewed from tlie centre lias the ocean 

 on the right side. 



(3) While the above two factor are essentially thermal or 

 thermodynamical, there remains the third one to be considered, 

 wdiich may conveniently be called ' ' hydrodynamical and topo- 

 graphical." This consists in the effect of the forced ascending air 

 current brought al)Out by the discontinuity of the horizontal flow of 

 air across the coast, due to the difference of " friction "'^ over land 

 and w^ater, or flat land and mountains. This effect has been dis- 

 cussed in the previously cited paper and may be summarized as 



follows : In the annexed figure, AB is 

 the coast line bordering the land on its 

 left side. PQ makes with AB a certain 

 angle depending on the coefficient of 

 friction on land and water. The ascend- 

 ing air current is induced w^hen the 

 gradient of the l)arometric pressure is 

 directed toward the B side of PQ, while 

 the descending current occurs wdien the 

 gradient points to the A side. The ab- 

 solute intensity of the current is maxi- 

 mum when the gradient is perpendicular 

 to PQ, while it is zero Avhen the gradient 

 coincides with OP or OQ. This influence will appear most con- 

 spicuous wdiere the land is in the shape of a narrow strip having 

 a large extent of water bodies on both sides, provided the temper- 

 ature and humidity on the two sides are not very different. Along 

 the coast of a continent, how^ever, the thermal and geographical 

 influence mentioned under the previous paragraph will generally 

 combine with the hydrodynamical influence, so that the relation 

 may vary widely according to season or the physical conditions 

 of the continent. 



Fio-. 43. 



In the sense of Guldberg and Mohn's theory. 



