METEOKOLOGY. 543 



cumulus clouds, water spouts, tornadoes, &c. " Iu respect to the rate 

 of diminution of temperature with ascent there is a notable difference 

 between the front and back of a normal depression moving eastward. 

 In the front part the south winds bring warm air, and even more in the 

 middle than in the lowest atmospheric strata, because the currents 

 move faster at an altitude, and because their direction is different, for 

 they flow both from the south and from tbe minimum, bringing with 

 them the latent beat of the condensation of the precipitation going on 

 within the inner portion of the degression, while the cooler air on the 

 earth's surface is drawn in from the regions that have not yet been 

 reached by the minimum. Therefore, here the temperature diminution, 

 with altitude at least in the lower half of the strata, is slow; but the 

 general excess of heat over the region causes a general gradient directed 

 outwards and a slow ascent of the air, the result of which is cloudy 

 sky and continuous precipitation. In the rear of the depression it is 

 otherwise. Here the wind in the middle strata of the atmosphere has, 

 indeed, in general, the same direction, but the swifter motion of the 

 upper layers and the contact of the lower layers with the surface of 

 the earth still warm from the preceding mild weather [and the rapid 

 evaporation of freshly fallen rainj cause even here an important dif- 

 ference in the relations of the upper and lower portions of the stream 

 of air, and especially a decidedly more rapid cooling of the upper re- 

 gion ; a very rapid fall of temperature witn ascent for the lower half of 

 the atmosphere,, is the consequence. Hence the air in the rear of a 

 minimum acquires the characteristic interchange of shower and sun- 

 shine, due to the many local upbursts of the warm lower strata over 

 the whole region where cold air is in the neighborhood of warmer, 

 moister air to the eastward." 



A third type of gradient is that presented by the high or anti cyclonic 

 areas, within which the temperature gradient and atmospheric motions 

 are, in general, directed downwards, but the lowest stratum, or sixth, 

 the part of the atmosphere which, of course, has been stopped in its 

 descent, its warming and its drying is found moving slowly outwards. 

 Iu such anti-cyclones the perfect freedom from clouds favors the radia- 

 tion of heat from the earth's surface (in winter and at nights), produc- 

 ing such great cooling as to lead to complete inversion of the normal 

 vertical distribution of temperature and to the formation of fog in the 

 lowest part of the stratum. Koppen concludes as follows: "The cold 

 over the continents directly causes the increased density of the air and 

 the initiation of barometric maxima and their descending air currents. 

 For even although the temperature does not suffice to explain the aver- 

 age distribution of pressure with latitude, and mechanical influences 

 have to be. added thereto, still for the annual variations in the pressure 

 over continents and ocean, the temperature is the deciding factor." (Z. 

 0. G. M., xvn, p. 92.) 



Dr- W. Koppen, in discussing the question of the monthly range of 



