THE ATMOSPHERE. 87 



Ann. duration. Force. Xo. of obs. 



30^ and 25° S 124 days. 5.6 miles.* 19,817 



25Oand20O 157 „ 5.7 „ 20,762 



20^ and 15° 244 „ 5.9 „ 17,844 



15° and 10° 295 „ 6.3 „ 14,422 



10° and 5° 329 „ 6.1 „ 13,714 



5° and 0° 314 „ 6.0 „ 15,463 



It thus appears tliat the south-east trade-winds of the Atlantic 

 blow with most regularity between 10° and 5°, and with most 

 force between 10'' and 15°. 



226. Tlie air sloughed off from the counter trades, moist air. — On 

 the polar side of 35°-40°, and in the counter trades (§ 204 [7]), a 

 different process of sloughing off and turning back is going on. 

 Here the winds are blowing towards the poles; they are going 

 from parallels of large to parallels of smaller circumference, while 

 the upper return current is doing the reverse ; it is widening out 

 with the increasing circumference of parallels, and creating room 

 for more air, while the narrowing current below is crowding out 

 and sloughing off air for its winds. 



227. The air sloughed off from the iijjper trade current dry. — In 

 the other case (§ 224), it was the heavy dry air that was sloughed 

 off to join the winds below. In this case it is the moist and 

 lightest air that is crowded out to join the current above. 



228. ITie meteorological influences of ascending columns of moist 

 air. — This is particularly the case in the southern hemisphere, 

 where, entirely around the globe between the parallels of 40° 

 and 60° or 65°, all, or nearly all, is water. In this great austral 

 band the winds are in contact with an evaporating surface all the 

 time. Aqueous vapour is very much lighter than atmospheric 

 air : as this vapour rises, it becomes entangled with the particles 

 of air, some of which it carries up with it, thus producing, 

 through the horizontal flow of air with the winds, numerous little 

 ascending columns. As these columns of air and vapour go up, 

 the superincumbent pressure decreases, the air expands and cools, 

 causing precipitation or condensation of the vapour. The heat 

 that is set free during this process expands the air still farther, 

 thus causing here and there in those regions, and wherever it 

 may chance to be raining, intumescences, so to speak, from the 

 wind stratum below; the upper current, sweeping over these 

 protuberances, bears them off in its course towards the equator, 



* Distance per hour that vessels average while sailing tlirough it. 



