CIRCULATION OF THE ATMOSPHERE 455 



is a considerable deviation from it in the upper levels. In the tem- 

 perate zones the normal vertical temperature gradient is only about 

 5.40° Centigrade, though it may be considerably more or considerably 

 less according to the circumstances. It may be generally said that, 

 except in restricted regions, the air does not cool as fast in going up- 

 wards as it should if it were caused by mere vertical expansion. The 

 upper levels of the air are too warm; warmer than they should be if 

 that law prevailed. In the temperate zones they are very much too 

 warm, and that is why the vertical gradient is less than it should be 

 according to that law. The fact is that the warm masses of air which 

 flow from the tropics towards the poles retain their heat above what 

 they should have for the given latitude, and in that way the upper 

 levels of the atmosphere are maintained at a considerably higher heat 

 than would be expected. When the air has once cooled to about 70° 

 below zero, Centigrade, it seems disinclined to cool much further, and 

 in the levels from 12,000 to 16,000 meters high there has been dis- 

 covered a tendency for the air to be somewhat warmer than it is in 

 the levels below, say from 8,000 to 12,000 meters high. It is generally 

 thought that this phenomenon is due to radiation in some of its effects, 

 but it is still a subject of discussion. If we should assume as the 

 average vertical gradient for the entire atmosphere a rate of about 7° 

 Centigrade per 1,000 meters then we should find that the temperatures 

 in the tropics fall off too fast, and in the temperate zones too slow to 

 conform to this average gradient. Now the mathematical law shows 

 that if the lower levels of the atmosphere are relatively too warm for 

 the upper levels there will be a westward drift as in the tropics, and if 

 the upper levels are too warm relatively for the lower levels there will 

 be an eastward drift as in the temperate zones. Speaking a little more 

 broadly still, in order to avoid discontinuity, that is to say, changes by 

 jumps in the atmosphere as regards the barometric pressure at the 

 different levels, since the warm air has less density than the cold air, 

 it follows that the warm air must move faster over the surface of the 

 earth than does the cold air. Hence it is that in the tropics the air is 

 too warm for its altitude, and it must move off faster than it otherwise 

 would in the tropics. The westward drift in the lower levels compen- 

 sates for this excessive temperature, and in the upper levels of the 

 temperate zones the excess of motion compensates for the higher tem- 

 perature. We find exactly the same principle working in the formation 

 of hurricanes and tornadoes. Hurricanes develop in the northern hemi- 

 sphere in the late summer and early autumn, and this is the season when 

 the cool air of the northern latitudes begins to spread southward 

 towards the equator as the sun begins its southward march into the 

 southern hemisphere. At first the cool air flows over the warm air in 

 the higher levels. This in a general way increases the vertical tern- 



