THE WINDS 



effect is slight, and affects the atmosphere as a whole. 

 Its maximum effect on a column of mercury is only a 

 hundred and thirtieth of an inch. But the other kind of 

 atmospheric tide is a major influence in the world's 

 atmosphere. It is a ''heat tide" which follows the sun in 

 its apparent circling of the earth, and is an elevation or 

 crest of air along a meridional line, which moves steadily 

 around the earth. As with the twice-daily oceanic tides, 

 this "heat tidal wave" is related to a "cool tidal wave" 

 on the opposite side of the earth. 



These tidal waves, one kind gravitational and insig- 

 nificant and the other kind caused by the sun's heat and 

 affecting the atmosphere powerfully, are periodic. They 

 merge with, or are affected by, wind modifications 

 caused by irregularities in the world's land surfaces, so 

 that the processes involved in the movements of the winds 

 are, in detail, infinitely complex. The entire atmosphere 

 is continually in motion, like the sea itself, as air currents, 

 moving in every conceivable direction, struggle for 

 ascendancy or battle for "right of way". 



A curious phenomenon of air motion is called the 

 Coriolis effect, after Gaspard G. de Coriolis (i 792-1 843), 

 the eminent French mathematician who first investi- 

 gated it. Any wind flowing "downhill" in the northern 

 hemisphere tries to turn in an anti-clockwise direction, 

 across the isobars, moving towards the nearest "valley" 

 of low pressure available. But a new factor enters — one 

 which remained obscure and perplexing until Coriolis 

 explained it. The earth's surface in our northern hemi- 

 sphere is steadily turning in an anti-clockwise direction — 

 leftward around the pole. But the air above the earth's 

 surface (being relatively free) tends to move straight 

 onwards, urged by inertia, and this movement of the air, 

 seen from beneath it, appears to us to be clockwise, because 

 we, as observers, are on the earth's leftward turning 

 surface. 



As we contemplate the extreme complexity of the 



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