ScHAW. — Australasian Weather-charts and N.Z. Storms. 61 



Art. III. — Australasian Weather-charts and Neiu Zealand 



Storms. 



By Major-General Schaw, K.E., C.B. 



[Read before the Wellingto7i Philosophical Society, 9th September, 1896.] 



Plate V. 



There are three great forces which produce movements in 

 our atmosphere — heat, gravitation, and the rotation of the 

 earth on its axis. 



Heat primarily causes motion through the heating of the 

 equatorial zone of the earth by the direct rays of the sun. 

 This heat is communicated to the layer of air in contact with 

 it, which expands, and, being thus lighter than the air not so 

 heated, rises, and the cooler air flows in below. The lighter 

 air flows north and south towards the poles, whence cooler 

 and heavier air flows in to take its place. This is the first 

 and simplest cause of motion in the atmosphere, and it is the 

 result of sun-heat and the attraction of gravitation. 



But the rotation of the earth causes the stream of air 

 flowing north and south to diverge from meridian lines. 

 Popularly w^e see why when we consider that air at the 

 equator is moving eastwards with the earth's circumference — 

 which there is nearly twenty-four thousand miles, or at 

 the rate of a thousand miles an hour — while at the poles 

 there is no eastward motion, and at all intermediate circles 

 of latitude the eastward rate of motion is graduated according 

 to the distance from the equator, that at 60° being just half 

 the equatorial rate. Hence all currents of air flowing towards 

 the poles are moving also more and more quickly than the 

 surface of the earth towards the east, and they seem to be 

 from the west of north or south, while all currents of air 

 flowing from the poles towards the equator are also apparently 

 moving in the reverse direction, from the east of north or south ; 

 because, as they advance equatorwards, the surface of the 

 earth is moving more and more rapidly to the east, and the 

 currents of air lag behind. But the mathematical law is more 

 complete than this — viz., that in the Southern Hemisphere, 

 between the equator and the poles, any body freely moving 

 over the surface of the earth in any direction appears to be 

 deflected to the left, and, in the Northern Hemisphere, to 

 the right, the amount of deflection varying directly as the 

 sine of the angle of latitude. Hence the deflection is much 

 greater at the antarctic circle than it is, say, in our latitude. 

 This is important for us to bear in mind when we come to 



