300 HOW RAIN IS FORMED. 



distribution of ntinosplieric pressure as indicated by tbe barometer is 

 the result of the sun's action in equatorial regions. It is this that gives 

 the motive j^ower to the whole system, so far as we have as yet traced 

 it, and it is this that produces those great inequalities of atmospheric 

 pressure that I have so far described. 



It remains now to see how storms are generated by these westerly 

 winds. In so far as they retain any southing, they are still moving 

 towards the pole in the northern hemisphere; that is to say, they are 

 advancing from all sides towards a mere point. Some portion of them 

 must therefore be continually turning back as the circles of latitude be- 

 come smaller and smaller. But they are now surface-winds, and in 

 order so to return they must rise and flow back as an upper current. 

 This they do by forming great eddies, or air-whirls, in the center of 

 which the barometer is very low, and over which the air ascends, and 

 these great air-whirls are the storms of the temperate zone and of our 

 latitudes. It is the ascent and dynamic cooling of the air in these great 

 eddies that cause the prolonged rain-fall of wet stormy w^eather. How 

 the eddies originate, or rather what particular circumstance causes them 

 to originate in one place rather than another, we can scarcely say, any 

 more than we can say how each eddy originates in a rapidly-flowing 

 deep river. Some very small inequality of pressure probably starts 

 them, but when once formed, they often last for many days, and travel 

 some thousands of miles over the earth's surface. 



Two such storms are represented on the charts of February 1 and 2, 

 1883, one on the coast of Labrador, the other to the southwest of the 

 British Isles. The first of these appears on the chart of January 28, in 

 the North Pacific, off tl-e coast of British Columbia. On the 29th it liad 

 crossed the Rocky Mountains, and was traversing the western i)art of 

 the Hudson's Bay Territory. On the 30th it had moved to the south- 

 east, and lay just to the west of the Great Lakes, and on the 31st be- 

 tween Lake Superior and Hudson's Bay. On February! it had reached 

 the position on the coast of Labrador shown in the chart, and on the 

 2nd had moved further to northeast, and lay across Davis's Straits, 

 and over the west coast of Greenland. After this it again changed its 

 course to southeast, and on February 4 passed to the north of Scotland, 

 towards Denmark, and eventually on to Russia. 



The second storm had originated off the east coast of the United States 

 between January 28 and 29, and on the following days crossed the At- 

 lantic on a course somewhat to north of east, till, on February 2, it lay 

 over England. 



These storms always move in some easterly direction, generally be- 

 tween east and northeast, and often several follow in rapid succession 

 on nearly the same track. It is this knowledge that renders it possible 

 for the Meteorological OfiQce to issue the daily forecasts that we see in 

 the newspapers. Were it possible to obtain telegraphic reports from a 

 few stations out in the iS^orth Atlantic, these storm w arnings could be 



