32 



NATURE 



l^May 9, 1878 



speedily exhausts the air volume immediately affected by 

 the storm of its humidity, that the lines of equal annual 

 rainfall on this section of the coast are very close together, 

 marking a decrease of precipitation inland. The energy 

 of rotation increases here as the pressure at the storm 

 centre falls. This energy concentrates at the northern 

 end of the depression, and the area of low barometer is 

 drawn, as it were, around the centre so formed as it 

 passes eastward over the first range of mountains. After 

 passing over all the intervening ranges of the great 

 plateau toward the line of the Rocky Mountains in Mon- 

 tana and the British territory northward thereof, the 

 storm as a moving atmospheric vortex is attended by 

 only a very little rain or snow. TheVegion over which it 

 passes cannot furnish any supply of humid air, and the 

 storm becomes again disorganised into a great depression 

 during and after its passage over the mountains, until its 

 centre has reached the eastern slopes. But here it enters 

 a new region so circumstanced in its topographical rela- 

 tions Avith the east and south, as to derive a full and 

 uninterrupted iiow of humid air from the great river 

 valleys, the lake regions, and the distant Gulf of Mexico. 

 There are no intervening mountain barriers between 

 these sources of humidity and the north-western prairies 

 to interrupt the atmospheric flow toward the depression 

 extending over them, but the storm reorganises slowly at 

 first as the conditions necessary to induce a strong in- 

 draught of air to its centre are of very gradual develop- 

 ment. When, however, they come into requisite combi- 

 nation, the indraught winds increase, and coming from 

 the north-east and east, are deflected southward and 

 south-eastward by the mountains, until a feeble but 

 decided vortex is developed in the centre of the depres- 

 sion. The centripetal winds now begin to increase with 

 the inflow of humid air, and the newly organised storm- 

 centre moves eastward along its track, toward the region 

 of the Mississippi Valley or the lakes. In doing so it 

 descends the gradient of the plains through air of in- 

 creasing density, and acquires greater energy every mile 

 it advances. High pressures to the northward and 

 southward of the storm-centre constantly feed it with 

 fresh volumes of air, which being of different conditions 

 of temperature and humidity, produce the rainfall that 

 generally begins when the eastern margin of the depres- 

 sion enters the Missouri Valley. In the great region of 

 the plains the storm finds free scope for development as 

 well as an unfailing supply of atmospheric moisture. It 

 usually attains its greatest energy when passing over 

 Iowa, Illinois, Ohio, and Kentucky, toward the Upper 

 Ohio Valley, and the narrow neck of territory between 

 Lake Ontario and the Pennyslvanian section of the 

 Alleghany Mountain Range. This mountain wall in- 

 fluences the course of the storm by deflecting it toward 

 the north-east from the Middle Ohio Valley region, and 

 thence over New England to Nova Scotia. The districts 

 eastward of the Alleghany Mountains and southward of 

 New York are rarely traversed by storm-centres coming 

 as I have described, from the north-west, but receive 

 the rainfall of the eastern margin of the storm as its 

 centre passes north-eastward beyond the mountains, into 

 the St. Lawrence Valley or the New England States. 



But the mountains cause a profuse rainfall on their 

 western slopes, and when the storm reaches the Atlantic 



the precipitation has been nearly exhausted. Its energy, 

 therefore, decreases, when crossing from Oswego to 

 Portland or Eastport, Maine, and does not recover until 

 it receives from the Gulf Stream Region a new supply of 

 humid air. I have endeavoured to describe the course of 

 a storm-centre from the Pacific to the Atlantic, across the 

 Continent, and have made no detailed explanation of the 

 relation to its movement of the areas of high pressure. 

 This I regard as of the highest importance, and will treat 

 of fully under a special head. The course of the storm 

 across the Atlantic, as well as its movement over Europe, 

 will be governed only, I may say, by the high pressures. 

 These being distributed from south to north, in a series 

 of continuous, but movable zones, mark the directions of 

 the storm's advance so clearly, as to enable an observer 

 at this side of the ocean to predict with general accuracy, 

 the section on the European coast on which the storm- 

 centre will arrive, as well as the time of its arrival. 

 Another type of Pacific storm is that which arrives on 

 the southern and central section of the California coast 

 as a great depression, and entering the continent, pours 

 its rains over California, and becomes divided into two 

 sub-areas of low barometer by the Sierra Nevada range. 

 One of these sub-areas, and nearly always the largest, 

 takes a south-easterly direction across Southern Nevada, 

 into Arizona, and crosses the Rocky Mountains in New 

 Mexico to Northern Texas, where it is organised into a 

 storm in the same manner, but much more rapidly, as 

 the previously described area crossing into Montana. 

 The other sub-area passes from Central California 

 to Idaho, and thence across the Rocky Mountains, 

 into the Yellowstone River Valley in Montana, pur- 

 suing a track, thereafter, which sometimes brings 

 the depression into the Lower Missouri Valley, but 

 usually towards the Upper Lake Region. This sub- 

 area of low pressure also becomes organised into 

 a storm, but one of much less energy than that of 

 Northern Texas. This can be accounted for by the fact 

 that the crossing of the mountains by both sub-areas being 

 almost simultaneous, the northern depression cannot 

 receive any considerable atmospheric flow from the 

 southward, as it is intercepted and drawn toward the 

 southerly vortex. It sometimes occurs that the two 

 centres of disturbance unite in a common depression west 

 of the Mississippi River, but usually they preserve their 

 identity, and become separated gradually by an inter- 

 vening zone of relatively high barometer developed 

 between them by their joint influence. The northern 

 centre moves away to the north-east, over the lakes and 

 Canada, with diminishing energy, but the southern storm 

 centre advances into the Lower Mississippi Valley, and 

 soon dominates th6 weather conditions over all the region 

 southAvard of the lakes. In this position its isobars extend 

 eastward to the Georgia coast, and even into the Atlantic, 

 but the centre moves towards the Ohio Valley, westward 

 of the Alleghany Mountains. The consequence is that a 

 section of the depression near the Atlantic coast is cut 

 off" by the high range of the Alleghanies, and another 

 sub-area is formed which is speedily organised into a dis- 

 tinct storm centre by the impinging of the ondraught 

 winds from the east, north-east, and south-east on the 

 mountains, in the same manner as I have already 

 described. 



