.164 



RA1MUNDI 



RAIN 



Kaimondl. See MAKCANTONIO. 



Kain. Whatever lowers the temperature of 

 tin' air below the point of saturation, or the dew- 

 point, may be regarded as a cause of rain. Various 

 causes may conspire to bring about this change of 

 temperature, hut by far the most important of 

 these originate in winds and other movements of 

 the atmosphere. The more prominent principles 

 of the connection of the winds to the rainfall are 

 these : ( 1 ) When the winds have traversed a con- 

 siderable extent of ocean before reaching land the 

 rainfall is large; (2) when the winds, on arriving 

 at the land, advance into higher latitudes or into 

 colder regions the rainfall is largely increased, for 

 the simple reason that the air is now more rapidly 

 brought In-low the point of saturation ; (3) if the 

 wind.-, even though they arrive directly from the 

 ocean, have not traversed a considerable breadth 

 of it, the rainfall is not large indeed, in the case 

 of the sea-board of I/ower California the mean 

 annual amount, as at San Diego, is only 10 inches ; 

 (4) if the winds, even though they have crossed a 

 great extent of ocean, yet on arriving at the land 

 at once advance into lower latitudes or into warmer 

 regions, the rainfall is small; (5) if a range of 

 mountains lie across the onward course of the 

 winds, the rainfall is largely increased on the side 

 facing the winds, but reduced over the region on 

 the other side of the range ; the reason being that, 

 as the air on the windward side of the ridge is 

 suddenly raised to a greater height in crossing the 

 ridge, the temperature is still more reduced by 

 mere expansion, and a more copious precipitation 

 is the consequence. On the lee side, as the air 

 descends to lower levels, it gradually gets drier, 

 and hence the rainfall of necessity diminishes itli 

 every stage of the descent to lower levels. 



Attention may lie here drawn to the diminished 

 velocity of the wind over land as compared with 

 the open sea, as has been fully shown bv the obser- 

 vations of the Challenger expedition. From these 

 it has been proved that an envelope of stiller air, 

 or air of less velocity as compared with that of 

 the ocean, broods over the land, and bv its presence 

 forces the wind blowing across the land to a greater 

 height, thus augmenting the rainfall. This drag- 

 ging effect of the land on the wind, and the import- 

 ant consequences resulting from it, explain how it 

 is that during north-easterly storms of rain the 

 foreshores of the Firth of l-'orth, Moray Firth, and 

 Pentland Firth, which look to the north-east, 

 receive a much heavier rainfall than other purls of 

 Scotland in these circumstances. On the Ayrshire 

 coast the annual rainfall ut Ayr is 38 inches, but 

 at Uirvan it rises to 51 inches, lloth stations are 

 close to the coast, the only difference being that 

 the hills to the eastward approach much nearer the 

 coast ut ( iirvaii. 



For short ]ieriods the heaviest rainfalls occur 

 with thunderstorms, and with tornadoes, water- 

 spnnt-, and other foim- of tin; whirlwind, for the 

 reason that not only is their rapid expansion due 

 to tin- rapid ascent of the air, but also to great 

 rarefaction produced by the extreme velocity of the 

 gyrations of the air round the axis of the whirl- 

 wind. One of the heaviest rainfalls yet recorded 

 in the British Islands was 2'24 inches in 40 minutes 

 at Lednathie, Korfarsliire, during a severe thunder 

 storm on 18th June 1887. At OmbwwtU, Ixmdon, 

 3'12 inches fell in 2 bourn 17 minutes on 1st August 

 1846. Of heavy falls during one day the following 

 may be mentioned : lien NVvis Olwervatorv, 7"29 

 indies, 3d October 1890; Sealliwaile. (i 7H inches, 

 -'h May 1884 ; Tongue, 6-00 inches. 7th Septemln-r 

 IN70; Newport, Wale*, fl'33 inches, 14th .July 

 1875; and ( 'amusinas, Argyllshire, .V60 inches, 

 24th January 1868. 



In the United States where severe thunder- 



storms and tornadoes more frequently occur, the 

 daily rainfalls rejieatedty exceed these amounts. 

 Thus, during the years 1885-89 the following heavy 

 rainfalls of one day were recorded : Brownsville, 

 Texas, 12-94 inches in September 1886; Pensacolu, 

 10-70 inches, June 1887; Key West. 7 Ml indies, 

 Septemln-r 18K9 : Chattanooga, 7"61 inches. March 

 1886; Shrcveport, 7'54 inches, January 1885; and 

 a day's rainfall of from 5 to 7 inches is repeatedly 

 recorded in the United States. These amounts are 

 greatly exceeded in lower latitudes. Thus, on the 

 Khasi Hills, India, 30 inches on each of five suc- 

 cessive days have been recorded ; at Bombay, 24 

 inches in one night ; and at Gibraltar, 33 inches in 

 26 hours. 



As regards the ocean little is yet known from 

 observation. This, however, is clear, that in the 

 equatorial belt of calms between the regions swept 

 by the two trades the rainfall of the ocean reaches 

 the maximum ; and the parts of the Atlantic and 

 Pacific which are longest within the belt of calms 

 as it shifts northward and sonthwnrd with season 

 have the heaviest ocean rainfall. But, though the 

 cloud -screen is unquestionably dense and the ruin- 

 fall frequent and heavy, the observations of the 

 Challenger and the Kurtira show that the state- 

 ments generally made as to these are very greatly 

 exaggerated. Over the open sea in the regions of 

 the trades the rainfall is everywhere small, owing 

 to the circumstance that these winds are an im 

 mediate outflow from anti-cyclonic regions, their 

 dry ness being further increased since their course 

 is directed in regions that become constantly 

 warmer. 



The trades, however, deposit a larger rainfall 

 over islands and other land-surfaces which they 

 traverse ; the amounts being pro|>ortional to the 

 height of the land, but more particularly to the 

 degree in which the high land ranges lie across 

 the paths of the winds. Thus, at Ascension, 

 which is within the south-east trades the whole 

 year round, the annual rainfall is 8'85 inches ; and 

 at St Helena on the coast 5-36 inches, but on the 

 high land in the interior of the island it rises to 

 24 inches. In the western division of the Pacific, 

 for some distance on each side of the equator, the 

 rainfall is very slight, and extensive guano deposits 

 are formed on Maiden and other islands scattered 

 over that region. In Mauritius, on the weather 

 shore of the island, the annual rainfall is about 

 30 inches ; but at < 'limy. 16 miles inland, it is 146 

 inches, the latter place lieing in the neighbourhood 

 of extensive forest-clad mountains. 



The heaviest rainfalls of the globe are brought 

 by the winds which have traversed the greatest 

 extent of ocean within the tropics. These con- 

 ditions are most completely fulfilled from June to 

 September by the winds which, beginning their 

 course from aliout 30 S. lat., blow home on 

 southern Asia as the south-west monsoon, which 

 accordingly distributes a heavier rainfall over a 

 larger portion of the earth's surface than occurs 

 anywhere else in any season. On these summer 

 winds the rainfall of India chiefly depends, and 

 the ' lie ' of the mountain-systems with respect to the 

 winds intensifies the effects. The following in 

 inches are the annual amounts at different points 

 in the west from Surat southward : Surat, 41 ; 

 Itotnbay, 74 ; Mahabaleshwar, 263 ; Banca, 251 ; 

 Ilonawar. 139; Calicut anil Cochin, 115. In the 

 west of Ceylon the rainfall is also large, being at 

 Colombo 88, at (Julie 90, at liatnapnra, inland 

 among the hills, ITiO ; but in the east of the island, 

 l>efore reaching which the monsoon is deprived of 

 much of its moisture, it is considerably less, being 

 at Mannar 91, at Jaffna 49, and at Trinconialee 62. 

 The rainfall is also very large in Burma, lieing at 

 Akyab 194, Sandoway 214, Diamond Island 119, 



