402 



NA TURE 



[AuGUbT 25, 1904 



to a serious extent the crop returns over large areas, and 

 the rsiinfall was not compensatory. In four of these years 

 the drought was so severe and Vifidely spread as to occasion 

 famine, with its attendant calamities, over large areas. 

 Severe droughts and famines occur at very irregular 

 intervals. A noteworthy feature is that they frequently 

 follow in pairs ^separated by intervals of two to' four years. 



The previous statement of the meteorology of India has 

 mdicated the chief conditions which affect the crop returns 

 seriously or disastrously over large areas in India. They 

 may be summed up briefly as follows : — 



(a) The dry monsoon. Absence or unusual feebleness of 

 cold-weather storms. 



(h) The wet monsoon. General feebleness of the monsoon 

 current, due either to corresponding feebleness of the south- 

 east trades, or to unusual diversion to East Africa ; or local 

 feebleness in a part of India, due to local conditions, or to 

 abnormal diversion to other rainfall areas in South Asia. 

 These conditions g-ive rise in the areas affected to one or 

 more of the following features : — 



(1) Prolonged delay in the commencement of the rains. 



(2) .Scanty rainfall during the season, with prolonged 

 perioas of fine, clear, hot weather. 



(3) Early termination of the rains. 



These features are as a rule more marked in the drier 

 districts of the interior than in the coast districts. The 

 effect on crop production is greatest and most disastrous 

 in the following areas : — 



(i) Central Burma. 



(2) The Deccan, including the Bombay and Madras 

 Deccan districts, and Hyderabad. 



(3) North-Western and Central India, more especially the 

 South Punjab, East Rajputana, and the United Provinces. 



The following important inferences are based upon the 

 preceding presentation of facts and the experience of the 

 past thirty years ; — 



(i) The lower air movement of the south-west monsoon 

 is the northward extension of the lower movement of the 

 south-east trades. The latter is a permanent feature of 

 the Indo-oceanic region, and the former a periodic invasion 

 of the Southern Asian seas and peninsulas initiated over 

 equatorial regions and propagated northwards to the 

 southern mountain barrier of the Central Asian plateau. 



(2) The primary factors determining this impulse across 

 the equator (the first stage of the establishment of the 

 south-west monsoon) are to be sought in the permanent 

 field of the south-east trades, and are not due to actions 

 in the heated areas of Southern or Central Asia. 



(3) The pressure conditions in the heated areas of Southern 

 .\sia and North-East kW\ca determine the direction, volume, 

 and intensity of the advance over the Indian seas to what 

 may be termed three competing areas for rainfall (viz., 

 Abyssinia, India, and Burma). These conditions are hence 

 miportant factors in the third stage of the advance of the 

 south-west monsoon current. 



(4) The movement when fully established by these actions 

 over the Southern Asian seas and peninsulas'is continued — 

 first, by the momentum of the lower circulation ; secondly, 

 by the release of energy accompanying aqueous vapour 

 condensation ; and thirdly, by thermal actions in Southern 

 .Asia, due to direct solar activity. The termination of the 

 lower horizontal current by vertical movement occurs 

 irregularly over the areas of frequent heavy rain in Southern 

 Asia and Abyssinia, and not over a heated area in Central 

 Asia. 



(5) The total volume of aqueous vapour brought up by 

 this circulation not only varies in amount from month to 

 month during the season, but also from vear to vear. The 

 largest variations (seasonal and annual)' depend' chiefly, if 

 not entirely, upon actions in the source of suppiv — viz., the 

 Indian Ocean. If those actions determine an increased or 

 diminished supply across the equator into the Indian seas, 

 there is a corresponding variation in the total precipitation 

 of the three competing areas. ."Vmongst such causes and 

 actions may be prolonged and untimelv diversion of the 

 south-east trades into East Africa, as in 1896, or general I 

 weakness of the air movement over the Indian Ocean, 

 probably accompanying a displacement and decreased 

 intensity of the southern anticvclone. as in iSqci. 



((1) The relative distrihuli:,n of the total rainfall in the 



NO. 1817, VOL. 70] 



three areas of discharge of the aqueous vapour of the 

 monsoon currents probably depends upon the relative in- 

 tensities of the pressure conditions established during the 

 hot weather, which are continued for a part or the whole 

 of the monsoon by actions depending on the rainfall result- 

 ing from the initial pressure conditions — an example of the 

 persistence of meteorological conditions and actions which 

 is a prominent feature of Indian meteorology. The total 

 rainfall of each of the three areas may differ considerably 

 from the normal, but there may be partial or complete com- 

 pensation on the whole. Thus it is the general (but not 

 the invariable) rule that the rainfall variations in Burma 

 and Assam are usually inverse to those of North-Western 

 India and also of India as a whole. 



(7) The distribution of the rainfall in any one of the 

 three competing areas (but more especially in India as the 

 largest) may vary widely from the normal — considerable 

 deficiency in some areas accompanying considerable excess 

 in others. This in India is undoubtedly due to local con- 

 ditions — e.g., local excess or deficiency of pressure at the 

 commencement of the period and established during the 

 previous hot weather. These pressure variations usually 

 accompany abnormally prolonged and heavy snowfall or 

 very scanty snowfall in the Western Himalayas. 



(8) Local or general drought in India during the south- 

 west monsoon may hence be due to — 



(a) General weakness of the south-east trades circulation. 



(i) Diversion of an unusually large proportion of the 

 south-east trades to South-East or East Africa during the 

 monsoon period. 



(c) Larger diversion than usual of the monsoon currents 

 to Burma or .Abyssinia. 



{A) \'ery unequal distribution in India itself, due to local 

 conditions established during the antecedent hot weather. 



These factors are given in the probable order of their 

 importance. 



(9) Scanty rainfall or drought during the dry season or 

 north-east monsoon in Northern India results from absence 

 or unusual feebleness of the cold weather storms which are 

 the sources of rainfall at that time. 



(10) The most prolonged and severe droughts in North- 

 western and Central India are due to the partial or complete 

 failure of the rainfall of at least two seasons in succession. 



(ti) .As the two circulations in the Indian oceanic region 

 have a common goal in the dry season (more especially from 

 December to March), it is probable that variations in the 

 strength of one circulation (more especiallv the larger) will 

 modify the field and strength of the other circulation. It 

 appears that this relation would be shown most strongly 

 between the southern circulation and the upper movement 

 of the northern circulation. And, as cold-weather storms 

 are disturbances in that upper movement, it is possible — 

 if not probable — that the larger variations in the number 

 and intensity of the cold-weather storms and the amount 

 of the cold-weather precipitation may be related to con- 

 ditions in the south-east trades regions. 



(12) There appears to be little or no relation between the 

 position and intensity of the Central Asian anticyclone and 

 the number of the cold-weather storms and rainfall of 

 Northern India in any season. 



The meteorology of the period 1892— 1902 is of especial 

 interest for its confirmation of the above inferences, more 

 especiallv the phenomena of the variations of rainfall in 

 India and the causes or actions to which they are due. 

 The year i8gt was noteworthy for a severe local famine 

 in Rajputana and the adjacent districts to the north and 

 east consequent on prolonged and excessive snowfall in the 

 Western Himalayas during the winter of 1890-91. The 

 following gives a brief summary of the moi'e prominent 

 features of the meteorology of this unique period : — 



(i) The eleven-year period 1892-1902 corresponds in length 

 to the sun-spot period, and it may be divided into two 

 periods of unequal length — a short period of excessive rain 

 and a long period of deficient precipitation. The maximum 

 of the first period was in 1893. The second period had 

 three strongly marked minima in 1896, 1899, and 1901, that 

 of 1899 being the absolute minimum. The following table 

 gives, for convenience of reference, data of the mean annual 

 and seasonal variations of rainfall of the Indian land area 

 for each ye.ir of the period : — 



