CONDITIONS OF THE TRADES-MONSOON AREA. 159 



ENQUIRY INTO THE PRESSURE AND RAINFALL CON- 

 DITIONS OF THE TRADES-MONSOON AREA. 



BY W. L. DALLAS. 



{Read April 14, igoj.) 



In 1900 the writer undertook the discussion of the seven mon- 

 soon seasons 1893 to 1899 and showed that during those seven 

 years there occurred a series of oscillations of pressure and that be- 

 tween these oscillations and the monsoon rainfall over India there 

 existed a very distinct and marked relationship. The data used in 

 this discussion consisted of the mean monthly and seasonal varia- 

 tions of pressure over India, derived from all the stations employed 

 in the Daily Weather Report of the Meteorological Department, 

 and the mean monthly and seasonal variations of pressure over the 

 Equatorial Belt and the Arabian Sea as given by the pressure obser- 

 vations recorded (1) at the Seychelles, Zanzibar and Mauritius and 

 (2) on board ships traversing the Arabian Sea and the South-east 

 Trades Region. 



The relationship as established for those seven monsoon seasons 

 was as follows: (1) The Indian monsoon rainfall was in defect 

 during the rising portions of these pressure oscillations and in 

 excess during the falling portions while the amount of the rainfall 

 variation agreed directly with the rapidity of the pressure changes. 

 (2) The pressure oscillations exhibited a periodicity of about four 

 years. 



It was carefully pointed out at the time that the discussion dealt 

 solely with the seven years under review so that, though the agree- 

 ment there disclosed was exact and clear, it was obvious that a 

 much longer series of observations would be required before it 

 would be safe to assert that the period of the oscillations and 

 the relationship between the pressure oscillations and the rainfall, 

 as disclosed in the discussions, could be accepted as having a gen- 

 eral application. As a matter of fact, before the publication of the 

 paper, it had already become apparent that the relationship had 

 not been maintained, while a simple examination of the existing 

 rainfall data of India showed that there does not obtain any simple 



