Meteorological Features. 127 



Sutton and Mr. Stewart * strongly oppose the prevalent theory which 

 maintains that the moisture-laden winds from the Indian Ocean are 

 the primary source of the South African rains. " Our rain," writes 

 Mr. Sutton (for Cape Colony) " originates on the Equator, being 

 carried hither in the upper atmospheric currents flowing from West 

 and North. The rain begins on the East Coast when the moist upper 

 stratum meets the lower bodies of air, damp with the moisture from 

 the Indian Ocean. Then it gradually works back from the East coast 

 as the Eastern air becomes heavier with vapour." 



The idea that the rains are due to a mixture of the upper and 

 lower currents seems to me to explain, partly, at least, what takes place 

 in South-Western Rhodesia. Rain occurs here mostly when the 

 conditions for such a mixture are fulfilled, i.e., when there is a 

 barometric depression and an increase of temperature during the 

 periods of North or veering winds. There is then a double reason for 

 an upward movement of the lower air. Judging from the cloud 

 observations taken at 8 a.m. and 8 p.m. during the last two years, I 

 think that the shell of the atmosphere affected by the lower currents 

 must be rather shallow, and that a small change in the temperature 

 and pressure will, on that account, force very rapidly the lower 

 current mto the upper one. The higher and middle clouds move, 

 we may say, invariably in the same direction, which is Eastwards 

 from a point perhaps slightly North of West. This direction is, as 

 far as I can judge, constant the whole year round. 



As I shall show later, the mean surface wind is almost exactly 

 opposite, blowing from a point between E. and E.S.E. This, 

 however, leaves open the question whether the moisture comes 

 originally from the upper West or North-West current or from the 

 lower easterly one. I incline to believe that a great part, the greater 

 part, perhaps, of the moisture comes direct from the Indian Ocean by 

 the East and South-East winds. These, as a rule, it is true, give us 

 clear weather, but there are several indications which point out that 

 they must be rich in moisture which is to be condensed and brought 

 down as rain. There is, first, the fact that the mean relative 

 humidity is generally higher in the East wind periods than in the 

 others, and that, even in the rainy season, when there is little or no 

 rain in the former, whilst theie is often a large amount in the latter. 

 It would, however, be advisable to study the dew point and the 

 absolute humidity in connection with the relative humidity to settle 

 satisfactorily the question whether the East winds are really more 

 moist than the others. Another fact also is that a drop in the 

 temperature readily brings forth clouds on East wind days. There 

 is here a particular kind of cold weather for which the natives have 

 a special name; they call it " Amakassa.'' It is exclusively 

 connected with East to South-East winds. On these occasions we have 

 very low, fast-moving cumulus clouds of ragged appearance. They 

 often coalesce into a cumulostratus, covering the whole sky and 

 invariably move in the same direction as the surface wind. They 



"An introduction to the study of S.A. Rainfall." T. R. Sutton. Travst. of 

 f^.A. PMl. Soc. Vol. XV. P.I. "Meteorology in South Africa," Ch. Stewart. 



