30 Report S.A.A. Advancement of Science. 



the West. Nearly all our clouds come from some westerly direction, 

 and since they pass over one of the driest areas in the world, 

 depositing very little moisture on the way, it follows that they must be 

 nearly or quite as rich in moisture as when they left the high levels 

 over the Atlantic. Therefore such difference as there is between the 

 absorption and radiation over land and sea must be more pronounced 

 in the lower levels than in the upper. 



Regarding the curious diurnal variations of pressure characteristic 

 of valleys and mountains, it has long been recognised that it is a 

 thermal result quite independent of the primary causes of the general 

 barometric oscillations over the earth's surface. 



Recently Prof. Bigelow has attempted a comprehensive theory of 

 the diurnal oscillations of all the chief meteorological elements. Here 

 again I quote at some length : — 



" Let us illustrate the formation of the double diurnal period 

 at the earth's surface and the single period in the cumulus level 

 by considering the behaviour of the absolute humidity, that is, 

 the number of grams of water vapor per cubic centimeter. The 

 first diurnal effect of the radiation from the earth is to raise the 

 vapor content of the atmosphere from the low level occupied by 

 it at night to a higher level during mid-day. This absorbing 

 screen of water vapor, visible or not, rises and falls once daily 

 through looo or 2000 meters, taken as a whole. While the warm 

 air rises by convection from the surface to the level of 1500 

 meters, the vapor rises with it and endeavours to saturate the unit 

 volumes of the higher strata at the prevailing lower temperatures, 

 the depleted lower volumes being fartiallv filled up again by 



fresh evaporation from the water and land surfaces 



There is a decrease in actual temperature with the elevation, and 

 therefore the saturated unit-volume content decreases. The 

 vapor sheet rises to higher levels, and this, together with the 

 fresh supply by evaporation from the surface, can refill the 

 depleted volume again, especially during the forenoon hours. 

 After the noon hour the continued increase of temperature gives 



rise to larger vapor capacity per unit-volume But 



while the rising vapor sheet keeps the upper volumes filled, the 

 lower, which are drained by the ascension of the water vapor, 

 can not be supplied by evaporation at the surface at a sufficiently 

 rapid rate to keep them full, because the prevailing surface 

 moisture has been taken up at an earlier hour. The same remarks 

 are true for the relative humidities. The result is that the upper 

 volumes are always full or relatively full, and have an increasing 

 actual content up to the early afternoon, about 2 p.m., so that 

 the diurnal curve at some distance above the ground has a single 

 maximum and minimum as observed. On the other hand, while 

 the 10 a.m. surface volumes are kept filled, or relatively filled, 

 they are actually depleted in the afternoon, and are not 

 replenished by evaporation up to the original relative humidity of 

 the morning, and therefore the curve shows a depression in the 

 early afternoon, and is doubly periodic. The second maximum 



