217 

 The cross section made from August 23- 30, which extended in a line 

 closely parallel to the direction of flow and indicates well the modifications 

 introduced as the air moved downstream over the sea surface ig illustrated 

 in Figures k to 6. It starts in a position near 10 N, and 57 E, very close 

 to theQcenter of colder temperatures, and extends to a position near 20TT, 

 and 63 E. Figure h shows the temperature field. The base of the inversion 

 or top of the moist layer appears at the 96O mb level in the upstream end; 

 it rises slightly in the first 6OO miles, and more rapidly afterward. In 

 the downstream end it was observed at the 850 mb level. The depth of the 

 stable layer did not vary much gownstream. At the surface there was an 

 increase in temperature from 2k to 26 C between the upstream and downstream 

 end, which no doubt resulted from heating from the water surface. At the 

 850 mb level the temperature decreased from 20-21 C in the upstream end to 

 around 16 C in the northeastern or downstream end. 



The field of potential temperature. Figure 5? indicates as one of its 

 most interesting features that the |00°K line followed closely the level of 

 the base of the inversion. The 30^ K isoline also followed closely varia- 

 tions in height of the inversion layer. This is evidence of rising adiabatic 

 motion as the monsoon air moved downstream over the ocean, since in the lower 

 layer we can presume with a great degree of validity that the flow followed 

 closely along the cross section line. At upper levels it cannot be similarly 

 assumed that the flow followed parallel to the cross section. The potential 

 temperature lines above the inversion were displaced generally downward 

 downstream. An attempt to construct an isentropic chart for a level in the 

 dry air mass aloft was not too successful, but there appeared to be a tendency 

 for downslope motion between the west and east sides of the sea, a tendency 

 that is also supported by the data in Figure 5- 



The most importajit feature of the moisture distribution ( Figure 6) 

 is the increase in moisture content downstream near the surface, evidence 

 also of the exchange processes between the sea surface and the air above . 

 The tendency for higher moisture content in the upper levels near 500 mb in 

 the eastern end of the section is also evident. In computations carried out 

 over the central Arabian Sea average evaporation rates of 60O-7OO cal cm 

 day" were obtained. 



An analysis of the distribution of the height of the base of the inversion 

 or mean depth of the surface moist layer is shown in Figure 7- It indicates 

 a shallow moist layer or low inversion in the western section of the Arabian 

 Sea, specially near the coasts of . Arabia and Somali, and a rise eastward 

 toward the coast of India. 



In the western half of the Arabian Sea all available soundings revealed 

 the presence of the inversion near the surface . A few aircraft soundings 

 available in the northern comer near Arabia and Pakistan revealed a very 

 pronounced inversion close to the surface. In the eastern third, near the 

 coast of India, most of the available observations showed absence of the 

 inversion. The stratification was generally indicative of disturbed weather 

 conditions with lapse rates close to the moist adiabatic and high moisture 

 content extending to 500 mb and above. To the south, near the equator, the 



