TRADE WINDS 



13 



cloud — it was difficult to recognize, and increased the 

 other difficulties which are inherent to observing on 

 ship. 



The flights were made when conditions were favor- 

 able for observing, and naturally the sky had little cloud 

 in the anticipated direction of the balloon's flight. Fre- 

 quently, however, clouds were present in other quarters 

 of the sky. Cumulus clouds were almost exclusively re- 

 corded, and mostly of the fair-weather type (cumulus- 

 humilis). It would appear that strato -cumulus clouds 

 were included under the term cumulus. From latitude 

 20° north to 20° south in the equatorial region cumulus 



clouds predominated, frequently in broad strata from 

 1.2 to 2.2 km and from 3.7 to 5.5 km. In the North and 

 South Pacific outside this tropical belt the most common 

 occurrence of clouds was from 0.8 to 1.5 km. 



Observation of cirrus clouds during balloon flights 

 was rare. In flight 59 (latitude 13.°0 south, longitude 

 119f8 west) the balloon was lost in cirrus clouds at 7.6 

 km, and in flight 66 (latitude 17.°1 south, longitude 135.°5 

 west) at 10.9 km. 



Stratus-cloud heights were measured on several oc- 

 casions in the northern Pacific at heights varying from 

 0.4 to 2.6 km. 



TRADE WINDS 



Variation in Velocity with Height 



In flights 40 to 49, made where the trades are well 

 developed off the South American coast, the maximum 

 mean wind velocity, 7 m per second, occurred at the 

 second minute of the flight, and the most rapidly moving 

 stratum occurred between a height of 200 and 400 m 

 above the surface. Farther west, midway between the 

 Tuamotu Archipelago and South America, where the 

 trades are most strongly developed in the South Pacific, 

 the mean maximum velocity of 10.3 m per second oc- 

 cxirred at the fourth minute, and the whole stratum from 

 400 to 1200 m had a mean velocity of 10 m per second. 

 Above this stratum the wind velocity decreased rapidly 

 to 3 m per second at the level of 3 km, at which the 

 smallest wind velocities occurred over the eastern trade- 

 wind region traversed by the Carnegie during the south- 

 ern summer. Farther west in the Pacific the Carnegie 

 observations indicate that the height of air stratum of 

 minimum velocity is somewhat higher, namely, 4 km 

 (fig. 45b). This agrees with the upper -wind velocities 

 determined only on days comparatively free from clouds 

 at Apia in longitude 171f8 west, which indicate a broad 

 minimum from 1.5 to 4 km with the lowest value of 3.9 

 m per second at 3.5 km. 



Stratification of Trade Winds 



The winds over the Atlantic Ocean have been recog- 

 nized by almost all observers to have a stratiform char- 

 acter. At certain levels, good for all latitudes, changes 

 in the structure of the air strata over the ocean are 

 clearly marked in temperature records, but they can al- 

 so be seen in changes in wind direction and less clearly 



in wind velocity. These wind shifts are not those asso- 

 ciated with cyclones, anticyclones, and large scale pres- 

 sure distributions, but arise from turbulence and fric- 

 tion in air strata. 



Since the horizontal projection of the balloon's posi- 

 tion was determined only for each minute, close approx- 

 imation of the height of a discontinuity cannot be given. 

 The criterion used in determining the discontinuity level 

 was a wind shift, especially when accompanied by a 

 change in velocity. An inspection of the plotted points 

 (tab. 5) representing the balloon's position on a horizon- 



Table 5. Heights where wind shifts occur, indicating 

 change in air stratum 



tal projection (figs. 6-27) shows that they lie usually on 

 a succession of straight lines representing various dis- 

 tinct strata of air. In some flights the personal factor 

 enters considerably in determining the minutes where 

 the strata may be considered to begin or end. The writ- 

 er and another person studied each flight to avoid or at 

 least to reduce the personal element. 



LITERATURE CITED 



1. Hann, J. v. Lehrbuch der Meteorologie, 4th ed., p. 



151 (1926). 



2. Ault, Captain J. P. Jour. Terr. Mag., vol. 34, pp. 



254-255 (1929). 



3. Perlewitz, P. Ann. Hydrogr ., vol. 40, p. 454 (1912). 



4. Knoche, K. Klimakunde vonSudamerika, p. 279 (1930). 



5. Sverdrup, H. U. Der Nordatlantische Passat, Veroff. 



Geophys. Inst. Univ. Leipzig, vol. 2, Heft 1, p. 51 

 (1917). 



6. Beals, E. A. Mon. Weath. Rev., vol. 55, pp. 224-226 



(1927). 



