10 



UPPER -WIND OBSERVATIONS OF LAST CRUISE OF CARNEGIE 



trades with the relatively high velocity up to 10 m per 

 second at a height of 3.5 km. When the other two flights 

 reached above 3 km in latitudes 7° and 8° south, the 

 winds were very light and decreased sharply from the 

 velocities at 2 and 2.5 km. This velocity decrease 

 doubtless is owing to the thinning out of the northeast 

 trade air mass as it projects itself southward over the 

 equator. The presence of northeast winds so far south 

 of the equator at this season is not unusual, as a few 

 unpublished observations from Nassau Island have indi- 

 cated. 



Flights 167 to 171, 

 November 14 to 17, 1929 



These flights were made northeast of the Samoan 

 Islands between latitudes 10° and 14° south. Light 



breezes from north to east were continuous from No- 

 vember 11 to 15, when southeast and south airs were 

 recorded in the afternoon, and though interrupted by long 

 periods of calm, were observed until November 17. On 

 this date, after slowly going through southwest, west, and 

 and west by north, the winds settled down from north- 

 west. The weather continued beautifully clear and with- 

 out rain, so that the balloons were followed to consider- 

 able heights. The winds altogether were extremely var- 

 iable owing to the conflict between northeast and south- 

 east trades. Thus, at a height of 0.25 and 0.50 km on 

 November 11, 15, and 16, winds were northeast, whereas 

 on November 14 and 17 southwest winds occurred at 

 these levels. In general the northeast trades dominated 

 the situation southward to latitude 13° south up to a level 

 of 4 km, but on November 15 and 16 southeast winds oc- 

 curred above the northeast current from 3.5 to 8 km. 



GENERAL CIRCULATION OF WINDS OVER THE TROPICAL REGIONS OF THE PACIFIC 



The pilot-balloon observations made over consider- 

 able areas are advantageously discussed together to ob- 

 tain a picture of the general circulation of the atmos- 

 phere. The upper winds in individual flights reflect not 

 only the permanent or seasonal conditions, but also the 

 passage of ephemeral centers of high or low pressure. 

 These passing conditions of weather have been dealt with 

 in the previous section and only the permanent state of 

 atmospheric movement will be discussed here. Fortu- 

 nately, most of the Carnegie flights were made within 

 the tropics where the regularity of weather conditions is 

 unbroken except for the passage of cyclones. Little gen- 

 eralization can be made from the flights in temperate 

 latitudes imtil further data which will include tempera- 

 ture and humidity have been obtained. The onlyaerolog- 

 ical observations over the South Pacific that have been 

 published are those taken by Dr. Harry Meyer in 1909- 

 11 and analyzed by P. Perlewitz [3]. 



Shallowness of Southeast Trade Winds 

 Off the Peruvian Coast 



The southeast trade winds in the eastern South Pa- 

 cific from the equator to at least 15° south become a 

 very shallow layer as they near the coast of the South 



Table 3. Frequency of winds from northwest quadrant 

 at Peruvian stations 



American continent. Within 200 km of the Peruvian 

 coast the northwest wind develops definitely at levels of 

 about 1 km. In the few mountain stations in Peru, where 



wind observations are recorded, the surface winds as 

 shown in table 3 blow mainly from between north and 

 west [4]. 



The wind shift from southeast to northwest is ex- 

 tremely abrupt, so that the top of the southeast trades 

 lies only 200 to 500 m below the stratum of northwest 

 winds. The shift is regularly from southeast through 

 south and west. 



A series of cloud observations by R. deC Ward made 

 at Arequipa, Peru in October -November showed without 

 exception 194 cirrus observations from the northwest 

 quadrant. Cirrus clouds occur in equatorial latitudes 

 between 10 and 15 km high, and, since both balloon and 

 cloud are in agreement in showing northwest winds at 

 these heights, the northwest current, at least during the 

 months September to December, forms a deep current, 

 which is constant in direction from a height of 10 km or 

 lower up to the stratosphere. In the Carnegie observa- 

 tions made between latitudes 85° and 90° west the mean 

 height of the wind shift from the southeasterly direction 

 to northerly occurs at about a height of 1 km. Just south 

 of the equator the discontinuity surface between trades 

 and antitrades slopes up more abruptly nearer the con- 

 tinent, and occurred at 1.8 km in flight 5, which was made 

 500 km off the coast. In flight 6 at 1000 km from the 

 coast there was no wind shift up to 6.2 km, whereas flight 

 7, farther to the west, showed southeast winds to 5 km. 

 The upper limit of the easterly drift over great areas of 

 the central South Pacific lies laetween 3.5 and 6 km, but 

 the southeast trades on many flights do not reach 1.5 km. 



The origin of these northwest winds within 20° of the 

 South American coast arises from the flow of northeast 

 trades across the equator, where their direction is 

 changed by the force of the earth's rotation. In the east- 

 ern Atlantic off the coast of Africa the trades are simi- 

 larly shallow, and are also found at greater heights as 

 one goes west to the central and western Atlantic [5]. The 

 northeast trades are made up in large measure of a great 

 current of air which has crossed Central America at low 

 points in the mountain range. This outflow represents 

 the emptying of polar air, which, in its previous history, 

 traversed the central United States and finally here com- 

 bines in the great equatorial circulation. 



