5.7 SOUTHERN HEMISPHERE wl ‘ ak. Be allitiachaoie, it . Despite the fact that the Marshall Islands are very near the southern hemisphere (only 11 degrees north of the equator), the only reports of ‘activity in excess of 300 d/m in the southern hemisphere came from stations essentially on the equator; namely _ Canton Island and Quito, Equador. The activity collected in the § 5 : Ter biased WS ene a low - values = anaes southern hemisnhere is probably toward because got eS rem we See er wh the practical restrictions imposed on organizing a radiological -metwork placed the guamed papers in dry areas. Stations selected on the basis of a hizh precipitation frequency would collect samples more representative of southern hemisphere maxima. The IVY network, however, does provide acceptable evidence that the - fallout in the southern hemisphere is relatively insignificant. The interesting question of cross-equator flow is best considered in two sections - the trade wind layer and the levels above the The northern hemisphere trade winds have a component toward the south, the southern hemisphere trades a component toward the “north (see figures 5.1, 5.2, and 5.3) so that low-level debris approaching the southern hemisphere from Eniwetok could not, on the average, penetrate very far into the southern hemisphere. Rising currents in the convergence zone might carry material aloft -and spew it into the southern hemisphere a few degrees south latitude above the trades, but as the debris returned to earth it would enter the southeast trades and be carried northward. In addition to dilution associated with such a complex path, the depletion by rainout would be large because the mixing zone between the trade winds is likewise the zone of great rainfall. It is, therefore, difficult to conceive of the trade wind portion of any atomic cloud being a significant source of fallout in the southern hemisphere. ' Radioactivity initially in the upper troposphere and the stratosphere is transported zonally, that is, to the east or west nanatad, rather than north-south. There are, to be sure, appreciable north and south components, but these are the consequences of waves On a zonal current that rarely have an amplitude, insofar as is known, greater than fifteen degrees of latitude, the trajectory returning to its original latitude more frequently than not. The intensity of the north-south motions, and thereby their effectiveness in transporting debris poleward, increases with increasing latitude because of the storminess of temperate latitudes. For this reason, more debris is transported to Canara than to Australia, despite the closer proximity of the latter to the test site. High-- level data-and analyses for the tropics are meager, but there is . oO -