A comparison of tritium and strontium-90 fallout in the Southern Hemisphere

The histories of THO and Sr-90 fallout in the Southern Hemisphere are compared for the data available to June 1966.The seasonal variation pattern (July–June) for THO in mid-latitude oceanic rainwaters is fairly closely reproduced from year to year, and peaks in late winter–spring. The peak coincides with the absence of the temperature inversion at the polar tropo-pause, suggesting that the stratosphere-troposphere exchange of the bulk of Southern Hemisphere THO fallout takes place over the polar regions.For Sr-90 in mid-latitude rainwaters, the seasonal variation pattern is not so closely reproduced from year to year, but shows a broad summer maximum and winter minimum. It appears that the bulk of the bomb particulate products mixes southward within the lower stratosphere and enters the troposphere at mid-latitudes, the seasonal fallout variation being similar to that found for the Northern Hemisphere, but 6 months out of phase.Consideration of the balance of THO in the troposphere leads to a relation which allows a seasonal variation pattern for the stratosphere-troposphere exchange of THO to be deduced from the rainwater concentrations.Stratospheric measurements of bomb particulate products are taken into account in an assessment of Sr-90 southward mixing paths. An attempt is also made to deduce the origin and possible southward mixing paths for thermonuclear THO appearing in the Southern Hemisphere.The history of the THO/Sr-90 annual (July–June) mean activity ratio is quite erratic, and it is not constant over the entire Southern Hemisphere at any one time.