Geographic variations of the large-scale vertical exchange speed

From a global study of the radioactive fallout in the latitude range 30°–60°N during the period 1963 to 1967 five regularly spaced areas have been found in which the amount of nuclear debris is somewhat higher than the average. This phenomenon may be attributed to an inhomogeneity in the rates of exchange between the lower stratosphere and the mid-troposphere, possibly related to the standing waves of the polar jet stream. At lower altitudes the existence of a continental threshold layer, which is characterized by a lower coefficient of vertical exchange, gives rise to an excess of dry fallout over the oceans. This may explain the discrepancies in measurements of the amount of radioactive fallout over continental areas and over oceanic areas at the same latitude, as observed by various authors.