STATUS OF GLOBAL FALLOUT PREDICTIONS 387 Sundstrom! of Sweden has approached the same problem differ- ently. He assumes that the horizontal and the vertical mixing intensi- ties at a given point are both proportional to the variability of the horizontal component of the wind, an observable quantity. A single proportionality constant converts the wind variability to diffusion constants so that the latter may differ with altitude and latitude. In the first very simplified model, a single meridional circulation cell within each hemisphere was assumed to be present in the stratosphere. By a com- parison of the observed ozone distribution in the stratosphere with that expected from an estimate of photochemical production, the two unknowns can be found which yield a best fit between prediction and ob- servation, The first unknownis the factor converting wind variability to diffusion coefficients and the second is the sense and magnitude of the preassigned meridional circulation. There is one serious defect in the preceding approaches which,if corrected, increases the complexity of either the New York University — Weather Bureau iterative or Sundstrom’s statistical methods. This defect may be illustrated in Fig. 13. In this north—south meridional, cross-section isolines of concentration slope downward toward northerly latitudes as radionuclides frequently do in the northern hemisphere. It is assumed that the surface of most intense mixing occurs not along horizontal planes but parallel to sloping lines labeled M. Theflux of a tracer substance is directed from north to south along the gradient in the horizontal planes, but the flux along the M surface is from south to north. Since there is no a priori reason why the principal mixing to be oriented exactly along horizontal surfaces, a mixing model Fickian diffusion should be capable of allowing the mixing to take along any arbitrary surface. Davidson’s model incorporates this ought using place capa- UP 2) NORTH SOUTH Fig. 13—Schematic diagram of mixing along a sloping surface. Z = height, C = mixing ratio, and M = principal mixing surface.