A previous experimental study of the nature of dispersion from point sources down-wind of three-dimensional hills of various crosswind aspect ration (spanwise breadth/height) has been extended to the case when the approaching wind is not normal to the spanwise axis of the hill. Surface concentration patterns resulting from sources placed at various heights have been examined, with attention limited to cases which led to the greatest concentration for the normal wind direction (theta=0). Sufficient data have been obtained to determine the terrain amplification factor (i.e., the ratio of the maximum ground-level concentration in the presence of the hill to that in its absence) for various wind directions, hills and source heights, and also to find how the ground-level concentration at the position of its maximum value for theta=0 varies with wind direction. It is demonstrated that in some circumstances the amplification factor (A) for a particular source position actually increases with small changes in wind direction. In general, however, there is a monotonic decrease in A as theta deviates from zero, which is most rapid for hills of small aspect ratio. In the case of wider hills, it is possible for the amplification factor to be reduced below unity for large theta.

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EPA/600/J-88/335.