The snow-generating elements associated with most snow storms have been observed in previous radar studies only in vertical section. Constant-altitude upper-level maps, composed from a series of plan-position-indicator photographs at progressively increasing elevation angle, reveal the plan pattern formed by these cells at the generating level.
Individual cells are rather shapeless, but with all dimensions within a factor of two of 1 mile, and persist for hours. Their velocity equals the generating-level wind, with a mean difference (including errors of meausrement) of 1.6 knots. The cells occur in groups of several hundred, either randomly distributed over an area of about 104 mi2 or in line arrays each about 5 mi by 75 mi over the same area. Whether or not arrays are formed, and the orientation of the arrays relative to the generating level wind when they do occur, are related to the component of wind shear along the wind at the generating level.