Land slope also changes the shape of the plume. Dispersion is increased if prevailing winds carry the plume up-slope. Down-slope or down valley winds tend to concentrate odors in lower lying areas. Gravity pulls the plume into valleys and low spots. Cold air drainage plays a very large role in the movement of agricultural odors.
Most farmstead odors are heavier t han air, increasing their tendency to settle in low-lying areas.
Odors may travel great distance under such dispersion conditions, following the course of streams and valleys by cold air drainage. Weather conditions at 3 p.m. on April 16, 1998. Colors on the map correspond with the colors of the dispersion categories given in Chart 1. Wind travels in the direction from the barbed end of the staff toward the solid circle at the station site. Forecast map for dispersion conditions at 1 p.m. on April 17, 1998.