Smoke and odors rise in the air and move downwind. As the plume moves, it spreads vertically and laterally, sometimes with erratic and agitated motion, due to turbulence in the atmosphere. Turbulence is caused by heat rising from the earth's surface and by wind moving at different speeds above and below each other.
Turbulence causes outside air to mix with the plume.
Another term for the mixing caused by turbulence is dispersion . Odors are dispersed and diluted when outside air mixes with the plume. Odors are scattered far and wide under good dispersion conditions. Odor intensity decreases as you move away from the cause of odors in a well-dispersed plume. Weather Effects on Dispersion Figure 1 shows how weather factors affect the shape of an odor plume.