Odor Plumes and the Signal They Provide

Atmospheric turbulence causes vigorous mixing of the air at a wide range of spatial and temporal scales. The largest eddies are hundreds of meters in extent and, according to the wind speed, can take minutes to pass a fixed point. They give rise to the continual changes we notice in wind direction. The smallest eddies are tens of millimeters in size and can pass in a few milliseconds. Material released from a point is entrained into the wind and carried away from its source. As it flows, in a plume, it is taken up by the turbulence. The filaments of air containing the material are stretched and distorted by the turbulence until they form the kind of pattern we see, for example, in the smoke blowing from a chimney. The fine-scale structure in an odor plume arises, similarly, through the action of atmospheric turbulence. Consequently the concentration at a stationary point fluctuates, at a frequency of some tens of cycles per second (Hz) (Jones 1983; Murlis et al. 1992).