Figure 8. Wind speed estimate.
The effect of temperature on smoke is slight; however, the effect of different temperature gradients is
enormous. Under inversion (stable) conditions, smoke released in the wind close to the surface of the ground
tends to remain in the lower layers of air. As the turbulence in these lower layers is slight, the smoke streamers
spread slowly and are comparatively straight. Under inversion conditions, smoke streamers tend to travel parallel
downwind before they merge into a continuous blanket of smoke. Even after merging, this blanket of smoke lies
low to the ground and reduces visibility at ground level to an extremely low value; however, they might not rise to
sufficient height to cover building tops or other tall objects.
Under lapse (unstable) conditions, smoke released from a source near the ground produces a streamer that is
whipped about in an erratic manner by the turbulence of the air. In low winds, the streamer may rise abruptly
from the source. In higher winds, the streamer may pass a short distance downwind before rising and become
diffused as it rises.
Under a neutral condition, smoke behaves in a manner between its behavior under inversion and lapse
conditions; the direction of the streamers is steadier and the tendency to rise is less than under strong lapse, while