winds. The greater the turbulence, the higher the level of the steering winds; the less the turbulence, the
lower the level of the steering winds.
(a) Wind speed. Wind speed influences the consistency of the smoke screen. Wind
speeds in excess of 15 knots (27 kilometers per hour) tend to break up the smoke screen; therefore, more
generators are needed in high winds. Wind speeds below 5 knots (9 kilometers per hour) change
direction often, causing smoke streamers to merge close to the smoke source and requiring the smoke
generators to be positioned closer to the smoke objective.
(b) Wind direction. Wind direction (Figure 1-15) determines where the smoke will travel
and, therefore, the position and/or maneuver pattern of the smoke generators. Wind direction for smoke
operations is classified as head, tail, flank, and quartering. Tail winds blow directly at the smoke
objective. Head winds are the opposite of tail winds. Flank winds blow directly across both the smoke
source and the smoke objective. Quartering winds blow at an angle toward or away from the smoke
(3) Weather. Weather conditions that affect smoke are temperature gradient, precipitation,
and cloud cover.
(a) Temperature gradient. Temperature gradients are classified in three types: inversion
(stable), neutral (favorable), and lapse (unstable).
(1) Inversion. Inversion occurs when the air temperature becomes progressively
warmer as it rises, which causes the smoke streamers to travel long distances before merging and linger
close to the ground for long periods. Smoke screens produced during inversion conditions provide
excellent concealment close to the ground but usually will not rise to cover the tops of tall objects.
Inverse conditions exist at night.
(2) Lapse. Lapse occurs when the air becomes progressively cooler as it rises,
causing the smoke to break up and to rise quickly into the air. Smoke screens produced under lapse
conditions offer poor concealment close to the ground. Lapse conditions exist in the daytime.
Neutral conditions occur when the air and the ground are
approximately the same temperature at a height of 1 meter off the ground. Under neutral conditions the
smoke streamers spread evenly and rise slightly, making neutral conditions the most favorable for
smoke operations. Neutral conditions exist in the early morning and in the late evening.