A stable condition exists when the air temperature increases with an increase in altitude. This
condition creates stable air currents and makes smoke linger for long periods. Under stable conditions,
smoke streamers tend to travel parallel downwind for long distances before they spread and 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. Stable conditions may keep smoke from rising high enough to cover
the tops of buildings or other tall objects.
When there is little change in temperature based on altitude, conditions are neutral. When the sky
is more than 70 percent covered with clouds, neutral temperature gradient conditions prevail. A wind
speed of 9 kmph or greater produces a neutral temperature gradient. The atmosphere is moderately
stable, and conditions are generally favorable for smoke. Under neutral conditions, smoke streamers
have steadier direction, and there is less tendency for them to rise than when they are under unstable
conditions. Also, streamers tend to spread and rise more quickly than under stable conditions.
Therefore, neutral temperature gradients are best for smoke.
Unstable conditions are just the opposite of inversion -- air temperature decreases with an increase
in altitude. Unstable conditions make smoke break up, because the air is moving. In low winds, during
unstable conditions, the smoke streamers may rise abruptly from the source. In higher winds, the
streamer may pass only a short distance downwind before rising and becoming diffused.
Light rains decrease visibility; therefore, less smoke is needed for concealment. Heavy rains and
snow so reduce visibility that smoke is rarely necessary to provide concealment.
Terrain. Since smoke is carried by the wind, it normally follows the contours of the earth. On flat
or unbroken terrain and over water, smoke streamers take longer to spread out and mix. On the other
hand, trees and buildings tend to mix smoke streamers and increase smoke coverage. Large hill masses
and rugged terrain cause strong cross currents that disperse smoke, causing holes and unevenness.
Fuel and Fog Oil Supplies. The smoke generator uses approximately 3 gallons of gasoline per
hour. Smoke is produced as fog oil is injected into the engine tube while the engine is running. Fog oil
is atomized by the hot combustion gases moving rapidly past the fog oil injection nozzle. The generator
vaporizes 25 to 50 gallons of fog oil per hour; 40 gph is normal.
By multiplying the number of generators by the length of generator operation (time in use), you
can determine approximate fog oil and gasoline requirements.
Learning Event 3
DESCRIBE THE USE OF SMOKE ON THE BATTLEFIELD
The deliberate use of smoke and the generation of dust on the battlefield have always been of
concern to units in action. When used to one's advantage, smoke can greatly reduce the enemy's
effectiveness both in daytime and at night. Smoke, combined with other weapons, will provide