2-347. Obscuration missions are important functions for mortars. Smoke missions must be planned well in advance so that the mortar carriers are loaded with a sufficient number of smoke rounds.
2-348. Atmospheric stability, wind velocity, and wind direction are the most important factors when planning target effects for smoke and WP mortar rounds. The effects of atmospheric stability can determine whether mortar smoke is effective at all or, if effective, how much ammunition is needed. The considerations are—
During unstable conditions, mortar smoke and WP rounds are almost ineffective—the smoke does not spread but often climbs straight up and quickly dissipates.
Under moderately unstable atmospheric conditions, base-ejecting smoke rounds are more effective than BFV bursting WP rounds.
Under stable conditions, both red phosphorous and WP rounds are effective.
The higher the humidity, the better the screening effects of mortar rounds.
2-349. The terrain in the target area affects smoke and WP rounds. If the terrain in the target area is swampy, rain-soaked, or snow-covered, then burning smoke rounds may not be effective. These rounds produce smoke by ejecting felt wedges soaked in phosphorus. These wedges then burn on the ground, producing a dense, long-lasting cloud. If the wedges fall into mud, water, or snow, they can extinguish. Shallow water can reduce the smoke produced by these rounds by as much as 50 percent. The terrain in the target area affects BFV bursting WP rounds little, except that deep snow and cold temperatures can reduce the smoke cloud by about 25 percent.