ATP-3-09-30 Observed Fires Download

Special Munitions 28 September 2017 ATP 3-09.30 6-23 WEATHER 6-93. Atmospheric stability, wind direction, and wind speed are the major factors influencing the effectiveness of smoke. Observers should be aware of effects on civilian populations or friendly units downwind before employing these munitions. Atmospheric Stability 6-94. The weather conditions, the time of day, and wind speed all affect atmospheric stability. The effects of atmospheric stability can determine whether smoke is effective or not and if effective, how much ammunition is needed. (See table 6-15 for temperature gradient effects on smoke.) Table 6-15. Temperature gradient effects on smoke Time of Day and Weather Conditions Smoke Conditions (Temperature Gradient) Smoke Behavior (Wind Direction →) Night—until one hour after sunrise. Wind speed is less than 9 km per hour (5 knots). Cloud cover is less than 30%. IDEAL (stable or inversion) Day—most often between one and two hours before and after sunrise. Wind speed is 9 km per hour (5 knots) or more. Cloud cover is 30% or more. FAVORABLE (neutral) Day—beginning two hours after sunrise. Wind speed is less than 9 km per hour (5 knots). Cloud cover is less than 30%. MARGINAL (unstable or lapse) Note. ideal, favorable, and marginal are smoke conditions. Stable, inversion, neutral, and lapse are temperature gradients. Legend. kph—kilometers per hour 6-95. Temperature gradients are determined by comparing air temperatures at 0.5 meters and 4 meters. Stable, neutral, and unstable are the three general temperature gradients used. 6-96. Stable (or inversion) conditions exist when the air temperature increases with an increase in altitude. This condition greatly limits vertical air currents. Smoke produced during stable conditions lies low to the ground. 6-97. Neutral conditions exist when an increase in altitude is accompanied by little or no change in air temperature. Limited vertical air currents also cause neutral conditions when the wind speed is greater than 5 knots. 6-98. Unstable (or lapse) conditions exist when the air temperature decreases with an increase in altitude. Vertical air currents and turbulence characterize an unstable condition. 6-99. Under unstable conditions, HC and WP rounds are almost ineffective. The smoke does not spread but often climbs straight up in a pillar and quickly dissipates. 6-100. Under moderately unstable conditions, base ejecting smoke rounds are more effective than bursting WP rounds. 6-101. The higher the humidity, the better the screening effects of WP, RP, and HC rounds. Table 6-16 on page 6-24 compares the effectiveness of HC and WP to the relative humidity. 6-102. The observer can obtain detailed information and analysis of the meteorological conditions from the FDC.