When a liquid agent is used to cause casualties through contact with the liquid in crossing or occupying the area, its duration of effectiveness is greatest when the soil temperature is just above the agent's freezing point. This limits the rate of evaporation of the liquid. Other favorable conditions are low wind speed, wooded areas, and no rain.

Conversely, unfavorable conditions are high soil temperature, high wind speed, bare terrain, and heavy rain.

Favorable and unfavorable conditions for liquid agents for vapor concentration effects are much the same as those for chemical clouds. In woods, however, a high temperature with only a very light wind gives the highest vapor concentrations.

Duration of the effectiveness of initial liquid contamination may be affected by wind speed; stability, mixing height, and temperature; and precipitation.

Wind Speed

Wind direction is important in determining the upwind side of a target for release purposes but has little impact on the duration of effectiveness, regardless of the method of release The vapor created by evaporation of the liquid agent, however, moves with the wind. Therefore, the vapor concentration is greatest on the downwind side of the contaminated area. Vapors are moved by the wind as discussed earlier in this chapter.

Evaporation due to wind speed depends on the amount of the liquid exposed to the wind (the surface of the liquid) and the rate at which air passes over the agent. Therefore, the duration of effectiveness is longer at the places of greater liquid agent contamination and in places where the liquid agent is sheltered from the wind.

The rate of evaporation of agents employed for persistent effect in a liquid state is proportional to the wind speed. If the speed increases, evaporation increases, thus shortening the duration of effectiveness of the contamination. Increased evaporation, in turn, creates a larger vapor cloud. The vapor cloud, in turn, is dispersed by higher winds. The creation and dispersion of vapor are a continuous process, increasing or decreasing in proportion to wind speed.

Releasing agents for persistent effect by point dispersal via bombs, shells, rockets, or land mines results in an unevenly distributed contaminant. Heavier concentrations of the liquid are found around the point of burst. Lighter concentrations result farther from the bursting position. There probably will be small areas between the points of burst that are not contaminated, depending upon the number of munitions used and the uniformity of dispersal.

Liquid agents released in the form of a spray are fairly evenly distributed, exposing the maximum surface area of the contaminant to the wind. This results in a more rapid evaporation than when the liquid agent is unevenly dispersed (as with bursting munitions). With spraying, the duration of effectiveness decreases, and there is a corresponding increase in the vapor concentration downwind from the sprayed area.

Some chemical agents have no significant vapor pressure, and, consequently, their rates of evaporation are not affected by wind speed. Also, some of these agents are extremely toxic, so even a very slight surface concentration represents a massive overkill dosage. When agents of this category are released from spray munitions under low wind speeds, they cover only a narrow zone. When released under higher wind speeds, they cover wider areas more effectively. Thus, when downwind safety is not a limiting consideration, high wind speeds may be more desirable than low wind speeds for these very persistent agents.

Table 1-5. Summary of favorable and unfavorable weather and terrain conditions for liquid agent employment.

With agents that vaporize readily, high wind speeds may cause complete vaporization before the agent reaches the ground, creating only a vapor hazard. The resulting vapor cloud is nonpersistent and dissipates quite rapidly due to the high degree of mechanical turbulence associated with high wind speeds.

Turbulence has the same effect on agents employed for persistent effect, whether released from bombs, rockets, artillery shells, or land mines. Turbulence tends to reduce the duration of effectiveness in the liquid state by helping to increase the rate of evaporation. Temperature, rather than turbulence, has the greater effect on the duration of effectiveness of liquid agents. However, a contaminated area that has been subjected to pronounced turbulence does not remain contaminated as long as one that has been subjected to only slight turbulence with low wind speeds.

Turbulence also influences the spraying of agents employed for persistent effect. High winds and air movements divert the drops from the target or spread them over a larger area. Steep mountain regions sometimes produce large-scale eddies that prevent effective coverage of the target. Any vapor concentrations built up from sprayed areas are slight when the degree of turbulence is high.

Stability, Mixing Height, and Temperature

Unstable conditions are characterized by warmer surfaces. The solar heating then causes evaporation to be more rapid.

Temperature, velocity, and turbulence also affect the dispersion of spray. When stable (inversion) conditions prevail, there usually is little or no thermal turbulence, wind speeds are low, and the degree of mechanical turbulence is also low. Often stable conditions exist continually only near the ground. Above the top of the stable surface layer, wind speed and turbulence are increased. Wind direction here also may be substantially different from the surface wind direction. A chemical spray released below the top of the inversion falls fairly quickly. The height of the top of an inversion varies throughout the period of the surface inversion existence, and it may vary rapidly over large hills and mountains.

The mixing height is the capping inversion at the top of the mixing layer and serves as a lid. It prevents further upward vertical growth of a chemical vapor. A mixing height can also exist above unstable or neutral surface stability conditions. In radiation inversions, which commonly form at night, the top of the surface based (mixing) stable layer is very close to the earth's surface shortly after the neutral condition changes to a stable condition (soon after sunset). As the surface stable layer intensifies, its top rises, reaching its maximum elevation between 0200 and 0400 hours local time. Maximum elevation may be 400 meters in a very intense stable layer. In the morning, solar radiation heats the surface and causes a good mixing condition close to the ground. The mixing height and turbulence condition increase until they destroy the stable layer. The mixing height can extend from the earth's surface up to 2 kilometers in elevation on a hot summer day. On a calm, clear night, the mixing height may extend only 50 to 100 meters above the earth's surface.

If a chemical agent is released above the surface stable layer, most of the agent remains aloft in the turbulence layer, and most of it will dissipate before settling low enough to be effective. For this reason, most spray missions are flown at either sunrise or sunset to take advantage of a neutral temperature gradient. With this gradient, there is some vertical exchange of air, and the chemical spray, being relatively heavy, has a natural tendency to settle to the ground. The Air Weather Service or an assigned meteorologist can provide information on the mixing height and the height of the top of the surface stable layer.

Under unstable conditions, convection currents often catch many very small droplets and carry them upward above the level of release. As a result, the spray takes longer to reach the ground, and much of it may dissipate before reaching the target area.

Temperature is one of the most important factors affecting the duration of effectiveness and vapor concentration of liquid agents. Agents employed for persistent effect acquire the temperature of the ground and the air they contact. Their evaporation rates are proportional to the vapor pressure at any given temperature. The temperature of the ground surface in winter in temperate zones closely follows the air temperature with a range of only 10 to 20 degrees between day and night. In the summer in temperate zones, the surface temperature may be much higher than that of the air in the daytime and much cooler at night. Turbulence usually accompanies a high ground temperature. The result is that although the vapor concentration in the immediate area may be very high, it falls off rapidly a short distance away. Temperature of vehicles, buildings, and other surfaces may be warmer. This is because of internal heat sources and/or higher solar heating.

From a defensive viewpoint, a dangerous situation is likely to occur on a summer evening when the ground temperature is still high and a stable condition has started to set in. Under these conditions, a heavy vapor cloud produced by evaporation could be dangerous downwind to a distance of 2,000 meters or more. With ordinary concentrations, however, danger from vapor is somewhat less.

Another important temperature factor to consider is that people perspire freely and wear lightweight clothing in a warm climate. Thus, they are more susceptible to the action of chemical agents.

For effective tactical employment of bombs, shells, rockets, and land mines in releasing liquid chemical agents, the actual temperature of the agent itself is vitally important. Generally, liquid agents are not effective when used at temperatures below their freezing points. However, liquid agents can produce casualties when the frozen particles thaw.

Humidity has little effect on how long liquid agents are effective. However, high relative humidity, accompanied by high temperatures, induces body perspiration and, therefore, increases the effectiveness of these agents. Also, permeable protective clothing is less resistant when sweat-soaked than when dry. Since sweaty skin is more susceptible to the action of vapor, lower vapor dosages produce casualties when the humidity is high.

Precipitation

Light rains distribute persistent agents more evenly over a large surface. Since more liquid is then exposed to the air, the rate of evaporation may increase and cause higher vapor concentrations. Precipitation also accelerates the hydrolysis effect. Rains that are heavy or of long duration tend to wash away liquid chemical agents. These agents may then collect in areas previously uncontaminated (such as stream beds and depressions) and present an unplanned contamination hazard.

The evaporation rate of a liquid agent reduces when the agent is covered with water but returns to normal when the water is gone. Precipitation may force back to the surface some persistent agents that have lost their contact effectiveness by soaking into the soil or other porous surfaces. These agents may again become contact hazards.

Snow acts as a blanket, covering the liquid contaminant. It lowers the surface temperature and slows evaporation so that only very low vapor concentrations form. When the snow melts, the danger of contamination reappears.

Terrain Contours

Terrain relief has little direct effect on a liquid agent. However, a slope affects temperatures and winds, and these influence the evaporation rates of liquid agents. However, the slope or contour may affect the delivery means capable of most efficiently delivering the agent on an area (for example, reverse slopes are normally not good for artillery employment, and mountainous terrain may restrict use of spray tanks).

When persistent agents are used in vegetated areas, some of the contaminant clings to grass and leaves. This increases the surface agent exposed to the air and, hence, the rate of evaporation. Personnel become most susceptible to liquid chemical agents in vegetated areas, because they are more apt to come in contact with the agent by brushing against the foliage. Within shaded woods, however, despite the greater surface covered by the liquid chemical agent (because of the vegetation), the reduction in surface temperature and wind speed increases the duration of effectiveness.

When bombs or shells burst in woods, usually most of the liquid falls near enough to the ground to be effective. An exception is bursts in virgin forests with dense canopies that may extend to 50 meters high.

A thick jungle or forest canopy usually prevents liquid agent spray from airplanes from reaching the ground in quantities sufficient to produce significant casualties. When stable conditions exist above the forest canopy, however, enough vapor penetrates the canopy to cause casualties.

The soil on which liquid agents are placed influences the evaporation rate and the duration of effectiveness. Bare, hard ground favors short-term effectiveness and high-vapor concentration. If the surface is porous, such as sand, the liquid agent quickly soaks in; and the area no longer appears to be contaminated.

The rate at which liquid agents evaporate from a sandy or porous surface is about 1/3 less than the evaporation rate from nonabsorbent surfaces. Extended contact with a contaminated porous material is dangerous if unprotected. However, if there is no free liquid on the surface, the danger from brief contact is relatively small if protected. If a porous surface on which liquid contamination falls has been wet by rain, the contaminant does not soak in as readily, and the surface is initially more dangerous to touch than it would be if the liquid agent had soaked in. When a mustard agent (HD) falls onto a wet surface, it stays in globules; and a thin, oily film spreads over the surface, making contamination easier to detect.

Other Surfaces

Persistence of liquids on painted surfaces of vehicles is much shorter than on most terrain. This is due to a number of factors, including increased surface temperature, turbulence of airflow over the vehicles or other equipment, and greater spread of drops to give more surface area for evaporation.

Persistence varies greatly with surface material. Absorption, adsorption, and desorption also vary with surface material. Rubber absorbs most agents rapidly and desorbs slowly. Chemical agent resistant coating (CARC) absorbs very little agent.