Field Behavior of NBC Agents (Including Smoke and Incendiaries) - FM 3-6

8/20/2019 Field Behavior of NBC Agents (Including Smoke and Incendiaries) - FM 3-6

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Preface

Primary users of this manual are NBC staff officers, staff weather officers, firesupport coordination personnel, artillery officers, and others involved in planningNBC operations. These soldiers must understand what effect weather and terrainhave on nuclear, biological, and chemical (NBC) operations and smoke. This manualcontains general information and the basic principles on how to get the best results.Commanders and staffs involved in planning for use of incendiaries or smokeoperations will also benefit from the use of this manual along with other referencessuch as FM 3-50, FM 3-100, FM 3-3, FM 3-4, and FM 3-5.

On the battlefield, the influences of weather and terrain on NBC operationsprovide opportunities to both sides. To retain the initiative, friendly forces leaders andstaff officers must understand how weather and terrain can be used to theiradvantage.

FM 3-6 implements International Standardization Agreement (STANAG) 2103,Reporting Nuclear Detonations, Radioactive Fallout, and Biological and ChemicalAttacks and Predicting Associated Hazards.

This manual explains how weather and terrain influence nuclear, biological, andchemical operations and discusses the following topics for use when planningoperations:

Basic principles of meteorology as they pertain to NBC operations.

Influence of weather on the use and behavior of NBC agents.Local weather predictions and their use.Influence of terrain on the behavior of NBC agents.US Air Force Air Weather Service (AWS) forecasts and their use in planning for

operations in an NBC environment. (The Navy gets meteorological forecasts fromcomponents of the Naval Oceanography Command. Meteorological reportinformation is in the NAVOCEANCOMINST 3140.1 publications series. It alsocontains information on the behavior of smoke clouds and incendiaries. In addition, itdiscusses the influences of weather and terrain on the thermal, blast, and radiationeffects of a nuclear detonation.)

Staffs planning the use of chemical weapons and commanders approving strikesmust understand basic weather characteristics. Therefore, weather analysessignificantly influence the selection of agents and munitions for employment. Thetarget analyst must know his or her weather data needs and where to get thisinformation in a combat environment. Chapter 1 covers meteorology and the impact

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of weather on chemical agent use. The remaining chapters address the impact of weather on smoke, incendiaries, biological agents, and nuclear detonations.

Users of this publication are encouraged to recommend changes and submitcomments for its improvement. Key each comment to the specific page and paragraphin which the change is recommended. Provide a reason for each comment to ensureunderstanding and complete evaluation. To send changes or comments, prepare DAForm 2028 (Recommended Changes to Publications and Blank Forms) and forward itto Commandant, USACMLS, ATTN: ATZN-CM-NF, Fort McClellan, AL 36205-5020.Air Force comments go to HQ USAF/XOORF, Washington, DC 20330. Marine Corps

comments go to Commanding General, Marine Corps Development and EducationCommand (CO93), Quantico, VA 22134. Navy comments go to Chief of NavalOperations (OP-954), Navy Department, Washington, DC 20350.

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The fielddependent ontemperature,precipitation.

CHAPTER 1

Chemical Agents

behavior of chemical agents is understand the impact of chemical agents on theweather variables such as wind, battlefield, the soldier must also understand howair stability, humidity, and these agents are affected by weather and terrain.The influence of each variable The following paragraphs give an overview of the

depends upon the synoptic situation and is locally basic characteristics of chemical agents and howinfluenced by topography, vegetation, and soil. weather and terrain influence and have specific

Chemical agents may appear in the field in effects on them.different forms: vapors, aerosols, or liquids. To

Basic CharacteristicsVapors and small particles are carried by thewinds, while any large particles and liquid dropsfall out in a ballistic-like trajectory and are quicklydeposited on the ground. Many agents give off vapors that form vapor clouds. The speed at whichan agent gives off vapors is called volatility.Agents may be removed naturally from the air byfalling out (large particles fall out much morequickly), by sticking to the ground or vegetation, or

by being removed by precipitation. Once depositedupon vegetation or other ground cover, volatileagents may be re-released to the atmosphere forfurther cycles of travel and present a hazard untilsufficiently diluted or decontaminated.

During approximately the first 30 seconds, thesize and travel of an agent are determinedprimarily by the functioning characteristic of themunition or delivery system. Thereafter, the traveland diffusion of the agent cloud are determinedprimarily by weather and terrain. For example, inhigh temperatures, volatile agents producemaximum agent vapor in 15 seconds. Light windsand low turbulence allow high localconcentrations of agents. High winds and strongturbulence reduce the concentration and increasethe area coverage by more quickly carrying awayand diffusing the agent cloud.

Vapors

When a chemical agent is disseminated as avapor from a bursting munition, initially the cloud

expands, grows cooler and heavier, and tends toretain its form. The height to which the cloud rises,due to its buoyancy, is called the height of thethermally stabilized cloud. If the vapor density of the released agent is less than the vapor density of air, the cloud rises quite rapidly, mixes with thesurrounding air, and dilutes rapidly. If the agentforms a dense gas (the vapor density of thereleased agent is greater than the vapor density of air), the cloud flattens, sinks, and flows over theearth’s surface. Generally, cloud growth duringthe first 30 seconds is more dependent upon themunition or delivery system than uponsurrounding meteorological conditions.

Nevertheless, the height to which the cloudeventually rises depends upon air temperature andturbulence. These determine how much cooler,ambient air is pulled into the hot cloud (and, hence,determines its rate of cooling). The agentconcentration buildup is influenced by both theamount and speed of agent release and by existingmeteorological conditions.

Shortly after release, the agent cloud assumesthe temperature of the surrounding air and movesin the direction and at the speed of the surroundingair. The chemical cloud is subjected to turbulenceforces of the air, which tend to stretch it, tear itapart, and dilute it. The heavier the agent, thelonger the cloud retains its integrity. Underconditions of low turbulence, the chemical agentcloud travels great distances with little decrease inagent vapor concentration. As turbulence

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increases, the agent cloud dilutes or dissipatesfaster.

Aerosols

Aerosols are finely divided liquid and/or solidsubstances suspended in the atmosphere.Sometimes dissolved gases are also present in theliquids in the aerosols. Chemical agent aerosolclouds can be generated by thermal munitions andaerosol spray devices or as by-products of liquidspray devices and bursting munitions.

Airborne aerosols behave in much the samemanner as vaporized agents. Initially, aerosolclouds formed from thermal generators have ahigher temperature than clouds formed from othertypes of munitions. This may cause some initialrise of the cloud at the release point. Aerosol-

generated clouds are heavier than vapor clouds,and they tend to retain their forms and settle backto earth. Being heavier than vapor clouds, they areinfluenced less by turbulence. However, as theclouds travel downwind, gravity settles out thelarger, heavier particles. Many particles stick toleaves and other vegetative surfaces they contact.

Liquids

When a chemical agent is used for its liquideffect, evaporation causes the agent to form intovapor. Depending upon volatility, vapor clouds areusually of low concentration, have about the same

temperature as the surrounding air, and tend tostay near the surface because of high vapordensity. Additionally, vapor density governs theextent that the vapor will mix with the air. Liquidagents with high vapor density impact at groundlevel with very little evaporation of the agent.These agents are termed persistent agents. Whiledrops are airborne, and after impacting, the liquidcontinues to evaporate. Agent vapor pressure willgovern the rate at which the liquid will evaporateat a given temperature and pressure. Initialconcentrations are lower, since the vapor source isnot instantaneous as a vapor agent is but evolves

over a long period (until the liquid source is gone).Liquid agents may be absorbed (soaked into asurface) and adsorbed (adhered to a surface), andthey may also evaporate. Once the liquid is nolonger present on the surface, desorption (going back into the air) begins. The vapor concentrationover areas contaminated with a liquid agent tends

to be less than with newly formed vapor clouds,and downwind agent concentrations are notnearly as great as with other types of agents.

Atmospheric Stability

One of the key factors in using chemicalweapons is the determination of the atmosphericstability condition that will exist at the time of attack. This determination can be made from ameteorological report or by observing fieldconditions.

When a meteorological report is available, itshould contain a description of the current orprojected atmospheric stability condition. If thedata given are based on an atmosphericdescription, Figure 1-1 may be used to convert thedata into traditional atmospheric stability

categories/conditions. When meteorologicalreports are not readily available, the stabilitycondition can be derived by using the stabilitydecision tree shown in Figure 1-2. Figure 1-2 isentered at the top with the current observedweather conditions (or estimated weatherconditions). Follow the decision tree to determinethe stability condition. The stability conditionplus the wind speed indicates the dispersioncategory of an agent vapor cloud.

Unstable conditions will cause lower

concentrations and/or poorer target coverage.Stable conditions will cause greater agent stabilityand higher concentrations. Use Figure 1-2 asguidance for employing an agent by starting inupper left corner at the word START. Followarrowed line to the first question. Answerquestion “Is it nighttime?” by selecting,

thethethein

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accordance with the facts, the yes or no arrowindicating your decision. At each branch in thearrows, follow the arrow most nearly correct forthe conditions under which the stability categoryis required. As questions are encountered along

your path, answer each and proceed along themost nearly correct path until a dispersioncategory is identified. The result from Figure 1-2 isthe stability category. An example of the use of Figure 1-2 is if you are inland one hour beforesunset and the winds are calm, the stabilitycategory is neutral (N) (category 4).

The dispersion category, the wind speed inknots, and the wind direction are the mostimportant meteorological data for deciding theinfluence of weather on vapor cloud dispersion.For any given dispersion category, a lower windspeed will produce higher dosages, smaller area

coverage, and, consequently, higher toxic effects.This is because when the wind speed is lower, thecloud moves more slowly past the individual in thetarget area; and the individual is in the cloudlonger, yielding a higher dose of the agent. SeeTable 1-1 for the dispersion categories and windspeeds during which atmospheric conditions areeither generally favorable, marginal, orunfavorable for employment of chemical agents.Factors such as agent toxicity, targetvulnerability, and the amount of the agentreleased will determine the actual doses,casualties, and other effects. Elevated agentreleases will alter the table results somewhat, butthe same trends occur. The main effect to beconsidered for elevated release effectiveness over aspecific target is that the agent must be releasedfurther upwind to compensate for the drift as theagent comes down.

Table 1-1 is a general reference tool to providean estimate, based on dispersion category andwind speed, when it would generally be mosteffective to employ a chemical agent vapor. Table1-2 indicates the typical cloud widths at givendownwind distances from a point source releasefor a chemical agent vapor cloud. Note that thecloud width depends upon dispersion category andnot directly upon wind speed. The cloud widthdistances represented in Table 1-2 are the dosagecontours for 0.01 milligram-minutes per cubicmeter (mg-min\M³). If the agent is released from aline source (spray system), the line length should be added to the cloud width (Table 1-2) to determine

total cloud width for travel distances up to 1kilometer. For longer travel distances, the lengthof the line source loses its importance (due todissipation), and the total cloud width isrepresented by the values in Table 1-2. The

chemical cloud widths listed in Table 1-2 areestimates. The widths will vary depending on theweather and terrain of a specific area.

The following examples are cited to explainfurther the use of Table 1-2. Based on a chemicalagent vapor being released from a point source indispersion category 4, the chemical cloud width at7 kilometers downwind would be approximately2.3 kilometers. Based on a chemical agent vapor being released from a line source that is 0.1kilometer in length (dispersion category 2), thechemical cloud width at a 0.5 kilometer downwinddistance would be .850 kilometer (0.75+0.1).

Table 1-3 presents the relative center linedosages (mg-min/M³) at different distancesdownwind for different dispersion categories andwind speeds. Remember, low wind speeds at thesame dispersion category give higher dosages. Thedosages listed in Table 1-3 are estimates and willvary depending on the estimated category andwind speed in the target area. The dosage values inTable 1-3 are based on 100 kilograms of thenonpersistent nerve agent (GB) being released atground level from a point source.

The information reflected in Table 1-3 is thedosage that would be incurred if the target werestationary. The dosage would decrease if the targetwere moving through the downwind cloud hazardarea. Additionally, in general, if the sourcestrength (100 kg) were doubled, the dosage wouldalso double, and if the source strength were halved,the dosage would also decrease approximatelyone-half.

To aid in using Table 1-3, the followingexample is provided. With dispersion category 4,wind speed 8 knots, and a downwind distance of 2kilometers, the center line dosage would be 18.91mg-min/M³. With dispersion category 2, windspeed 3 knots, and at a downwind distance of 4kilometers, the center line dosage would be 1.030mg-min\M³.

Vapor Concentration and Diffusion

Agent concentration is governed by thevolume of the agent cloud. Since clouds

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continually expand, agent concentration levelsdecrease over time. Wind speed determines thedownwind growth of the cloud. Vertical andhorizontal turbulence determines the height andwidth of the cloud. The rate at which thedownwind, vertical, and horizontal componentsexpand governs the cloud volume and the agentconcentration.

To be effective the agent cloud, at a specificconcentration level, must remain in the target areafor a definite period. Wind in the target area mixesthe agent and distributes it over the target afterrelease. For ground targets, high concentrationsand good coverage can best be achieved with lowturbulence and calm winds when the agent is

delivered directly on target. A steady, predictablewind drift over the target is best when the agent isdelivered on the upwind side of the target.Conditions other than these tend to produce lowerconcentrations and/or poorer target coverage.However, unless weather conditions are knownwithin the target area, the effects of the agent ontarget will be approximations.

The concentration and diffusion of a chemicalagent cloud are also influenced by the factors of hydrolysis, absorption, adsorption, lateral spread,drag effect, and vertical rise.

Hydrolysis is the process of the agent reactingwith water vapor in the air. It does not influencemost agent clouds in tactical use because the rate

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of hydrolysis is too slow. However, hydrolysis can be important for smoke screens. See the discussionof the effect of humidity on increasing smokescreen effectiveness in Chapter 2.

Absorption is the process of the agent being

taken into the vegetation, skin, soil, or material.Adsorption is the adding of a thin layer of agent tovegetation or other surfaces. This is important indense vegetation. Both absorption and adsorptionof chemical agents may kill vegetation, thusdefoliating the area of employment.

When a chemical cloud is released into the air,shifting air currents and horizontal turbulence blow it from side to side. The side-to-side motion of the air is called meandering. While the agent cloudmeanders, it also spreads laterally. Lateralspreading is called lateral diffusion. Figure 1-3shows a cloud with lateral spread and

meandering. Table 1-2 indicates the amount of lateral spread that occurs under differentdispersion categories and distances downwind. Inmore unstable conditions, the lateral spread tendsto be greater than in stable conditions.

Wind currents carry chemical clouds along theground with a rolling motion. This is caused by the

differences in wind velocity. Wind speeds increaserapidly from near zero at the ground to higherspeeds at higher elevations above the ground. Thedrag effect by the ground, together with theinterference of vegetation and other ground

objects, causes the base of an agent cloud to beretarded as the cloud stretches out in length. Whenclouds are released on the ground, the dragamounts to about 10 percent of the vertical growthover distance traveled over grass, plowed land, orwater. It amounts to about 20 percent over gentlyrolling terrain covered with bushes, growingcrops, or small patches of scattered timber. Inheavy woods, the drag effect is greatly increased.The vertical spread of the cloud is illustrated inFigure 1-4.

Wind speeds can vary at different heights. Thewind direction can also change with an increase in

height. This is known as wind shear. Because of wind shear, a puff (or chemical cloud) may becomestretched in the downwind direction and maytravel in a direction different from that of thesurface wind. Additionally, a chemical cloudreleased in the air may be carried along faster thanit can diffuse downward. As a result, air near the

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ground on the forward edge of the cloud may beuncontaminated, while the air a few feet up may beheavily contaminated. This layering effect

becomes more pronounced and increasesproportionately with the distance of the forward

edge of the cloud from the source. Figure 1-5illustrates this. A small puff of agent cloudreleased from its source some time earlier has tiltedforward, while the bottom has been retarded due toslower winds caused by drag.

The vertical rise of a chemical cloud dependsupon weather variables, such as temperaturegradient, wind speed, and turbulence, and the

Vapors andWind, temperature, humidity, precipitation,

terrain contours, and surface cover influence thefield behavior of vapors and aerosols. Forexample, in a chemical attack on US forces (lstDivision) 26 February 1918 in the Ansauvillesection, extremely stable conditions, calm winds,

difference between the densities of the clouds andthe surrounding air. As mentioned earlier, thetemperature of both the cloud and the airinfluences their relative densities. Hotter gases areless dense and, therefore, lighter than cooler gases

and air. Therefore, they rise until they are mixedand somewhat diluted and attain the sametemperature and approximately the same densityas surrounding air.

The vapor cloud formed by an agent normallyemployed for persistent effect rises in a similarmanner, but vapor concentrations build up moregradually.

Aerosolsand heavy underbrush in the target area

contributed to the overall effectiveness of achemical attack. Several additional casualtiesresulted due to the increased chemical agentpersistency caused by the favorable weatherconditions. Favorable and unfavorable weather

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and terrain conditions for tactical employment of a chemical aerosol or vapor cloud are summarizedin Table 1-4.

If a chemical cloud is to be placed directly onan occupied area, the best possible weather

conditions are calm winds with a strong, stabletemperature gradient. Under these conditions, thecloud diffuses over the target with minimumdilution and does not move away. Such conditionsare most apt to occur on a calm, clear night. If asmall amount of air movement is required tospread the cloud evenly over the target area, a lowwind speed and stable or neutral conditions aremost favorable. These conditions most often occuron a clear night, a cloudy night, or a cloudy day.

When the desired effect is for the chemicalcloud to travel, the most favorable conditions arestable or neutral conditions with a low to medium

wind speed of 3 to 7 knots. These conditions may bepresent on a clear night, a cloudy night, or a cloudyday. The presence of low to medium wind speedskeeps the cloud traveling over the area without toomuch diffusion, and the stable or neutralconditions keep the agent concentration high andthe cloud close to the ground.

Favorable terrain conditions for a chemicalcloud are smooth or gently rolling contours orwooded areas. Unfavorable conditions forchemical clouds (usually found on clear days) areextreme or marked turbulence, wind speeds above10 knots, an unstable dispersion category, rain,and rough terrain.

Wind

High wind speeds cause rapid dispersion of vapors or aerosols, thereby decreasing effectivecoverage of the target area and time of exposure tothe agent. In high winds, larger quantities of munitions are required to ensure effectiveconcentrations. Agent clouds are most effectivewhen wind speeds are less than 4 knots and steady

in direction. The clouds move with the prevailingwind as altered by terrain and vegetation. Steady,low wind speeds of 3 to 7 knots enhance areacoverage unless an unstable condition exists. Withhigh winds, chemical agents cannot beeconomically employed to achieve casualties. Thechart at Figure 1-2 indicates the ef f ect of wind onstability categories. Tables 1-1, 1-2, and 1-3

indicate the effects of wind and dispersioncategories upon dosage and area coverage.

Unstable conditions, as indicated in Figure 1-2and Tables 1-1, 1-2, and 1-3, are the least favorableconditions. Unstable conditions (such as many

rising and falling air currents and greatturbulence) quickly disperse chemical agents.Unstable is the least favorable condition forchemical agent use because it results in a lowerconcentration, thereby reducing the area affected by the agent. Many more munitions are required toattain the commander’s objectives under unstableconditions than under stable or neutral conditions.

Stable conditions (such as low wind speedsand slight turbulence) produce the highestconcentrations. Chemical agents remain near theground and may travel for long distances before

being dissipated. Stable conditions encourage the

agent cloud to remain intact, thus allowing it tocover extremely large areas without diffusion.However, the direction and extent of cloud travelunder stable conditions are not predictable if thereare no dependable local wind data. A very stablecondition is the most favorable condition forachieving a high concentration from a chemicalcloud being dispersed.

Neutral conditions are moderately favorable.With low wind speed and smooth terrain, largeareas may be effectively covered. The neutralcondition occurs at dawn and sunset and generallyis the most predictable. For this reason, a neutraldispersion category is often best from a militarystandpoint.

Temperature

There will be increased vaporization withhigher temperatures. Also, the rate of evaporationof any remaining liquid agent from an explodingmunition can vary with temperature. Generally,the rate of evaporation increases as thetemperature increases. See FM 3-9/AFR 355-7 forspecific information on chemical agents, such astheir boiling and freezing points and vapor

density.

Humidity

Humidity is the measure of the water vaporcontent of the air. Hydrolysis is a process in whichcompounds reactchemical change.

with water resulting in aChemical agents with high

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hydrolysis rates are less effective under conditionsof high humidity.

Humidity has little effect on most chemicalagent clouds. Some agents (phosgene and lewisite)hydrolyze quite readily. Hydrolysis causes these

chemical agents to break down and change theirchemical characteristics. If the relative humidityexceeds 70 percent, phosgene and lewisite can not be employed effectively except for a surprise time-on-target (TOT) attack because of rapidhydrolysis. Lewisite hydrolysis by-products arenot dangerous to the skin; however, they are toxicif taken internally because of the arsenic content.Riot control agent CS (see glossary) alsohydrolyzes, although slowly, in high humidities.High humidity combined with high temperaturesmay increase the effectiveness of some agents because of body perspiration that will absorb the

agents and allow for better transfer.

Precipitation

The overall effect of precipitation isunfavorable because it is extremely effective inwashing chemical vapors and aerosols from theair, vegetation, and material. Weather forecasts orobservations indicating the presence of orpotential for precipitation present an unfavorableenvironment for employment of chemical agents.

Terrain Contours

Terrain contours influence the flow of chemical clouds the same as they influenceairflow. Chemical clouds tend to flow over lowrolling terrain and down valleys and settle inhollows and depressions and on low ground. Localwinds coming down valleys at night or up valleysduring the day may deflect the cloud or reverse itsflow. On the other hand, they may produceconditions favorable for chemical cloud travelwhen general area forecasts predict a calm.

A chemical cloud released in a narrow valleysubjected to a mountain breeze retains a highconcentration of agent as it flows down the valley.This is because of minimal lateral spread. Hence,high dosages are obtained in narrow valleys ordepressions. High dosages are difficult to obtainon crests or the sides of ridges or hills. After aheavy rain, the formation of local mountain orvalley winds is sharply reduced. In areas of adjacent land and water, daytime breezes from the

water and nighttime breezes from the land controlchemical cloud travel.

Surface Cover

Ground covered with tall grass or brushretards flow. Obstacles, such as buildings or trees,set up eddies that tend to break up the cloud andcause it to dissipate more rapidly. However, streetcanyons or spaces between buildings may havepockets of high concentrations. Flat country(during a neutral or inversion condition) or openwater promotes an even, steady cloud flow. Figure1-5 illustrates the horizontal and vertical spread of a cloud over flat country.

The amount and type of vegetation in the areaof the chemical operation also influence the travelof a chemical cloud. Vegetation, as it relates tometeorology or diffusion, is called vegetativecanopy or just canopy. The effects of canopies areconsidered below.

Woods are considered to be trees in full leaf (coniferous or deciduous forests). The term“heavily wooded canopy” denotes jungles orforests with canopies of sufficient density to shademore than 90 percent of the ground surface

beneath. For chemical operations, areascontaining scattered trees or clumps of bushes areconsidered to be open terrain although drag issomewhat increased. In wooded areas where treesare not in full leaf or where foliage has beendestroyed by previous attack so that sunlightstrikes the ground, the diffusion (stability)category will be similar to those in the open.

When bombs are dropped into a wooded area,some may be expected to burst in the treetops.Although the released aerosol and vapor settletoward the ground, some of the agent is lost,depending upon the thickness and height of thefoliage. The initial burst and pancake areas of chemical clouds released within woods or junglesare smaller than those released in the open.However, concentrations within the initial cloudsare higher in wooded areas, sometimes three timesthat of bursts in the open. The magnitude of concentration from ground bursts depends uponthe density of undergrowth and trees.

Generally, when conditions in the open aremost favorable for the use of chemical agents,conditions also are favorable in heavily woodedareas if dispersion occurs below the canopy. Low

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wind speeds under the canopies spread agentclouds slowly in a downwind and downslopedirection. Areas of dense vegetation also increasethe potential surface area for the deposition of chemical agents. If there are gullies and stream

beds within the woods, clouds tend to follow thesefeatures. This flow may be halted or diverted byupslope winds.

Vegetation absorbs some agents. However, foran attack against troops poorly trained in NBC

defense (where lethal dosages may be obtained in30 seconds or less), the amount of agent absorbed

by foliage will have little or no effect on the successof the attack. High concentrations of chemicalagents may destroy vegetation, since the leaves

absorb some of the agent. In some instances, theabsorbed agent may be released or desorbed whenthe vegetation is disturbed or crushed, creating asecondary toxic hazard.

LiquidsWeather, terrain contours, vegetation, soil,

and some other surfaces affect the rate of evaporation. That, in turn, influences thepersistence of a chemical agent liquid and theconcentration of the vapor. Most weather

conditions do not affect the quantity of munitionsneeded for an effective initial liquidcontamination. Table 1-5 summarizes favorableand unfavorable weather and terrain conditionsfor the employment of a liquid chemical agent.

When a liquid agent is used to cause casualtiesthrough contact with the liquid in crossing oroccupying the area, its duration of effectiveness isgreatest when the soil temperature is just abovethe agent’s freezing point. This limits the rate of evaporation of the liquid. Other favorableconditions are low wind speed, wooded areas, andno rain.

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

Favorable and unfavorable conditions forliquid agents for vapor concentration effects aremuch the same as those for chemical clouds. Inwoods, however, a high temperature with only avery light wind gives the highest vaporconcentrations.

Weather

Duration of the effectiveness of initial liquidcontamination may be affected by wind speed;stability, mixing height, and temperature; andprecipitation.

W i nd Speed

Wind directionthe upwind side of a

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is important in determiningtarget for release purposes but

has little impact on the duration of effectiveness,regardless of the method of release The vaporcreated by evaporation of the liquid agent,however, moves with the wind. Therefore, thevapor 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 (thesurface of the liquid) and the rate at which airpasses over the agent. Therefore, the duration of effectiveness is longer at the places of greaterliquid agent contamination and in places wherethe liquid agent is sheltered from the wind.

The rate of evaporation of agents employed forpersistent effect in a liquid state is proportional tothe wind speed. If the speed increases, evaporationincreases, thus shortening the duration of

effectiveness of the contamination. Increasedevaporation, in turn, creates a larger vapor cloud.The vapor cloud, in turn, is dispersed by higherwinds. The creation and dispersion of vapor are acontinuous process, increasing or decreasing inproportion to wind speed.

Releasing agents for persistent effect by pointdispersal via bombs, shells, rockets, or land minesresults in an unevenly distributed contaminant.Heavier concentrations of the liquid are foundaround the point of burst. Lighter concentrationsresult farther from the bursting position. Thereprobably will be small areas between the points of

burst that are not contaminated, depending uponthe number of munitions used and the uniformityof dispersal.

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than when the liquid agent is unevenly dispersed downwind from the sprayed area.(as with bursting munitions). With spraying, the Some chemical agents have no significantduration of effectiveness decreases, and there is a vapor pressure, and, consequently, their rates of corresponding increase in the vapor concentration evaporation are not affected by wind speed. Also,

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some of these agents are extremely toxic, so even avery slight surface concentration represents amassive overkill dosage. When agents of thiscategory are released from spray munitions underlow wind speeds, they cover only a narrow zone.

When released under higher wind speeds, theycover wider areas more effectively. Thus, whendownwind safety is not a limiting consideration,high wind speeds may be more desirable than lowwind speeds for these very persistent agents.

With agents that vaporize readily, high windspeeds may cause complete vaporization beforethe agent reaches the ground, creating only avapor hazard. The resulting vapor cloud isnonpersistent and dissipates quite rapidly due tothe high degree of mechanical turbulenceassociated with high wind speeds.

Turbulence has the same effect on agents

employed for persistent effect, whether releasedfrom bombs, rockets, artillery shells, or landmines. Turbulence tends to reduce the duration of effectiveness in the liquid state by helping toincrease the rate of evaporation. Temperature,rather than turbulence, has the greater effect onthe duration of effectiveness of liquid agents.However, a contaminated area that has beensubjected to pronounced turbulence does notremain contaminated as long as one that has beensubjected to only slight turbulence with low windspeeds.

Turbulence also influences the spraying of

agents employed for persistent effect. High windsand air movements divert the drops from thetarget or spread them over a larger area. Steepmountain regions sometimes produce large-scaleeddies that prevent effective coverage of the target.Any vapor concentrations built up from sprayedareas are slight when the degree of turbulence ishigh.

St abi l i t y , M ix i ng Hei ght ,and Temperat ure

Unstable conditions are characterized bywarmer surfaces. The solar heating then causesevaporation to be more rapid.

Temperature, velocity, and turbulence alsoaffect the dispersion of spray. When stable(inversion) conditions prevail, there usually islittle or no thermal turbulence, wind speeds are

low, and the degree of mechanical turbulence isalso low. Often stable conditions exist continuallyonly near the ground. Above the top of the stablesurface layer, wind speed and turbulence areincreased. Wind direction here also may be

substantially different from the surface winddirection. A chemical spray released below the topof the inversion falls fairly quickly. The height of the top of an inversion varies throughout theperiod of the surface inversion existence, and itmay vary rapidly over large hills and mountains.

The mixing height is the capping inversion atthe top of the mixing layer and serves as a lid. Itprevents further upward vertical growth of achemical vapor. A mixing height can also existabove unstable or neutral surface stabilityconditions. In radiation inversions, whichcommonly form at night, the top of the surface-

based (mixing) stable layer is very close to theearth’s surface shortly after the neutral conditionchanges to a stable condition (soon after sunset).As the surface stable layer intensifies, its top rises,reaching its maximum elevation between 0200 and0400 hours local time. Maximum elevation may be400 meters in a very intense stable layer. In themorning, solar radiation heats the surface andcauses a good mixing condition close to theground. The mixing height and turbulencecondition increase until they destroy the stablelayer. The mixing height can extend from theearth’s surface up to 2 kilometers in elevation on a

hot summer day. On a calm, clear night, themixing height may extend only 50 to 100 metersabove the earth’s surface.

If a chemical agent is released above thesurface stable layer, most of the agent remainsaloft in the turbulence layer, and most of it willdissipate before settling low enough to be effective.For this reason, most spray missions are flown ateither sunrise or sunset to take advantage of aneutral temperature gradient. With this gradient,there is some vertical exchange of air, and thechemical spray, being relatively heavy, has anatural tendency to settle to the ground. The Air

Weather Service or an assigned meteorologist canprovide information on the mixing height and theheight of the top of the surface stable layer.

Under unstable conditions, convectioncurrents often catch many very small droplets andcarry them upward above the level of release. As aresult, the spray takes longer to reach the ground,

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and much of it may dissipate before reaching thetarget area.

Temperature is one of the most importantfactors affecting the duration of effectiveness andvapor concentration of liquid agents. Agents

employed for persistent effect acquire thetemperature of the ground and the air they contact.Their evaporation rates are proportional to thevapor pressure at any given temperature. Thetemperature of the ground surface in winter intemperate zones closely follows the airtemperature with a range of only 10 to 20 degrees between day and night. In the summer intemperate zones, the surface temperature may bemuch higher than that of the air in the daytimeand much cooler at night. Turbulence usuallyaccompanies a high ground temperature. Theresult 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 bewarmer. This is because of internal heat sourcesand/or higher solar heating.

From a defensive viewpoint, a dangeroussituation is likely to occur on a summer eveningwhen the ground temperature is still high and astable condition has started to set in. Under theseconditions, a heavy vapor cloud produced byevaporation could be dangerous downwind to adistance of 2,000 meters or more. With ordinaryconcentrations, however, danger from vapor issomewhat less.

Another important temperature factor toconsider is that people perspire freely and wearlightweight clothing in a warm climate. Thus, theyare more susceptible to the action of chemicalagents.

For effective tactical employment of bombs,shells, rockets, and land mines in releasing liquidchemical agents, the actual temperature of theagent itself is vitally important. Generally, liquidagents are not effective when used at temperatures

below their freezing points. However, liquid agentscan produce casualties when the frozen particlesthaw.

Humidity has little effect on how long liquidagents are effective. However, high relativehumidity, 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 sweatyskin is more susceptible to the action of vapor,lower vapor dosages produce casualties when thehumidity is high.

Prec ip i t a t i on Light rains distribute persistent agents more

evenly over a large surface. Since more liquid isthen exposed to the air, the rate of evaporationmay increase and cause higher vaporconcentrations. Precipitation also accelerates thehydrolysis effect. Rains that are heavy or of longduration tend to wash away liquid chemicalagents. These agents may then collect in areaspreviously uncontaminated (such as stream bedsand depressions) and present an unplannedcontamination hazard.

The evaporation rate of a liquid agent reduceswhen the agent is covered with water but returns tonormal when the water is gone. Precipitation mayforce back to the surface some persistent agentsthat have lost their contact effectiveness bysoaking into the soil or other porous surfaces.These agents may again become contact hazards.

Snow acts as a blanket, covering the liquidcontaminant. It lowers the surface temperatureand slows evaporation so that only very low vaporconcentrations form. When the snow melts, thedanger of contamination reappears.

Terrain ContoursTerrain relief has little direct effect on a liquid

agent. However, a slope affects temperatures andwinds, and these influence the evaporation rates of liquid agents. However, the slope or contour mayaffect the delivery means capable of mostefficiently delivering the agent on an area (forexample, reverse slopes are normally not good forartillery employment, and mountainous terrainmay restrict use of spray tanks).

Vegetation

When persistent agents are used in vegetatedareas, some of the contaminant clings to grass andleaves. This increases the surface agent exposed tothe air and, hence, the rate of evaporation.Personnel become most susceptible to liquidchemical agents in vegetated areas, because theyare more apt to come in contact with the agent by

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brushing against the foliage. Within shadedwoods, however, despite the greater surfacecovered by the liquid chemical agent (because of the vegetation), the reduction in surfacetemperature and wind speed increases the

duration of effectiveness.When bombs or shells burst in woods, usuallymost of the liquid falls near enough to the groundto be effective. An exception is bursts in virginforests with dense canopies that may extend to 50meters high.

A thick jungle or forest canopy usuallyprevents liquid agent spray from airplanes fromreaching the ground in quantities sufficient toproduce significant casualties. When stableconditions exist above the forest canopy, however,enough vapor penetrates the canopy to causecasualties.

Soil

The soil on which liquid agents are placedinfluences the evaporation rate and the duration of effectiveness. Bare, hard ground favors short-termeffectiveness and high-vapor concentration. If thesurface is porous, such as sand, the liquid agentquickly 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 nonabsorbentsurfaces. Extended contact with a contaminatedporous 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 liquidcontamination falls has been wet by rain, thecontaminant does not soak in as readily, and thesurface is initially more dangerous to touch than itwould be if the liquid agent had soaked in. When amustard agent (HD) falls onto a wet surface, itstays in globules; and a thin, oily film spreads overthe surface, making contamination easier todetect.

Other Surfaces

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

Persistence varies greatly with surfacematerial. Absorption, adsorption, and resorptionalso vary with surface material. Rubber absorbsmost agents rapidly and desorbs slowly. Chemicalagent resistant coating (CARC) absorbs very littleagent.

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Smoke

CHAPTER 2

and Incendiaries

Smoke and incendiaries are combat long been employed as a means of concealingmultipliers. Their effective use on a target can battlefield targets. Additionally, incendiary fireprovide tactical advantages for offensive and damage causes casualties and materiel damagedefensive operations. For example, smoke has and can also impact psychologically.

SmokeChemical smokes and

obscurants can degrade theother aerosoleffectiveness of

sophisticated antitank guided missiles (ATGMs).The precision guidance systems of ATGMs aretypically electro-optical devices and generallyoperate in the near-, mid-, or far-infrared portionsof the electromagnetic spectrum, rather than in thevisible light band of the spectrum. The use of smoke in the target area can be a convincingcombat multiplier offensively and a dynamiccountermeasure defensively. Smoke should be of primary interest to all commanders and staff planners because the proper use of smoke canprovide many operational advantages.

Smoke has four general uses on the battlefield—obscuring, screening, deceiving, andidentify ing/signalling. Obscuring smoke is placedon an enemy to reduce vision both at, and out from,the position. Screening smoke is used in friendlyoperational areas or between friendly units andthe enemy. Deceiving smoke is used to mislead theenemy. Identifying/signalling smoke is a form of communication that has multiple uses. Overall,the objective of smoke employment is to increasethe effectiveness of Army operations whilereducing the vulnerability of US forces.Specifically, smoke can be used to accomplish thefollowing:

Deny the enemy information.

Reduce effectiveness of enemy targetacquisition.Disrupt enemy movement, operations,

command, and control.Create conditions to surprise the enemy.Deceive the enemy.

During offensive operations, smoke canscreen the attacker while an attack is carried out.

Some offensive applications include concealingmovement of military forces and equipment;screening locations of passages through barriers;and helping to secure water crossings,

beachheads, or other amphibious operations.For defensive operations, smoke can be

effectively used to blind enemy observation pointsto deprive the enemy of the opportunity to adjustfire, to isolate enemy elements to permitconcentration of fire and counterattack, and todegrade the performance of threat ATGMs.

There are generally two categories of smokeoperations on a battlefield-hasty and deliberatesmoke. Hasty smoke operations are conductedwith minimum prior planning, normally tocounter some enemy action or anticipated action of immediate concern to a commander. Hasty smokeis usually used on small areas, is of short duration,and is most often used by battalion or smallerunits. Deliberate smoke is planned in much greaterdetail. It is often employed over a large area for arelatively long period by brigades, divisions, orcorps. For further information on hasty anddeliberate smoke operations, refer to FM 3-50.

The following paragraphs on smoke operationcontain information on smoke characteristics,diffusion of smoke, weather effects, hasty anddeliberate smoke operations, and tacticalconsiderations.

Characteristics

Smoke is an aerosol that owes its ability toconceal or obscure to its composition of manysmall particles suspended in the air. Theseparticles scatter or absorb the light, thus reducingvisibility. When the density or amount of smoke

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material between the observer and the object to bescreened exceeds a certain minimum thresholdvalue, the object cannot be seen.

The effectiveness of smoke used to obscure orconceal depends primarily on characteristics such

as the number, size, and color of the smokeparticles. Dark or black smoke absorbs a largeproportion of the light rays striking individualsmoke particles. In bright sunlight, a largequantity of black smoke is required for effectiveobscuration because of the nonscatteringproperties of the particles. At night or under lowvisibility conditions, considerably less smoke isneeded.

Grayish or white smoke obscures by reflectingor scattering light rays, producing a glare. During

bright daylight conditions, less white smoke than black smoke is required to obscure a target. Years

of experience with smoke screen technology haveshown that white smoke is superior to black smokefor most applications. Available white smokeincludes white phosphorus (WP) and redphosphorus (RP) compounds, hexachloroethane(HC), and fog oil (SGF2). WP, RP, and HC arehydroscopic—they absorb water vapor from theatmosphere. This increases their diameters andmakes them more efficient reflectors andscatterers of light rays. Fog oils arenonhygroscopic and depend upon vaporization

techniques to produce extremely small diameterdroplets to scatter light rays. The reflecting andabsorbing qualities of smoke are illustrated inFigure 2-1.

Smoke, when placed between a target and a

viewer, degrades the effectiveness of target-acquisition and aiming systems. The amount of smoke necessary to defeat aiming and acquisitionsystems is highly dependent upon the prevailingmeteorological conditions, terrain relief, availablenatural light, visibility, and the attenuationeffects of natural particles in the atmosphere.Other factors that must be considered includesmoke from battlefield fires and dust raised bymaneuvering vehicles and artillery fire.

The ability to detect and identify a targetconcealed by such a smoke screen is, in turn, afunction of target-to-background contrast.

Additionally, the amount of available naturallight, the position of the sun with respect to thetarget, the reflectance of the smoke screen and thetarget, and the portion of the electromagneticspectrum to be attenuated below the thresholdcontrast for detection will impact on detecting andidentifying a target.

DiffusionThe diffusion of smoke particles into the

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atmosphere generally obeys physical laws.Diffusion is governed by wind speed, turbulence,stability of the atmosphere, and terrain. Thediffusion of smoke, as used on the battlefield,originates from four basic source configurations.These may be defined as continuous point sources,instantaneous point sources, continuous linesources, and area sources. A continuous pointsource may bethought of as a smoke release from asingle smoke generator or smoke pot. The burstingof a projectile containing WP is considered to be aninstantaneous source. A series of generators, setup crosswind, represent a line source. Munitionswhich scatter smoke-generating submunitions inan area are considered an area source.

Weather EffectsMeteorological conditions that have the mosteffect on smoke screening and munitionsexpenditures (including the deployment of smokegenerators) include wind direction, relativehumidity, visibility, and atmospheric stability. To

be effective, an obscuring screen must be placed inan advantageous position with respect to theprevailing wind direction. The target area to be

screened must be defined in terms of whether theprevailing wind direction is considered to be ahead or tail wind, a quartering wind, or a flankwind. Figure 2-2 illustrates these conditions. Itmust be remembered that flanking winds can befrom either the right or left side of the screeningarea and that there are four quartering-winddirections. Wind direction is critical fordetermining the adjustment or aim point forscreens deployed by artillery or mortars and alsofor the placement of generators if used to produceeither hasty or deliberate smoke.

As smoke is released into the atmosphere, it istransported and diffused downwind. The plume isdepleted quite rapidly by atmospheric turbulence.The obscuration power of the plume becomesmarginal at relatively short downwind distances

and must be replenished at each point where theattenuation of a line of sight approaches aminimum. The transport wind speed and directionfor a diffusing plume in the surface boundary layerof the atmosphere occurs at a height of about half of the plume height. Usually, this would be aheight of about 10 meters. For smoke operations,then, speeds and directions should be obtained fora height of about 10 meters above the surface.

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The relative humidity of the atmosphere isimportant to the use of smoke on a battlefield. Aspreviously stated, WP, RP, and HC smokecompounds are hydroscopic—they absorbmoisture from the atmosphere. As relativehumidity increases, the amount of screeningmaterial available for target obscurationincreases. For example, the HC compound isconsidered to be only about 70-percent efficient;that is, for every 100 grams of HC in a munition,only 70 grams are available for screening. If therelative humidity yield factor is then added in, thescreening power of HC increases. This is shown inTable 2-1. Applicable technical references indicatethe amount of HC or WP contained in variousmunitions. For example, the 105-millimeter WP(M416) round contains 6 pounds of WP; the 155-millimeter HC (Ml16A1) round contains 5.45pounds of HC; and the 76-millimeter WP (M361A1)round contains 1.38 pounds of WP (453.6 gramsequals 1 pound).

Phosphorous compounds are considered to be better screening agents than HC. This is becauseWP and RP have large yield factors for various

relative humidities. Yields for WP are also shownin Table 2-1. Upon ignition, WP burns at atemperature of about 800ºC to 850°C. As aconsequence, the smoke from a WP munitionpillars, creating an excellent vertical screen,especially with high relative humidities. However,only about 10 percent of the smoke generated fromWP munitions is available for screening near the

ground. This should be considered when planningsmoke missions.

Battlefield visibility can be practically definedas the distance at which a potential target can beseen and identified against any background.Reduction of visibility on a battlefield by anycause reduces the amount of smoke needed toobscure a target.

Turbulence, atmospheric instability, andwind speed can have an adverse effect upon smokeexpenditures. Unstable conditions are usuallyconsidered to be unfavorable for the use of smoke.Under calm or nearly calm conditions, the use of smoke is also sometimes unsatisfactory. Ingeneral, if the wind speed is less than 3 knots orgreater than 20 knots, smoke can be anunsatisfactory countermeasure on the battlefield.

Operations

Smoke operations are of two types: hasty anddeliberate.

Hasty Smoke

Hasty smoke generally is placed in the area to be screened by artillery, smoke pots, or mortarprojectiles. Obscuring smoke usually is employedon enemy forces to degrade their vision bothwithin and beyond their location. Screening

smoke is used in areas between friendly and enemyforces to degrade enemy ground and aerialobservation and to defeat or degrade enemyelectro-optical systems. Screening smoke also may be employed to conceal friendly ground maneuver.Deception or decoy smoke is used in conjunctionwith other measures to deceive the enemyregarding friendly intentions. Decoy smoke can beused on several approaches to an objective todeceive the enemy as to the actual avenue of themain attack.

In the offense, hasty smoke may be used toestablish screens, enabling units to maneuver

behind or under screens and deny the enemyinformation about strength, position, activities,and movement. Ideally, a screen should be placedapproximately 500 to 800 meters short of theenemy to allow for maximum visibility formounted forces during the final assault. Hastyscreens on the flanks also can be used. Flankingscreens can be produced with mechanized

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generators. Hasty obscuring smoke also may be ideal for this type of hasty smoke.placed on enemy strongpoints. Figure 2-3 shows the positioning of an

On defense, hasty smoke may be used to obscuring hasty smoke cloud on enemy forces forimpede and disrupt enemy formations. It also may tail wind and head wind conditions. Figure 2-4 be used beyond the forward line of own troops illustrates screening smoke for flank and

(FLOT) to silhouette Threat targets as they emerge quartering winds ahead of an advancing force.through the smoke and are engaged. Smoke Figure 2-5 is an example of mechanized unitsscreens also may be used to conceal defensive generating a smoke screen for a counterattackingpositions and cover disengaging and moving force.forces. Mechanized smoke generator units are

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Del i berat e Smoke

Large area smoke screens generally fallwithin the realm of deliberate smoke in that theyare usually planned well in advance of theoperation. Large area screening or theestablishment of a smoke blanket or haze isgenerally carried out by the use of smokegenerators. Generators usually are positioned in aline source configuration at a right angle to theprevailing wind direction. Usually, if the terrainallows it, the generators are evenly spaced alongthe smoke line. Generators are ideal for screeningriver crossings if the prevailing wind direction isupstream, downstream, or a tail wind.

The employment of large smoke is probably

most effective if the screen is generated beforesunrise when stable conditions and light-to-moderate winds are most likely. Screens generatedin these conditions will remain close to the groundwith only moderate vertical diffusion. Screens alsoreduce incoming solar radiation reaching theground so that convective turbulence issuppressed, similar to overcast weather

conditions. Thus, smoke hazes and blankets can be maintained and remain useful for longer timeperiods.

The use of large area smoke screens in anyarea depends upon the prevailing wind direction.Operators must be prepared to shift theirgenerators to preselected locations if the winddirection changes.

Tactical Considerations

In addition to the importance of winddirection, relative humidity, visibility, stability,and turbulence to the successful completion of asmoke mission, the effects of terrain and soilconditions should be considered. Terrain effects

discussed in Appendix C apply to smoke as well asNBC agents. A diffusing smoke plume also tendsto follow the terrain-influenced surface winds.Also, in forests and jungles smoke has a tendencyto be more evenly dispersed and to persist longerthan over more open terrain.

The condition of the soil influences theeffectiveness of artillery-delivered and mortar-

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delivered smoke but has very little direct effectupon screening or obscuring smoke. An impactingsmoke munition bursting in soft soil loseseffectiveness since part of the filling compound isdriven into the dirt. In some cases, totallyineffective screens result if smoke munitions aredelivered to a boggy or swampy target area.

A last point to consider involves winddirection effects upon smoke screens. Munitions

expenditures for a screen deployed in quarteringwind conditions must be increased by a factor of about 1.5 over a flank wind direction condition.For head and tail winds, expenditures are three tofour times those for flank winds. Thus, reductionin expenditures owing to visibility and relativehumidity effects may be negated by winddirections.

IncendiariesWeather conditions have little influence on

incendiary munitions themselves. Wind andprecipitation, however, may greatly influence thecombustibility of the target and its susceptibilityto fire spread. The purposes of incendiaries are tocause maximum fire damage on flammablematerials and objects and to illuminate. Initialaction of the incendiary munition may destroythese materials, or the spreading and continuingof fires started by the incendiary may destroythem. Incendiary materials used include gasolinegels, burning metals, incendiary mixes, and whitephosphorus.

To be effective, incendiary munitions should be used against targets susceptible to fire or heatdamage. A considerable part of the target must beflammable, so the fire can spread. Fire walls andcleared lanes offer some resistance to the spread of fires.

Winds assist in the effectiveness of incendiaries, increase the rate of combustion, andcan spread fires downwind more rapidly. Actually,each large fire can create a wind system of its own.This wind system results from the tremendousheat generated and the resulting vertical windcurrents. Incoming winds can feed more air to thefire. This increases the rate of combustion, which,in turn, can increase the wind. In extreme cases,this wind is called a fire storm and sometimesexceeds 60 knots.

Smoke, sparks, and flames fly in the directionof the wind. Incendiary strikes (at successivetargets) should be planned to begin with thefarthest downwind target and proceed upwind.This will prevent aiming points from becomingobscured by smoke traveling downwind of initialfires. Additionally, the position of friendly forcesor facilities that must not be damaged must be

considered (in relation to the wind direction) whenplanning incendiary strikes.

Temperature, temperature gradient, andclouds have little if any effect on incendiaries.Humidity also has little effect upon incendiarymunitions but may affect combustible material.Wood, vegetation, and similar material absorbsome moisture from the air over a period. If relativehumidities have been high for some time, as in thetropics, it may be more difficult to achievecombustion from incendiary action.

Rain or snowfall, even when light, can rendergrass and brush quite incombustible and make acontinuing fire unlikely. Heavy timbers are notaffected unless they have been exposed to longperiods of precipitation. Combustible materialsexposed to rain may be susceptible to fire damage,such as in mass incendiary attacks. In theseattacks, the heat of combustion may be sufficientto dry combustible materials in the target area.

In regions of high humidities, such as thetropics, mass incendiary attacks generatetremendous amounts of heat, causing verticalwind currents. This rising air can causethunderstorms, counteracting the effects of theincendiaries.

It is difficult to extinguish burning metalswith water; a spray actually speeds the burning.Water surrounding the area of burning metalsprevents fire spread. Water extinguishes burning

phosphorus, but unconsumed particles will burnagain when dry.Three elements of terrain affect the efficient

use of incendiaries. These are soil, vegetation, andtopography. The type of soil affects the impactingof the munition; combustibility of the vegetationaffects the efficiency of the incendiary; andtopography influences wind speed and direction.

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CHAPTER 3

Biological Agents and Nuclear Detonations

In a general war, US forces may be faced by anenemy capable of employing nuclear or biologicalweapons. The effects of weather and terrain on

BiologicalIn a general war, US forces may be faced by an

enemy capable of producing and employing biological agents. These include disease-causingmicroorganisms (pathogens) and toxins. Toxinsare biologically derived chemical substances that

have desirable characteristics for use as biologicalwarfare agents. Toxins may be natural orsynthetic.

Biological agents will most likely bedisseminated as an aerosol. Therefore, a basicknowledge of their field behavior is essential forestimating friendly vulnerability. These agentsdiffer from chemical agents in some aspects of field behavior. Pathogens decay as a result of factors such as weathering. They also require timeto invade a body and multiply enough to overcomethe body’s defenses. This is known as theincubation period. This period may vary from

hours to months, depending on the type of pathogen.The following paragraphs discuss biological

agent dissemination, weather effects, and terraininfluences, and they briefly summarize theinfluence of these on biological agent field

behavior.

Dissemination

Pathogens are most likely to be disseminatedas aerosols. Toxins, on the other hand, may bedisseminated as either aerosols or large liquid

drops. An aerosol is composed of particlescontaining pathogens or toxins. The force of thewind moves it along. At the same time, the aerosolspreads by turbulent diffusion.

Biological agents that die rapidly are said tohave a high decay rate. High wind speeds (10 to 20knots) carry these agents over more extensive

biological agent aerosols and on nuclear weaponsfollow.

Agentsareas during the agent survival period. Multiplewind shifts occur at low wind speeds. These shiftsmay cause more lateral spread and downwinddiffusion than higher speeds. Optimum effectdepends on the nature of the agent and

atmospheric conditions. Highly virulent(malignant) agents with low decay rates canspread over large areas (by low or high windspeeds) and still present a casualty threat.Virulent agents with higher decay rates employedunder the same atmospheric conditions are muchless effective.

Weather Effects

Air stability, temperature, relative humidity,pollutants, cloud coverage, and precipitation havean effect on biological agents.

Ai r S tab i l i t y

Atmospheric stability influences a biologicalcloud in much the same way it affects a chemicalcloud. However, biological agents may be moreeffective in lower concentrations than chemicalagents. This is because of their high potency. Astable atmosphere results in the greatest cloudconcentration and area coverage of biologicalagents. Under unstable and neutral stabilityconditions, more atmospheric mixing occurs. Thisleads to a cloud of lower concentration, but theconcentration is sufficient to inflict significant

casualties. The coverage area under unstablestability conditions is also reduced.

Tempera tu re

Air temperature in the surface boundary layeris related to the amount of sunlight the ground has

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received. Normal atmospheric temperatures havelittle direct effect on the microorganisms of a biological aerosol. Indirectly, however, anincrease in the evaporation rate of the aerosoldroplets normally follows a temperature increase.

There is evidence that survival of most pathogensdecreases most sharply in the range of -20°Cto -40°C and above 49°C. High temperatures killmost bacteria and most viral and rickettsialagents. However, these temperatures will seldom if ever be encountered under natural conditions.Subfreezing temperatures tend to quick-freeze theaerosol after its release, thus decreasing the rate of decay. Exposure to ultraviolet light—one form of the sun’s radiation—increases the decay rate of microorganisms. Ultraviolet light, therefore, has adestructive effect upon the biological aerosol. Mosttoxins are more stable than pathogens and are less

susceptible to the influence of temperature.

Rela t i ve Humid i t y

The relative humidity level favoringemployment of a biological agent aerosol dependsupon whether the aerosol is distributed wet or dry.For a wet aerosol, a high relative humidity retardsevaporation of the tiny droplets containing themicroorganisms. This decreases the decay rate of wet agents, as drying results in the death of thesemicroorganisms. On the other hand, a low relative

humidity is favorable for the employment of dryagents. When the humidity is high, the additionalmoisture in the air may increase the decay rate of the microorganisms of the dry aerosol. This is because moisture speeds up the life cycle of themicroorganisms. Most toxins are more stable thanpathogens and are less susceptible to the influenceof relative humidity.

Po l l u t an t s

Atmospheric pollutant gases can also affectthe survival of pathogens. Pollutant gases have been found to decrease the survival of manypathogens. These gases include nitrogen dioxide,sulfur dioxide, ozone, and carbon monoxide. Thiscould be a significant factor in the battlefield overwhich the air is often polluted.

Cloud Coverage

Cloud coverage in an area influences theamount of solar radiation received by the aerosol.Thus, clouds decrease the amount of destructiveultraviolet light the microorganisms receive.Cloud coverage also influences factors such asground temperature and relative humidity, asdiscussed in Chapter 1.

Prec i p i t a t i on

Precipitation may wash suspended particlesfrom the air. This washout may be significant in aheavy rainstorm but minimal at other times. Highrelative humidities associated with mists, drizzles,and very light rains are also an important factor,These may be either favorable or unfavorable,depending upon the type of agent. The low

temperatures associated with ice, snow, and otherwinter precipitation prolong the life of most biological agents.

Terrain Influences

Soil, vegetation, and rough terrain influence a biological agent aerosol.

Soi l and Veget at i on

Soil influences a biological agent aerosol asrelated to temperature and atmospheric stability.

Appendix C discusses the interrelationship between soil and these weather elements.

Vegetation reduces the number of aerosolparticles. Impact of the suspended particles upontrees and grass causes some particles to settle, andthis settling reduces agent concentration.However, vegetative cover reduces exposure toultraviolet light, increases relative humidities,and may reduce temperatures (while fostering aneutral temperature gradient). All these factorsfavor the survival of wet aerosols.

Rough Terrain

Rough terrain creates wind turbulence, andturbulence influences the vertical diffusion of aerosol. This turbulence reduces agenteffectiveness and area coverage. Terrain affectsthe path of the aerosolsurface concentration.

and the distribution of

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Nuclear DetonationsWhen a nuclear explosion occurs, blast

radiation and heat or thermal effects will occur.The influence of weather and terrain on theseeffects will be discussed in this section. When anuclear weapon detonates at low altitudes, afireball results from the sudden release of immensequantities of energy. The initial temperature of thefireball ranges into millions of degrees, and theinitial pressure ranges to millions of atmospheres.Most of the energy from a nuclear weapondetonation appears in the target area in the form of three distinct effects. These are nuclear radiation, blast, and thermal radiation.

Nuclear Radiat i on. Neutron and gammaradiation from the weapon detonation producescasualties and, in many cases, material damage aswell. Ionized regions, which may interfere ‘with thepropagation of electromagnetic waves associatedwith communication systems and radars, resultwhen the atmosphere absorbs nuclear radiation.

Blast. A blast wave with accompanying drageffects travels outward from the burst.

Thermal Radiat ion. Intense thermalradiation emits from the fireball, causing heatingand combustion of objects in the surrounding area.

In the detonation of a typical fission-typenuclear weapon, the percentage of the total energyappearing as nuclear radiation, blast, or thermalradiation depends on the altitude at which the

burst takes place (subsurface, surface, or air) andon the physical design of the weapon. For burstswithin a few kilometers above the earth’s surface,slightly more than 50 percent of the energy mayappear as blast, approximately 35 percent asthermal energy, and approximately 15 percent asnuclear radiation.

Certain weather conditions will influence theeffects of nuclear weapons. Likewise, differenttypes of terrain will also influence the effects of nuclear weapons. In addition to theseconsiderations, the type of operation can have adirect bearing on weather and terrain effects on

nuclear weapons use.Nuclear Radiation

When a nuclear explosion occurs, one usualresult is the well-known mushroom-shaped cloud.This cloud may extend tens of thousands of meters, and in the case of a surface burst or

shallow subsurface burst, it is a tremendousvertically developed aerosol cloud bearingradioactive material. The effect of wind speed anddirection at various altitudes is of particularinterest. These factors are of great importance inpredicting the location(s) of the fallout that mayresult from a nuclear explosion.

The effects of weather and terrain apply to both the initial and residual effects of nuclearexplosions, although this section will primarilyaddress the residual aspects. For moreinformation on the effects of weather on bothinitial and residual effects, refer to FM 3-3.

Precipi tat ion

Precipitation scavenging can cause the

removal of radioactive particles from theatmosphere. This is known as rainout. Because of the uncertainties associated with weatherpredictions, the locations that could receiverainout cannot be accurately predicted. Rainoutmay occur in the vicinity of ground zero or thecontamination could be carried aloft for tens of kilometers before deposition. The threat of rainoutespecially exists from a surface or subsurface burst. Vast quantities of radioactive debris will becarried aloft and be deposited downwind.However, rainout may cause the fallout area toincrease or decrease and also cause hot spots

within the fallout area.For airbursts, rainout can increase theresidual contamination hazard. Normally, theonly residual hazard from an airburst is a smallneutron induced contamination area around GZ.However, rainout will cause additionalcontaminated areas in unexpected locations.

Yields of 10 kilotons or less present thegreatest potential for rainout, and yields of 60kilotons or more offer the least. Additionally,yields between 10 kilotons and 60 kilotons mayproduce rainout if the nuclear clouds remain at or below rain cloud height.

Rain on an area contaminated by a surface burst changes the pattern of radioactiveintensities by washing off higher elevations,

buildings, equipment, and vegetation. Thisreduces intensities in some areas and possiblyincreases intensities in drainage systems; on lowground; and in flat, poorly drained areas.

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W i nd Speed and D i rect i on

Wind speed and direction at various altitudesare two factors that determine the shape, size,location, and intensities of the fallout pattern on

the ground because contaminated dirt and debrisdeposit downwind. The principles and techniquesof fallout prediction from winds-aloft data are inFM 3-3. Surface winds also play an important rolein the final location of fallout particles. Just assnow falls on pavements or frozen surfaces andsurface winds pile it in drifts, so, too, can localwinds cause localization of fallout material increvices and ditches and against curbs and ledges.This effect is not locally predictable, but personnelmust be aware of the probability of these highlyintense accumulations of radioactive materialoccurring and their natural locations.

Clouds and Ai r Densi t y

Clouds and air density have no significanteffects on fallout patterns.

Terra in Contours

Ditches, gullies, small hills, and ridges offersome protection against the gamma radiationemanating from the contaminated area. Terraincontours also cause local wind systems to develop.These wind systems will affect the final

disposition of fallout on the ground, creating bothhot spots and areas of pattern.

H eavy Fo l i age

Heavy foliage can

low-intensity within the

stop some of the falloutfrom reaching the ground. This may reduce theintensity on the ground.

Soil

Soil surface materials (soil) at the burst sitedetermine particle size (large or small). Theparticle size helps determine when and where mostof the fallout will reach the ground, the largerparticles settling first. Composition of the soil nearground zero will materially affect the size anddecay rate of the pattern of residual radiationinduced by neutrons from the weapon.

Type of O perat i on

Temperature and terrain can also influencethe effects of nuclear radiation on tacticaloperations. The effects of cold weather, desert,

jungle, mountain, and urban operations onnuclear defense planning follow.

Cold Weather Operations

Weather conditions limit the number of passable roadways. Radiological contaminationon roadways may further restrict resupply andtroop movement. Seasonal high winds in the arcticmay present a problem in radiologicalcontamination predictions. These winds mayreduce dose rates at ground zero. At the same time,they extend the area coverage and create a

problem for survey/monitoring teams. Hot spotsor areas of concentrated accumulation of radiological contamination may also occur inareas of heavy snow and snow drifts.

Desert Operations

Desert operations present many varyingproblems. Desert daytime temperatures can vary between 90°F to 125°F (32°C to 52°C). Thesetemperatures create an unstable temperaturegradient. However, with nightfall, the desert coolsrapidly and a stable temperature gradient results.

A possibility of night attacks must be consideredin all planning.Nuclear defense planning in a desert is

generally much the same as in other areas, with afew exceptions. Lack of vegetation and permanentfixtures, such as forests and buildings, makes itnecessary to plan for and construct fortifications.Construction may be difficult because of inconsistencies of the sand. However, sand, incombination with sandbags, gives additionalprotection from radiation exposure. Blowingwinds and sand make widespread radiologicalsurvey patterns likely. The varying terrain may

make radiological survey monitoring verydifficult.

Jungle Operations

Radiation hazards also may be reduced because some of the falling particles are retained by the jungle canopy. Subsequent rains, however,

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will wash these particles toconcentrate them in waterRadiation hot spots will result.

Mountain Operations

the ground andcollection areas.

In the mountains, the deposit of radiologicalcontamination will be very erratic because of rapidly changing wind patterns. Hot spots mayoccur far from the point of detonation, and low-intensity areas may occur very near it. Limitedmobility makes radiological surveys on the grounddifficult, and the difficulty of maintaining aconstant flight altitude makes air surveys highlyinaccurate.

Urban Operations

Buildings provide a measure of protection

against radiological contamination. Taking thisinto consideration, troops who must move in orthrough a suspected contaminated urban areashould travel through buildings, sewers, andtunnels to reduce contamination risk. However,they should consider the dangers of collapse because of blast. They should also considerhazards of debris and fire storms resulting fromruptured and ignited gas or gasoline lines.

Blast

Most of the materiel damage and a

considerable number of the casualties caused byan airburst result from the blast wave. For thisreason, it is desirable to consider the phenomenaassociated with the passage of a blast wavethrough air.

The expansion of the intensely hot gases atextremely high pressures within the fireballcauses a blast wave to form in the air, movingoutward at high velocities. The maincharacteristic of the blast wave is the abrupt rise inpressure above ambient conditions. Thisdifference in pressure with respect to the normalatmospheric pressure is called the overpressure.

Initially, the velocity of the shock front ismany times the speed of sound. However, as thefront progresses outward, it slows down andmoves with the speed of sound.

The magnitude of the air blast parameters isdependent on the yield of the weapon, height of burst, and the distance from ground zero.

The blast wave may last from tenths of asecond to seconds, depending on the yield and thedistance from the burst. Weather, surfaceconditions, topography, and the type of operation being conducted all affect the blast wave.

Weather

Rain and fog may lessen the blast wave because energy dissipates in heating andevaporating the moisture in the atmosphere.

Surface Condit ions

The reflecting nature of the surface over whicha weapon is detonated can significantly influencethe distance to which blast effects extend.Generally, reflecting surfaces, such as thin layersof ice, snow, and water, increase the distance to

which overpressures extend.

Topography

Most data concerning blast effects are basedon flat or gently rolling terrain. There is no quickand simple method for calculating changes hilly ormountainous terrain produce on blast pressures.In general, pressures are greater on the forwardslopes of steep hills and are diminished on reverseslopes when compared with pressures at the samedistance on flat terrain. Blast shielding is nothighly dependent on line-of-sight considerations

because the blast waves will bend or diffractaround obstacles. The influence of small hills orfolds in the ground is considered negligible fortarget analysis. Hills may decrease dynamicpressures and offer some local protection fromflying debris.

Type of Operat ion

Temperature and terrain can also influencethe effect of blast on tactical operations. Theeffects of cold weather and jungles or forests onoperations follow.

Cold Weather OperationsAt subzero temperatures, the radius of damage

to material targets can increase as much as 20percent. These targets include such items as tanks,APCs, artillery, and military vehicles. Anincreased dynamic pressure can result from a

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precursor wave over heat-absorbing surfaces.However, tundra, irregular terrain features, and

broken ice caps break up the pressure wave.Blast effects can drastically interfere with

troop movement by breaking up ice covers andcausing quick thaws. These effects can causeavalanches in mountainous areas. In flat lands,the blast may disturb the permafrost to such anextent as to restrict or disrupt movement.

Jungle or Forest Operations

Initial effects of nuclear detonations are notsignificantly influenced by the dense vegetation.However, the blast wave will probably causeextensive tree blowdown and missile effects.Forests, in general, do not significantly affect the

overpressure but do degrade the dynamic pressureof an air blast wave.

Thermal Radiation

Thermal radiation results from the heat andlight produced by the nuclear explosion. During anuclear explosion, the immediate release of anenormous quantity of energy in a very small spaceresults in an initial fireball temperature thatranges into millions of degrees. For a given type of weapon, the total amount of thermal energy

available is directly proportional to the yield.Within the atmosphere, the principalcharacteristics of thermal radiation are that it—

Travels at the speed of light.Travels in straight lines.Can be scattered.Can be

Documents

Field Behavior of NBC Agents (Including Smoke and Incendiaries) - FM 3-6