FM 3-6 Field Behavior of NBC Agents

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Preface

Primary u sers of this m anu al are NBC staff officers, staff weath er officers, fire

support coordination personnel, artillery officers, and others involved in planning

NBC operations. These soldiers must und erstand wh at effect weather and terrainhave on nu clear, biological, and chemical (NBC) opera tions and smoke. This man ual

contains 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 references

such 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 and

staff officers mu st und erstand how w eather and terrain can be u sed to theiradvantage.

FM 3-6 implements International Standardization Agreement (STANAG) 2103,

Reporting Nuclear Detonations, Radioactive Fallout, and Biological and ChemicalAttacks and Predicting Associated Hazard s.

This man ual explains how w eather an d terrain influence nuclear, biological, and

chemical operations and discusses the following top ics for use when plann ingoperations:

Basic principles of meteorology as they p ertain to N BC opera tions.

Influence of weather on the use and behavior of NBC agents.Local weather pred ictions 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 N avy gets m eteorological forecasts from

comp onents of the N aval Oceanograph y Com man d. Meteorological reportinformation is in the N AVOCEANCOMIN ST 3140.1 pu blications series. It alsocontains information on the behavior of smoke clouds and incendiaries. In addition, it

discusses the influences of weather and terrain on the thermal, blast, and radiation

effects of a nuclear detonation.)

Staffs planning the use of chemical weapons and commanders approving strikesmu st understand basic weather characteristics. Therefore, weather analyses

significantly influence the selection of agents and mu nitions for emp loyment. Thetarget analyst must know his or her weather data needs and where to get thisinformation in a combat env ironment. Chapter 1 covers meteorology and the imp act

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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 pu blication are encouraged to recomm end changes and submitcomments for its improvement. Key each comment to the specific page and paragraph

in wh ich the change is recommend ed. Provide a reason for each comment to ensu re

und erstanding and comp lete evaluation. To send changes or comments, prepare DA

Form 2028 (Recommend ed Changes to Pu blications and Blank Forms) and forward it

to Comm andant, USACMLS, ATTN: ATZN-CM-NF, Fort McClellan, AL 36205-5020.

Air Force comm ents go to HQ USAF/ XOORF, Washington, DC 20330. Marine Corp s

comments go to Comm anding General, Marine Corps Development and EducationCommand (CO93), Quantico, VA 22134. Navy comments go to Chief of NavalOperations (OP-954), Navy Depar tment, 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 w ind, battlefield, the soldier must also understand howair stability, hu midity, 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 an d terrain influence and have sp ecific

Chemical agents may app ear in the field in effects on them .different forms: vapors, aerosols, or liquid s. 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 quicklydep osited on the groun d. Many agents give offvapors that form vap or cloud s. The speed at wh ichan agent gives off vapors is called volatility.Agents may be removed n aturally from the air byfalling out (large particles fall out much morequickly), by sticking to the ground or vegetation, orby being removed by precipitation. Once depositedup on vegetation or other grou nd cover, volatileagents may be re-released to th e atmosph ere for

further cycles of travel and present a hazard untilsufficiently diluted or d econtaminated.

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 vap or in 15 seconds. Light wind sand low turbulence allow high localconcentrations of agents. High winds and strongturbulence reduce the concentration and increase

the 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,du e to its buoyancy, is called th e height of thethermally stabilized cloud . If the vap or density ofthe released agent is less than the vapor d ensity ofair, the cloud r ises qu ite rapid ly, mixes with thesurrounding air, and dilutes rapidly. If the agentforms a d ense gas (the vap or d ensity of thereleased agent is greater than th e vapor density ofair), the cloud flattens, sinks, and flows ov er theearths surface. Generally, cloud growth duringthe first 30 second s is more d epend ent up on the

munition or delivery system than uponsurround ing meteorological conditions.

Nevertheless, the height to which the cloudeventually rises depends upon air temperature andturbu lence. These d etermine how mu ch 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 surround ing

air. The chemical cloud is subjected to tu rbulenceforces 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 vap or 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 d issolved gases are also p resent 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 mu ch the samemanner as vaporized agents. Initially, aerosolcloud s formed from therm al generators have ahigher temperature than clouds formed from othertypes of mu nitions. This may cause some initialrise of the cloud at the release point. Aerosol-generated cloud s are heavier than vap or clouds,and they tend to retain their forms and settle backto earth. Being heavier than vapor clouds, they areinfluenced less by turbulence. However, as thecloud s travel down wind , gravity settles out thelarger, heavier p articles. Many par ticles stick toleaves and other vegetative sur faces they contact.

Liquids

When a chemical agent is used for its liqu ideffect, evaporation causes the agent to form intovapor. Depending u pon volatility, vapor cloud s areusually of low concentration, have about the same

temperatu re as the surroun ding air, and tend tostay near the surface because of high vapordensity. Additionally, vapor density governs theextent that the vap or 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 (goingback into the air) begins. The vap or concentrationover areas contaminated with a liquid agent tends

to be less than with n ewly formed vap or cloud s,and dow nw ind agent concentrations are notnearly as great as with other typ es of agents.

Atmospheric Stability

One of the key factors in u sing chemicalweapons is the determination of the atmosphericstability condition that will exist at the time ofattack. This determination can be m ade from am eteorological repo rt or by o bserving fieldconditions.

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

categories/ conditions. When m eteorologicalreports 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 th e current observedweather cond itions (or estimated w eatherconditions). Follow the decision tree to determinethe stability condition. The stability conditionplus the w ind sp eed 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 inup per left corner at the word START. Followarrowed line to the first question. Answerquestion Is it nighttime? by selecting,

theth eth ein

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accordance with the facts, the yes or no arrowindicating your d ecision. 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 p ath, answer each and proceed along them ost nearly correct path un til a dispersioncategory is identified. The result from Figure 1-2 isthe stability category. An example of the u se ofFigure 1-2 is if you are inland one hou r beforesunset and the winds are calm, the stabilitycategory is neu tral (N) (category 4).

The dispersion category, the wind speed inknots, and the w ind d irection are the mostimportant meteorological data for deciding theinfluence of weath er on vapor cloud d ispersion.For any given dispersion category, a lower windspeed will produce higher dosages, smaller area

coverage, and , consequently, h igher toxic effects.This is because when the wind speed is lower, thecloud moves more slowly past the ind ividual in thetarget area; and the individual is in the cloudlonger, yielding a h igher d ose of the agent. SeeTable 1-1 for the dispersion categories and w indspeeds d uring w hich 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 agent

releases will alter the table results somewhat, butthe sam e trend s occur . The m ain effect to beconsidered for elevated release effectiveness over aspecific target is that the agen t mu st be releasedfurther up wind 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 vap or. Table1-2 indicates the typ ical cloud wid ths at givendownwind distances from a point source releasefor a chemical agent vapor cloud. Note that thecloud w idth depends up on 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\ M). If the agent is released from aline sou rce (spray system), the line length shouldbe added to the cloud width (Table 1-2) to determine

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

chemical cloud width s listed in Table 1-2 areestimates. The w idths w ill vary d epending on theweather and terr ain of a specific area.

The following exam ples are cited to explainfurther the use of Table 1-2. Based on a chemicalagent vap or being released from a point source indispersion category 4, the chemical cloud width at7 kilometers downwind would be app roximately2.3 kilometers. Based on a chemical agent vap orbeing 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/ M) at different d istancesdownwind 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 dep ending 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 were

stationary. The dosage would decrease if the targetwere moving through the downwind cloud hazardarea. Ad d itionally, in gen eral, if the sou rcestrength (100 kg) were doubled, the dosage wouldalso double, and if the source strength were halved,the d osage 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 w ould be 18.91mg-min/ M. With dispersion category 2, windspeed 3 knots, and at a dow nw ind d istance of 4kilometers, the center line dosage would be 1.030mg-min\ M.

Vapor Concentration and Diffusion

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

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continually expand, agent concentration levelsdecrease over time. Wind sp eed d etermines 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 groun d targets, high concentrationsand good coverage can best be achieved w ith lowturbulence and calm w inds wh en the agent is

delivered d irectly on target. A steady, pred ictablewind drift over the target is best when the agent isdelivered on the up w ind side of the target.Conditions other than these tend to produce lowerconcentrations an d/ or poorer target coverage.How ever, un less weather conditions are know nwithin the target area, the effects of the agent ontarget will be approximations.

The concentration an d diffusion of a chemicalagent cloud are also influen ced by the factors ofhyd rolysis, absorption, adsorp tion, 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 canbe important for smoke screens. See the discussionof the effect of humidity on increasing smokescreen effectiveness in Chapter 2.

Absorption is the p rocess of the agent being

taken into the vegetation, skin, soil, or material.Adsorption is the ad ding of a thin layer of agent tovegetation or other surfaces. This is imp ortant 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 turbu lenceblow it from side to side. The side-to-side motion ofthe 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 oflateral spread that occurs under differentdispersion categories and distances downwind. Inmore unstable conditions, the lateral spread tendsto be greater than in stable cond itions.

Wind currents carry chemical clouds along th eground 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 agen t cloud to beretarded as the cloud stretches out in length. Whenclouds are released on th e ground , the dragamounts to about 10 percent of the vertical growthover distance traveled over grass, plowed land, orwater . It amou nts to about 20 percent over gentlyrolling terrain covered with bushes, growingcrops, or small patches of scattered timber. Inheavy w oods, the d rag effect is greatly increased.The vertical spread of the cloud is illustra ted 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 ofwind shear, a puff (or chemical cloud) may becomestretched in the dow nwind direction and maytravel in a d irection d ifferent from that of thesurface wind . Add itionally, a chemical cloudreleased in th e 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 effectbecomes more p ronounced and increasesproportionately with the distance of the forward

edge of the cloud from the source. Figure 1-5illustrates th is. A sm all pu ff of agent cloudreleased from its source some time earlier has tiltedforward, while the bottom has been retarded due toslower wind s caused by d rag.

The vertical rise of a chemical cloud dep endsupon weather variables, such as temperaturegradient, wind speed, and tu rbulence, and the

Vapors andWind, temp erature, hum idity, precipitation,

terrain contour s, and surface cover influen ce thefield behavior of vapors and aerosols. Forexamp le, in a chemical attack on US forces (lstDivision) 26 Febru ary 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 somewh at diluted an d attain the sametemperature and approximately the same densityas surrounding air.

The vapor cloud formed by an agent normallyemp loyed for per sistent effect rises in a similarmanner, but vapor concentrations build up moregradually.

Aerosolsand heavy underbrush in the target areacontribu ted to the ov erall effectiveness of achemical attack. Several additional casualtiesresulted d ue to the increased chem ical agentpersistency caused by the favorable weatherconditions. Favorable and unfavorable weather

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

If a chemical cloud is to be placed directly onan occup ied 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 m ost apt to occur on a calm, clear nigh t. If asmall amount of air movement is required tospread the cloud evenly over the target area, a lowwind speed and stable or neutral cond itions 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 cond itions 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 med ium w ind speedskeeps the cloud traveling over the area without toomuch diffusion, and the stable or neutralconditions keep the agent concentration h igh andthe cloud close to the ground .

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

Wind

High w ind speeds cause rapid d ispersion ofvap ors or aerosols, thereby d ecreasing effectivecoverage of the target area and time of exposure tothe agent. In high w inds, larger qu antities ofmunitions are required to ensure effectiveconcentrations. Agen t clouds are m ost effectivewhen w ind speeds are less than 4 knots and steady

in direction. The clouds move with the prevailingwind as altered by terrain and vegetation. Steady,low w ind 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 effect 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 d isperse chemical agents.Unstable is the least favorable condition forchemical agent u se because it results in a lowerconcentration, thereby redu cing th e area affectedby the agent. Many more mu nitions are required toattain the commanders objectives und er unstableconditions than under stable or neutral conditions.

Stable conditions (such as low wind speedsand slight turbu lence) pr odu ce the highestconcentrations. Chemical agents remain near theground and may travel for long distances beforebeing d issipated . Stable cond itions encourage the

agent cloud to rem ain intact, thu s allowing it tocover extremely large areas withou t d iffusion.However, the direction and extent of cloud travelund er stable conditions are not predictable if thereare no dependable local wind data. A very stablecondition is the m ost favorable cond ition forachieving a high concentration from a chem icalcloud being dispersed.

Neutral conditions are moderately favorable.With low w ind speed an d sm ooth 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 thetemperatu re increases. See FM 3-9/ AFR 355-7 forspecific information on chem ical agents, such astheir boiling an d freezing p oints and v apor

density.

Humidity

Hu midity is the measure of the water vap orcontent of the air. Hydrolysis is a process in whichcomp ound s reactchemical change.

with water resulting in aChemical agents with high

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hydrolysis rates are less effective under conditionsof high hum idity.

Hu mid ity has little effect on m ost 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 notbe employed effectively except for a surprise time-on-target (TOT) attack because of rapidhyd rolysis. Lewisite hyd rolysis by-products arenot dangerou s 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 hu midity combined with high temperatu resmay increase the effectiveness of some agentsbecause of body perspiration that will absorb the

agents and allow for better tran sfer.

Precipitation

The overall effect of precipitation isun favorable because it is extremely effective inwashing chemical vapors an d aerosols from theair, vegetation, and m aterial. Weather forecasts orobservations ind icating the p resence of orpotential for precipitation present an unfavorableenvironment for employment of chemical agents.

Terrain Contours

Terrain contours influence the flow ofchemical clouds the same as they influenceairflow. Chemical cloud s tend to flow over lowrolling terrain and down valleys and settle inhollows and depressions and on low ground. Localwind s coming dow n valleys at night or up valleysdu ring the day may d eflect 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 d osages 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 ofadjacent land and water, daytime breezes from the

water and nighttime breezes from the land controlchemical cloud travel.

Surface Cover

Ground covered w ith tall grass or bru shretard s flow. Obstacles, such as bu ildings or trees,set up edd ies that tend to break up the cloud andcause it to dissipate more rapidly. However, streetcanyons or spaces between bu ildings m ay havepockets of high concentrations. Flat coun try(during a neutral or inversion condition) or openwater promotes an even, steady cloud flow. Figure1-5 illustrates the horizontal and vertical spread ofa 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 to

meteorology or diffusion, is called vegetativecanopy or just canopy. The effects of canop ies areconsidered below.

Woods a re considered to be trees in full leaf(coniferous or d eciduous forests). The termheavily wooded canopy denotes jungles orforests with canopies of sufficient density to shadem ore than 90 percent of the grou nd surfacebeneath. 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 d iffusion (stability)category will be similar to those in the open.

When bombs are dropped into a wooded area,some may be expected to bu rst in the treetops.Although the released aerosol and vap or settletoward the groun d, some of the agent is lost,depend ing upon the thickness and height of thefoliage. The initial burst and pancake areas ofchemical clouds released within woods or junglesare smaller than those released in the op en.However, concentrations within the initial cloudsare higher in wooded areas, sometimes three timesthat of bursts in the op en. The m agnitude ofconcentration from ground bursts depends uponthe density of und ergrowth and trees.

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

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wind speeds und er the canopies spread agentclouds slowly in a downwind and downslopedirection. Areas of dense vegetation also increasethe p otential surface area for the d eposition ofchemical agents. If there are gu llies and stream

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

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

defense (where lethal dosages may be obtained in30 second s or less), the am ount of agent absorbedby foliage will have little or no effect on the su ccessof the attack. H igh concentrations of chemicalagents may destroy vegetation, since the leaves

absorb som e 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 ofevaporation. That, in tu rn, influences thepersistence of a chemical agent liquid and theconcentration of the vapor. Most weather

conditions do not a ffect the quan tity of mu nitionsneeded for an effective initial liquidcontamination. Table 1-5 sum marizes favorableand un favorable 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 wh en the soil temp eratu re is just abovethe agen ts freezing p oint. This limits the rate ofevaporation of the liquid. Other favorableconditions are low wind speed, wooded areas, andno rain.

Conversely, unfavorable conditions are highsoil temperatur e, high w ind speed, bare terrain,and heavy rain.

Favorable and unfavorable conditions forliquid agen ts for vapor concentra tion effects aremu ch the same as those for chem ical cloud s. Inwoods, however, a high temperature w ith only avery light wind gives the highest vaporconcentrations.

Weather

Duration of the effectiveness of initial liquidcontamination m ay be affected by w ind sp eed;stability, mixing height, and temp erature; andprecipitation.

Wind Speed

Wind d irectionthe upw ind side of a

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is imp ortant in d eterminingtarget for release purposes but

has little impact on the d ura tion of effectiveness,regard less of the method of release The vaporcreated b y evap oration of the liqu id agent,how ever, moves with the w ind. Therefore, thevapor concentration is greatest on the downwind

side of the contaminated area. Vapors are movedby the wind as discussed earlier in this chapter.Evaporation du e to wind speed depends on the

amoun t of the liquid exposed to the wind (thesurface of the liquid ) and th e rate at w hich airpa sses over the agen t. Therefore, the dur ation ofeffectiveness is longer at the places of greaterliquid agent contamination an d in places wh erethe liquid agent is sheltered from the wind.

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

effectiveness of the contamination. Increasedevaporation, in turn, creates a larger vap or cloud .The vapor cloud, in tur n, is dispersed by higherwind s. 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 un evenly d istributed contaminant.Heav ier 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 up onthe num ber of munitions used and the uniformityof dispersal.

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than when the liquid agent is unevenly dispersed down wind from the sprayed area.(as with bursting munitions). With spraying, the Some chemical agents have no significantdu ration of effectiveness decreases, and there is a vapor pressure, and, consequently, their rates ofcorresponding increase in the vapor concentration evaporation are not affected by w ind sp eed. Also,

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some of these agents are extremely toxic, so even avery slight su rface 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 un der higher wind speeds, theycover wid er areas m ore effectively. Thu s, 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 windspeed s may cause comp lete vaporization beforethe agent reaches the ground, creating only avap or hazard . The resulting vapor cloud isnonpersistent and dissipates quite rapidly du e tothe high degree of mechanical turbulenceassociated with high w ind 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 ofeffectiveness 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.How ever, a contaminated area that has beensubjected to pronounced turbulence does notremain contaminated as long as one that has beensubjected to only slight tu rbu lence with low w indspeeds.

Turbulence also influences the sp raying of

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

Stability, Mixing Height,and Temperature

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 p revail, there u sually islittle or no thermal tu rbulence, wind speeds are

low, and th e degree of mechanical turbu lence isalso low. Often stable conditions exist continuallyonly near the grou nd. Above the top of the stablesurface layer, wind speed and turbulence areincreased. Wind d irection h ere also m ay be

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

The mixing height is the capping inversion atthe top of the mixing layer and serves as a lid. Itprevents further u pw ard vertical growth of achemical vapor. A m ixing height can also existabove unstable or neutral surface stabilityconditions. In rad iation inversions, whichcommon ly form a t night, the top of the surface-

based (mixing) stable layer is very close to theearths 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 th emorn ing, solar rad iation heats the surface andcauses a good mixing condition close to theground . The m ixing height and turbulencecondition increase until they destroy the stablelayer. The mixing height can extend from theearths surface up to 2 kilometers in elevation on a

hot summer day. On a calm, clear night, themixing height m ay extend only 50 to 100 metersabove the earths surface.

If a chemical agent is released above th esurface stable layer, most of the agent remainsaloft in the tu rbulence 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 temperatu re grad ient. With this grad ient,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 h eight and theheight of the top of the surface stable layer.

Und er u nstable 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 grou nd,

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and much of it may dissipate before reaching thetarget area.

Temperature is one of the most importantfactors affecting the d uration of effectiveness andvap or concentration of liqu id agents. Agents

emp loyed for persistent effect acqu ire 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 airtemp eratu re with a range of only 10 to 20 degreesbetween day and night. In the sum mer intemperate zones, the surface temperatu re may bemu ch higher than that of the air in the daytimeand mu ch 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 offrapidly a short d istance away. Temperature ofvehicles, buildings, and other su rfaces may bewarmer. This is because of internal heat sou rcesand/ or higher solar heating.

From a defensive viewpoint, a dangeroussituation is likely to occur on a summer eveningwh en the ground temperature is still high and astable condition has started to set in. Under thesecond itions, a heavy vapor cloud prod uced byevaporation could be dangerous downwind to adistance of 2,000 meters or m ore. With ord inaryconcentrations, however, danger from vapor is

somewhat less.Another imp ortant temp erature factor to

consider is that p eople perspire freely and w earlightweight 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 temperatu re of theagent itself is vitally imp ortant. Generally, liquidagents are not effective when used at temperaturesbelow 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,indu ces body perspiration and , therefore,increases the effectiveness of these agents. Also,permeable protective clothing is less resistant

when sweat-soaked than wh en dry. Since sweatyskin is more susceptible to the action of vapor,lower vapor d osages prod uce casualties when thehumidity is high.

Precip i ta t ionLight rains distribute persistent agents more

evenly over a large sur face. 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 longdu ration tend to w ash away liquid chemicalagents. These agents may then collect in areaspreviously uncontaminated (such as stream bed sand depressions) and present an unp lannedcontamination hazard.

The evaporation rate of a liquid agent reduces

when the agent is covered with water but returns tonormal w hen th e water is gone. Precipitation m ayforce back to the sur face some p ersistent agentsthat hav e lost their contact effectiveness bysoaking into the soil or other p orous su rfaces.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 ofliquid agents. However, the slope or contour mayaffect the d elivery m eans capable of mostefficiently delivering the agent on an area (forexample, reverse slopes are norm ally not good forartillery emp loyment, and m ountainous terrainmay restrict use of spray tanks).

Vegetation

When p ersistent agents are u sed in vegetatedareas, some of the contaminant clings to grass andleaves. This increases the surface agent exposed tothe air and , hence, the rate of evapora tion.Personnel become m ost susceptible to liquidchemical agents in vegetated areas, because theyare more ap t to come in contact w ith the agent by

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

du ration of effectiveness.When bombs or shells burst in woods, usually

most of the liquid falls near enough to the groundto be effective. An exception is bursts in virginforests with d ense canopies that m ay extend to 50meters high.

A th ick jun gle or forest canopy usu allyprevents liquid agent spray from airplanes fromreaching th e groun d 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 w hich liquid agents are p lacedinfluences the evaporation rate and the duration ofeffectiveness. Bare, hard ground favors short-termeffectiveness and high-vap or concentration. If thesurface is porou s, such as sand , the liquid agentquickly soaks in; and the area no longer appears tobe contaminated.

The rate at which liquid agents evaporatefrom a sandy or p orous surface is about 1/ 3 less

than the evaporation rate from nonabsorbentsur faces. Extend ed contact with a contam inatedporou s material is dangerous if unp rotected.However, if there is no free liquid on the surface,the danger from brief contact is relatively small if

protected. If a p orous su rface on w hich liquidcontamination falls has been w et 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 amu stard agent (HD) falls onto a w et surface, itstays in globules; and a thin, oily film spreads overthe surface, making contamination easier todetect.

Other Surfaces

Persistence of liquids on p ainted su rfaces ofvehicles 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 ofdrop s to give more surface area for evaporation.

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

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Smoke

CHAPTER 2

and Incendiaries

Smoke and incendiaries are combat long been emp loyed as a means of concealingmultipliers. Their effective use on a target can battlefield targets. Add itionally, incendiary fireprovide tactical advantages for offensive and dam age causes casualties and m ateriel damagedefensive operations. For examp le, 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 are

typically electro-optical devices and generallyoperate in the near-, mid-, or far-infrared por tionsof the electromagnetic spectrum, rather than in thevisible light band of the spectrum. The use ofsmoke in the target area can be a convincingcombat multiplier offensively and a dynamiccountermeasure defensively. Smoke should be ofprimary interest to all command ers and staffplanners because the p roper u se of smoke canprovide many operational advantages.

Smoke has four general uses on thebattlefieldobscuring, screening, deceiving, andidentify ing/ signalling. Obscuring smoke is placedon an enemy to redu ce vision both at, and out from,the position. Screening smoke is used in friendlyoperational areas or between friendly u nits andthe enemy. Deceiving smoke is used to mislead theenemy. Identifying/ signalling smoke is a form ofcomm unication that has m ultiple uses. Overall,the objective of smoke emp loyment is to increasethe effectiveness of Army operations w hilereducing 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 cond itions to surp rise the enemy.Deceive the enemy.

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

Some offensive ap plications includ e 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 op erations, smoke can beeffectively used to blind enemy observation pointsto depr ive the enemy of the opportu nity to adjustfire, to isolate enemy elements to permitconcentration of fire and counterattack, and todegrade the performance of threat ATGMs.

There are generally tw o categories of smokeoperations on a battlefield-hasty and deliberatesmoke. Hasty sm oke operations are conductedw ith m inimu m p rior planning, normally tocounter some enemy action or anticipated action ofimmediate concern to a commander. Hasty smokeis usually used on small areas, is of short d uration,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 p eriod by brigad es, 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 suspen d ed in the air. Thesepar ticles scatter or absorb the light, thus red ucingvisibility. 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 u sed to obscure orconceal depend s 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 requ ired for effectiveobscuration because of the nonscatteringproperties of the particles. At night or under lowvisibility conditions, considerably less sm oke isneeded.

Grayish or white smoke obscures by reflectingor scattering light rays, prod ucing a glare. Duringbright daylight conditions, less white smoke thanblack smoke is required to obscure a target. Years

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

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

Smoke, when p laced between a target and a

viewer, d egrad es the effectiveness of target-acquisition and aiming systems. The amount ofsmoke necessary to defeat aiming and acquisitionsystems is highly d ependent up on the p revailingmeteorological 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 w ith 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 p articles into the

surface and planetary boundary layers of the

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atmosphere generally obeys physical laws.Diffusion is governed by wind speed, turbulence,stability of the atm osphere, 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 p oint sources, continu ous 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 aninstantaneou s source. A series of genera tors, setup crosswind, represent a line source. Munitionswhich scatter smoke-generating submunitions inan area are considered an area source.

Weather Effects

Meteorological conditions that have the mosteffect on smoke screening and mun itionsexpenditu res (includ ing the deployment of smokegenerators) include wind direction, relativehumidity, visibility, and atmospheric stability. Tobe effective, an obscuring screen must be placed inan ad vantageou s position w ith respect to theprevailing w ind direction. The target ar ea to be

screened mu st be defined in terms of whether theprevailing w ind 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 th e right or left side of the screeningarea and th at there are four qu artering-winddirections. Wind direction is critical fordetermining the ad justm ent or aim p oint forscreens deployed by artillery or mortars and alsofor the placement of generators if used to prod uceeither 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 d ownw ind d istances

and mu st 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 atmosph ere occurs at a height of about halfof 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 isimpor tant to th e use of smoke on a battlefield. Aspreviously stated, WP, RP, and H C smokecompounds are hydroscopicthey absorbm oisture from th e atmosp here. As relative

hu mid ity increases, the amoun t 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 variousmu nitions. For example, the 105-millimeter WP(M416) roun d contains 6 pou nd s of WP; the 155-millimeter HC (Ml16A1) round contains 5.45pounds of HC; and the 76-millimeter WP (M361A1)roun d contains 1.38 pou nd s of WP (453.6 gram sequals 1 pound).

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

relative hum idities. Yields for WP are also shownin Table 2-1. Upon ignition, WP burns at atemperature of about 800C to 850C. 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 m issions.

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.

Turbu lence, atmosp heric instability, andwind speed can have an ad verse effect upon smokeexpenditures. Unstable conditions are usuallyconsidered to be unfavorable for the use of smoke.Under calm or nearly calm conditions, the use ofsmoke 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 tobe screened by artillery, smoke pots, or mortarprojectiles. Obscur ing smoke usu ally is employedon enem y forces to degrad e their vision bothwithin and beyond their location. Screeningsmoke is used in areas between friendly and enemyforces to degrade enemy ground and aerialobservation and to defeat or degrad e enemyelectro-optical systems. Screening smoke also maybe 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 movem ent. Ideally, a screen should be p lacedapp roximately 500 to 800 meters short of theenemy to allow for m axim um 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 p ositioning of an

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

(FLOT) to silhouette Threat targets as they emerge quartering wind s ahead of an advan cing 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 u nits are

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Deliberate Smoke

Large area sm oke screens generally fallwithin the realm of deliberate smoke in that theyare usu ally plann ed w ell in ad vance of theoperation. Large area screening or theestablishment of a smoke blanket or haze isgenerally carried out by the use of smokegenerators. Generators usu ally are positioned in aline source configuration at a right angle to thepreva iling w ind direction. Usually, if the terrainallows it, the generators are evenly spaced alongthe smoke line. Generators are id eal for screeningriver crossings if the prevailing wind direction isup stream, downstream, or a tail wind.

The employm ent of large smoke is probablymost 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 alsoredu ce incoming solar rad iation reaching theground so that convective turbulence issuppressed, similar to overcast weather

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

The use of large area smoke screens in anyarea depend s upon th e prevailing wind d irection.Operators mu st 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 effectsdiscussed 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 tenden cyto be more evenly dispersed and to persist longerthan over more open terrain.

The cond ition of the soil influences theeffectiveness of artillery-delivered and mortar-

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delivered smoke bu t has very little direct effectupon screening or obscuring smoke. An impactingsmoke m un ition b ursting in soft soil loseseffectiveness since part of th e filling compou nd isdriven into th e dirt. In some cases, totally

ineffective screens result if smoke munitions aredelivered to a boggy or sw ampy target area.

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

expend itures for a screen dep loyed in quarteringwind conditions must be increased by a factor ofabou t 1.5 over a flank w ind d irection condition.For head and tail winds, expenditures are three tofour times those for flank winds. Thus, reduction

in expenditures owing to visibility and relativehu mid ity effects may be negated by w inddirections.

IncendiariesWeather cond itions have little influence on

incend iary munitions them selves. Wind an dprecipitation, however, may greatly influence thecombustibility of the target and its susceptibilityto fire spread . The pu rposes of incendiaries are to

cause maximu m fire d amage on flammablematerials and objects and to illum inate. Initialaction of the incend iary mu nition m ay d estroythese 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 mu nitions shou ldbe used against targets susceptible to fire or heatdamage. A considerable part of the target must beflamm able, so the fire can spread. Fire walls andcleared lanes offer some resistance to the spread offires.

Winds assist in the effectiveness ofincend iaries, 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 th e tremendou sheat generated and the resulting vertical windcurrents. Incoming w inds 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) shou ld be planned to begin w ith thefarthest downw ind target and p roceed upw ind.This will pr event aiming p oints from becomingobscured by smoke traveling d ownw ind of initialfires. Add itionally, the position of friend ly forcesor facilities that mu st not be dam aged m ust be

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

Temp erature, temp erature grad ient, 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 wh en light, can rendergrass and bru sh qu ite 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 w ater; a spray actually speeds the burn ing.Water surround ing the area of burn ing metalsprevents fire spread. Water extinguishes burning

phosphorus, but u nconsumed p articles will burnagain when dry.Three elemen ts of terra in affect th e 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 Nu clear Detonations

In a general w ar, 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 prod ucing and em ployingbiological agents. These include disease-causingmicroorgan isms (pathogen s) and toxins. Toxinsare biologically derived chemical substan ces 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 offield behavior. Pathogens decay as a result offactors such as weathering. They also require timeto invade a body and multiply enough to overcomethe bodys defenses. This is known as theincubation period. This period m ay vary from

hou rs to month s, dep end ing on the type ofpathogen.The following paragraphs discuss biological

agent dissemination, weather effects, and terraininfluences, and they briefly summarize theinfluence of these on biological agen t fieldbehavior.

Dissemination

Pathogens are most likely to be d isseminatedas aerosols. Toxins, on the other hand , may bedisseminated as either aerosols or large liquidd rop s. An aerosol is comp osed of pa rticlescontaining pathogens or toxins. The force of thewind moves it along. At the same time, the aerosolspreads by turbulent diffusion.

Biological agents th at d ie rapid ly are said tohave a high decay rate. High wind speeds (10 to 20knots) carry th ese agents over m ore 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 w ith low d ecay rates canspread over large areas (by low or high windspeed s) and still present a casu alty 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.

Air Stability

Atmospheric stability influences a biologicalcloud in mu ch the same w ay it affects a chemicalcloud . How ever, biological agents may be mor eeffective in low er concentrations th an chemicalagents. This is because of th eir 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 th econcentrat ion is sufficient to inflict sign ificant

casualties. The coverage area under unstablestability conditions is also redu ced.

Temperature

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

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received. Normal atmosph eric temperatures havelittle direct effect on the microorganisms of abiological aerosol. Indirectly, however, anincrease in the evapora tion rate of the aerosoldroplets normally follows a temperature increase.

There is evidence that su rvival of most pathogensdecreases most sharply in th e range of -20Cto -40C and above 49C. High temperatures killmost bacteria and most viral and rickettsialagents. However, these temperatures will seldom ifever be encountered under natural conditions.Subfreezing temperatures tend to quick-freeze theaerosol after its release, thus decreasing the rate ofdecay. Exposure to ultraviolet lightone form ofthe suns radiationincreases the decay rate ofmicroorganisms. Ultraviolet light, therefore, has adestructive effect upon the biological aerosol. Mosttoxins are more stable than pathogens and are less

susceptible to the influence of temperatu re.

Relat ive Humidi ty

The relative humidity level favoringemployment of a biological agent aerosol dependsupon whether the aerosol is distributed wet or dry.For a w et aerosol, a high relative humid ity retardsevaporation of the tiny d roplets containing themicroorganisms. This decreases the decay rate ofwet agen ts, 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 hum idity is high, the add itionalmoisture in the air may increase the decay rate ofthe m icroorganisms of the dry aerosol. This isbecause moistu re speed s up the life cycle of themicroorganisms. Most toxins are more stable thanpathogens and are less susceptible to the influenceof relative humidity.

Pollutants

Atmosph eric pollutan t gases can also affectthe su rvival of pathogens. Pollutant gases havebeen foun d to d ecrease the survival of ma nypath ogens. These gases includ e nitrogen d ioxide,sulfur d ioxide, ozone, and carbon m onoxide. Thiscould be a significant factor in the battlefield overwhich the air is often polluted.

Cloud Coverage

Cloud coverage in an area influen ces theamou nt of solar rad iation received by the aerosol.Thus, clouds decrease the amount of destructiveultraviolet light the m icroorganisms receive.Cloud coverage also influences factors such asground temperature and relative humidity, asdiscussed in Chapter 1.

Precipi tat ion

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 un favorable,depending upon the type of agent. The low

temperatures associated with ice, snow, and otherwinter pr ecipitation prolong th e life of mostbiological a gents.

Terrain Influences

Soil, vegetation, and rough terrain influence abiological agent aerosol.

Soil and Vegetation

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

App endix C d iscusses the interrelationshipbetween 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.How ever, vegetative cover redu ces 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 turbu lence, andturbulence influences the vertical diffusion ofaerosol. This turbulence reduces agenteffectiveness an d area coverag e. Terrain affectsthe path of the aerosolsurface concentration.

and the distribution of

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Nuclear Detonations

Wh en a nu clear explosion occu rs, blastrad iation and h eat or therm al 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 th einitial pressure ranges to m illions of atmosph eres.Most of the energy from a nu clear weap ondetonation app ears in the target area in the form ofthree d istinct effects. These are nu clear radiation,blast, and thermal radiation.

Nuclear Radiat ion. Neutron and gamm aradiation from the weapon d etonation prod ucescasualties and, in many cases, material damage aswell. Ionized regions, which may inter fere with th epropagation of electromagnetic waves associatedwith comm un ication systems and radars, resultwhen the atmosphere absorbs nu clear rad iation.

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

Thermal Radiation. Intense thermalrad iation emits from the fireball, causing h eatingand combustion of objects in the surrounding area.

In the detonation of a typical fission-typenuclear weapon, the p ercentage of the total energyappearing as nuclear radiation, blast, or thermalradiation depends on the altitude at which theburst 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 m ayapp ear as blast, approximately 35 percent asthermal energy, and approximately 15 percent asnuclear radiation.

Certain weather conditions will influence theeffects of nu clear w eapon s. Likewise, d ifferenttypes of terrain will also influen ce the effects ofnuclear w eapons. In addition to theseconsiderations, the type of operation can have adirect bearing on weath er and t errain effects onnuclear weapons use.

Nuclear Radiation

When a nuclear explosion occurs, one usu alresult is the well-known mushroom-shaped cloud.This cloud may extend tens of thousands ofmeters, and in the case of a sur face burst or

shallow subsurface burst, it is a tremendousvertically d eveloped aerosol cloud bearingradioactive material. The effect of wind speed anddirection at various a ltitud es is of par ticularinterest. These factors are of great importance inpredicting the location(s) of the fallout that m ayresult from a nuclear explosion.

The effects of weather and terrain apply toboth the initial and residual effects of nuclearexplosions, although th is section w ill pr imarilyaddress the residual aspects. For moreinformation on the effects of weather on bothinitial and residual effects, refer to FM 3-3.

Precipitation

Precipitation scavenging can cause the

removal of radioactive particles from theatmosphere. This is known as ra inout. Because ofthe uncertainties associated with weatherpredictions, the locations th at could receiverainout cannot be accurately predicted. Rainoutmay occur in the vicinity of ground zero or thecontamination could be carried aloft for tens ofkilometers before deposition. The threat of rainoutespecially exists from a surface or subsurfaceburst. Vast quantities of radioactive d ebris will becarried aloft and be deposited downwind.How ever, rainou t m ay 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 residu al hazard from an airburst is a smallneutron indu ced contamination area around GZ.However, rainout will cause additionalcontaminated areas in unexpected locations.

Yield s of 10 kiloton s or less pr esent th egreatest potential for rainout, and yields of 60kilotons or more offer the least. Add itionally,yields between 10 kilotons an d 60 kilotons m ayprod uce rainout if the nuclear cloud s remain at orbelow rain cloud height.

Rain on an area contaminated by a su rfaceburst changes the pattern of radioactiveintensities by w ashing off higher elevations,buildings, equipment, and vegetation. Thisredu ces intensities in some areas and possiblyincreases intensities in drainage systems; on lowground; and in flat, poorly drained areas.

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W ind Speed and Direction

Wind speed and direction at various altitudesare two factors that d etermine the shap e, size,location, and intensities of the fallout pattern on

the ground because contaminated d irt and d ebrisdeposit 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 wind s p ile it in d rifts, so, too, can localwinds cause localization of fallout material increvices and ditches and against curbs and ledges.This effect is not locally predictable, but personnelmu st be aware of the probability of these highlyintense accum ulations of rad ioactive materialoccurring and their natural locations.

Clouds and Air Density

Clouds and air density have no significanteffects on fallout patterns.

Terrain 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 ofpattern.

Heavy Foliage

Heav y foliage can

low-intensity within the

stop some of the falloutfrom reaching the groun d. This may redu ce theintensity on the ground .

Soil

Soil surface materials (soil) at the burst site

d etermine pa rticle size (large or sm all). Theparticle size helps determine when and w here mostof the fallout will reach the groun d , 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 Operation

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

jungle, mountain, and urban op erations onnuclear defense planning follow.

Cold Weather Operations

Weather conditions limit the n um ber ofpassable road ways. Rad iological contam inationon roadw ays may further restrict resupp ly 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 ofradiological contamination may also occur inareas of heavy snow an d snow drifts.

Desert Operations

Desert op erations p resent m any varyingproblems. Desert daytime temperatures can varybetween 90F to 125F (32C to 52C). Thesetemp eratures create an unstable temp eraturegrad ient. However , with nightfall, the desert coolsrapidly and a stable temperatu re gradient results.

A possibility of night attacks must be consideredin all plann ing.N uclear d efense planning in a d esert is

generally mu ch 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 ofinconsistencies of the sand . How ever, sand, incombination with sandbags, gives additionalprotection from ra d iation exposu re. Blowingwinds and sand make widespread radiologicalsurvey patterns likely. The varying terrain may

make radiological survey monitoring verydifficult.

Jun gle Operations

Radiation hazar ds also may be redu cedbecause some of the falling particles are retainedby 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 ofrapidly changing w ind p atterns. 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 p rovide a measu re of protection

against rad iological contam ination. Taking thisinto consideration, troops wh o mu st move in orthrough a suspected contaminated urban areashould travel through buildings, sewers, andtunn els to red uce contamination risk. However,they shou ld consider the d angers of collapsebecause of blast. They shou ld also considerhazard s 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 w ithin 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. How ever, as thefront progresses outward, it slows down andmoves with the speed of sound.

The magnitude of the air blast parameters isdep endent on the yield of the weapon, height ofburst, and the distance from ground zero.

The blast wav e may last from tenths of asecond to seconds, depend ing on the yield and thedistance from the burst. Weather, surfaceconditions, topograp hy, and th e type of operationbeing conducted all affect the blast wave.

Weather

Rain an d fog m ay lessen the blast w avebecause energy dissipates in heating andevaporating the moisture in the atmosphere.

Surface Conditions

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 d istance to

which overpressures extend.

Topography

Most d ata concerning b last effects are ba sedon 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 th e blast wav es will bend or d iffractarou nd 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 Operation

Temp erature and terrain can also influencethe effect of blast on tactical operations. Theeffects of cold w eather an d jun gles or forests onoperations follow.

Cold Weather OperationsAt subzero temperatures, the radius of damage

to material targets can increase as mu ch 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, andbroken ice caps break up the pressure w ave.

Blast effects can drastically interfere withtroop m ovement by breaking up ice covers andcausing quick thaws. These effects can causeavalanches in mountainous areas. In flat lands,the blast may d isturb th e permafrost to such anextent as to restrict or disrup t movemen t.

Jun gle 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, d o 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 ofweap on, the total amount of thermal energy

available is directly proportional to the yield.Within the atmosphere, the principalcharacteristics of therm al radiation ar e that it

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

The therm al effects will be influenced byw eather, terrain, height of burst, and typ e ofoperation.

Weather

Any condition that significantly affects thevisibility or th e transp arency of the air a ffects thetransmission of thermal radiation. Clouds, smoke(including artificial), fog, snow, or rain absorb andscatter thermal energy. Depend ing on theconcentration, they can stop as much as 90 percentof the thermal energy. On the other hand , clouds

above the bu rst may reflect ad ditional thermalrad iation onto the target that would h aveotherwise traveled harmlessly into the sky.

Terrain

Large hill

Documents

FM 3-6 Field Behavior of NBC Agents