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 Influence of Weather, Terrain and Buildings In the dispersal of CW agents, a mixture of liquid droplets and gas is generated. The largest droplets fall and cause ground contamination whereas the very small droplets remain suspended as an aerosol. Together, the aerosol and the gas form a primary cloud which drifts in the wind. Evaporation of ground contamination causes a secondary cloud which also drifts in the wind. The ratio between the primary cloud and ground contamination will be different depending on the CW agent dispersed. Addition of thickeners, the dispersal method used, and the height of the dispersal will also influence distribution. A volatile substance will cause, e.g., a large proportion of primary cloud whereas a persistent substance leads to more ground contamination. Explosive dispersal will lead to a greater proportion of primary cloud than if the substance is dispersed by means of spray or discharged in bulk. Influence of Weather An attack with chemical weapons always causes injuries to unprotected people who are in or close to the target area. Strong wind, heavy rain or temperatures below 0.C may reduce effects. After the attack, the weather will be of great importance for the respiratory risks expected at different distances from the target. Similarly, weather conditions influence the effect of ground contamination. After an attack, the primary cloud will drift with the wind. Wind velocity will be decisive for how long it will take for the primary cloud to pass the given place. High wind velocity implies a short time of passage and thus fewer injuries to unprotected persons, whereas low wind velocity will lead to more injuries. Consequently, a weak wind may cause effects at greater distances than strong winds. Wind velocity also naturally influences how fast the primary cloud moves. If the wind is gentle, then there are better opportunities to provide warning in time. In very weak winds, however, the gas cloud will not move very far. In addition, the wind direction varies widely in such situations which is why a circular area must be alerted in an attack with CW agents. The gas/aerosol concentration in the primary cloud also depends to a high degree on the air exchange or turbulence of the atmosphere. In clear weather during the night, the ground surface is cooled and inversion is formed (stable temperature stratification). Inversion leads to weak turbulence and thus high concentration and great effect from the primary cloud. When the sun shines during the summer, the ground surface becomes warm, with considerable turbulence as a result (unstable temperature stratification). The effect of the primary cloud is then reduced strongly, particularly at long distances. The concentration in the primary cloud may also decrease in cold weather and particularly if the temperature is lower than -20C. This depends on a smaller amount of CW agents evaporating during dispersal which also implies that the share of ground contamination will be greater. Precipitation also reduces the concentration since some of the gas/aerosol is "washed" away by wet deposition. Low temperature implies that ground contamination will remain and cause risks of contact for long periods. In very low temperatures, however, the effect of some substances may cease since they will freeze (freezing-point for hydrogen cyanide -13 C, VX -39 C, mustard agent-lewisite ca. -35 C, soman -42 C). Thick winter clothing will give better protection than summer clothing. A major problem during the winter may be that contaminated snow on shoes and clothes is taken into tents, vehicles or buildings. Once in the warmth, the CW agent will evaporate and may cause dangerously high concentrations of gas. Light rain will cause ground contamination to be more dangerous since the pores in the soil become clogged and prevent the substance from penetrating down into the soil. Heavy rain, however, will flush off ground contamination whereas heavy snow will cover it. In both cases, the contact risk is decreased. There is frequently a residue of CW agents which have penetrated the soil after the above-mentioned periods. In experiments, residues have been found after three times longer than listed. Consequently, respiration risks may still be present within the target area since the substance in the soil evaporates slowly. In situations of longer stays and activities implying close contact with the soil for longer periods, then the contact risks may remain for the longer period. Terrain and Buildings Woodland and undulating terrain give shorter danger distances for the primary cloud since the wind will be exposed to greater turbulence. Woodland also adsorbs a certain amount of gas and aerosol through dry deposition. In or close to the target area, however, woodland, depressions, pits and narrow streets may lengthen the effect of an attack. Gas and aerosol will be retained in these areas, particularly in situations of weak wind and stable stratification. The longest danger distances are obtained if the cloud passes over plains or lakes, or follows the contours of a valley. The effect of a passing cloud of gas/aerosol will be delayed inside tents, buildings and vehicles. Owing to the lower air exchange in such places, it will take longer for the cloud to penetrate. A certain amount of the CW agent will be taken up and bound on walls and other surfaces, which also contributes to decreases in concentration. Consequently, it may be expected that there is a certain reduction in the effect of a passing cloud of gas. In ordinary buildings, the protection can be improved by closing doors and windows, turning off the ventilation and sealing all cracks with tape. It will, thus, take longer for the cloud of gas to penetrate a house, for example. For the same reason, the gas will remain longer in-doors when the rest of the cloud has passed by. Consequently, it is of the greatest importance to obtain information on when it is possible to start airing the building again. Ground configuration is also of importance for the contact risk in ground contamination. A dry, hard but porous surface, e.g., asphalt or concrete, will lead to fewer contact risks. On soft ground, e.g., grass, moss, sand or snow, it is easier to come into contact with CW agents which have penetrated the underlying surface. In dense woodland, the ground contamination is reduced and becomes uneven since the falling droplets are caught to some extent in the crowns of the trees. Terrain covered by bushes, on the other hand, may lead to major risks of contact. Influence of Weather on the Primary Cloud Factors diminishing the danger of the primary cloud: Influence of Weather on Ground Contamination Factors decreasing the danger of ground contamination
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