EFFECTS OF WEATHER AND TERRAIN CA79

OBJECTIVES Describe the effects of weather on the movement of agent vapor following an accidental release Describe how terrain factors influence movement of agent vapor following an accidental release CA80

EFFECTS OF WEATHER AND TERRAIN Weather elements and chemical properties determine how far a chemical warfare agent vapor plume will travel how concentrated the chemical warfare agent will be within the plume CA81

WINDSPEED One of the most important factors determining how far vapor plume will go is windspeed Higher windspeed: vapor plume disperses quickly and shorter downwind hazard distance Lower windspeed: vapor plume more likely remains intact, lengthening downwind hazard distance CA82

WIND DIRECTION Since vapor plume will ride wind current, wind direction is always important Vapor plumes do not drift upwind Mechanical turbulence, brought about by wind blowing over and around objects, tends to mix agent vapor with surrounding air, making concentration less, thereby reducing hazard Weather vane symbolizing wind direction CA83

VERTICAL TEMPERATURE GRADIENT The difference between temperature of air at ground level and at higher levels Perhaps most important element in determining vertical dispersion and travel of vapor plume During day solar radiation heats ground more than surrounding air air next to ground is warm and rises cooler air from above sinks to ground surface mixing called thermal turbulence CA84

THERMAL TURBULENCE Disperses chemical warfare agent vapors, thereby reducing likelihood of dangerous concentrations of chemical agent vapors in the plume Occurs during daylight hours and is more pronounced on sunny, summer days CA85

INVERSION At times, ground cooler than air overhead; creates a condition known as an inversion Thermal mixing does not occur during inversions Occurs mainly at night Thermal mixing does not occur during inversions CA86

Neutral conditions common at sunrise and sunset and on overcast days Thermal mixing moderate Occur when temperature of air overhead is somewhat cooler than air at ground level but temperature gradient not enough to produce strong vertical mixing Tend to occur at sunrise, sunset, and during cloudy, overcast days Neutral conditions common at sunrise and sunset and on overcast days CA87

STABILITY CLASSES CA88

Precipitation has complex effects on dispersion of agent Removes agent from atmosphere Water runoff containing chemical warfare agent collects in low places Agent is not neutralized some chemical changes may take place Snow may cover agent agent still toxic even at freezing temperatures When snow melts, chemical warfare agent contamination problem may remain Precipitation has complex effects on dispersion of agent CA89

TERRAIN In hilly terrain On open ground air near ground warms and moves uphill in daytime air near ground cools and flows downhill at night On open ground ground warms rapidly on sunny days increasing temperature gradient ground cools at night and temperature gradient decreases or reverses temperature gradient is more pronounced in desert areas CA90

VEGETATION Reduces spread of contamination Reduces windspeed absorption - penetration of chemical warfare agent into vegetation adsorption - build-up of chemical warfare agent on the surface of vegetation May give off agent vapors after absorption and adsorption Heavy forests tend to reduce amount of thermal turbulence under their canopies creating neutral conditions CA91

BODIES OF WATER Large bodies of water also affect temperature gradients because water retains heat better than soil Inlet of the Chesapeake Bay from Aberdeen Proving Ground CA92

DETERMINING THE HAZARD Hazard from a chemical release would depend on how much agent is released into the air the amount of agent involved the chemical properties of agent the temperature gradient Windspeed and direction would determine how soon an agent plume arrives at a given point CA93

COMPUTER MODELING The Army uses computer simulation models to estimate how far vapor chemical release will travel and the concentration of agent at various distances from the source of the release This information is used in the models to estimate extent of hazard Based upon model results, decisions such as if and where to evacuate, how soon to begin evacuation, and when to shelter, would be made CA94