1. EFFECTS OF WEATHER AND TERRAIN
CA79

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. STABILITY CLASSES
CA88

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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