EFFECTS OF WEATHER AND TERRAIN

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EFFECTS OF WEATHER AND TERRAIN
CA79
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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
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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
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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
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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
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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
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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

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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

• 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
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STABILITY CLASSES
CA88
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PRECIPITATION

• 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
• 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
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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
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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
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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
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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