Atmospheric Stability Charts
Inversion Temperature Gradient (Stable-S). This condition usually exists on a clear or partially clear night when middle and low clouds cover less than 30 percent of the sky, and on early mornings until about 1 hour after sunrise when the wind Speed is less than 5 kmph--ideal for enemy employment of chemical agents.
Neutral Temperature Gradient (Neutral-N). This condition usually exists on heavily overcast days or nights at 1 or 2 hours before sunset or 1 to 2 hours after sunrise when the middle and low clouds cover more than 30 percent of the sky. Independent of cloud cover and time of day, a neutral condition may also exist when the wind speed is greater than 5 kmph. Additionally, periods of precipitation are normally accompanied by a neutral condition. A neutral temperature gradient is most favorable for enemy use of biological agents.
Lapse Temperature Gradient (Unstable-U). This condition normally exists on a clear day when the middle and low clouds cover less than 30 percent of the sky and when the wind speed is less than 5 kmph. It is the least favorable condition for the enemy to employ chemical or biological agents. When a lapse condition exists, area coverage without diffusion will be enhanced with a steady low wind speed of 3 to 7 kmph.
To obtain the air stability category, refer to Table 2-1. Enter Table 2-2 with the category obtained from Table 2-1. Select the appropriate weather and terrain conditions from Table 2-2. Read across to where the columns intersect and extract the final stability category. For more information on field expedient behavior of chemical agents, see FM 3-6 or from CDM.
Once the proper stability condition is determined, use Table 2-3 to compute the downwind distance for Type A attacks.
Plotting Chemical Agent Hazards
Classification of chemical agents:
Chemical Hazard Plotting Steps
Air-Contaminating Agents--Type A
Type A agents normally are dispersed as aerosols or vapor clouds with little or no contamination on the ground.
Ground-Contaminating Agents--Type B
Type B agents normally are expected to be dispersed in liquid form to contaminate surfaces.
Adjusted Hazard Prediction
Adjust hazard predictions when:
- Windspeed change of 10 kmph or more
- Wind speed increases from less than 10 kmph to more than 10kmph or the reverse.
- Change of air stability category (Type A attacks only)
- Downwind direction change of 30° or more.
Persistency of Chemical Agents
Table 2-5 shows persistency of chemical agents for moderate contamination. As a rule of thumb, cooler conditions increase the persistency of chemical agents. Persistency triples as contamination levels increase from moderate to heavy. Chemical agent persistency data in Table 2-5 is for surface winds of 10 kmph and considers weathering only (without decon). For other surfaces use the following multiplication factors for the times given: alkyd paint = 1.3, bare soil = 4.0. Agent HL is not shown. To approximate HL, use GD persistency times. Concentrations of contamination are considered to be heavy at (10 grams per square meter. One week is considered to be 168 hours. One month (30 days) is equal to 720 hours.
Time of Arrival for Chemical Hazards
The earliest an agent can be expected to arrive at a location is determined by dividing the distance from the attack center by twice the wind speed. For example, if you are 10 kilometers from the attack center and the wind speed is 5 kilometers per hour, the earliest the agent cloud would arrive at your location would be one hour.
10 km ÷ (2 x 5 kmph) = 1 hour
|Join the GlobalSecurity.org mailing list|