High winds increase evaporation rates of liquid chemical agents and dissipate
chemical clouds.
Effects of wind speed on persistent-effect agents are
variable.
Nonpersistent agents are most effective in winds less than 15
knots.
Wind direction must be considered for target coverage and to
determine hazards to friendly troops.
Wind speed and direction are vital factors in estimating the amount of
equipment, munitions, and fog oil required for a smoke operations.
Wind
speeds from 4 to 10 knots are ideal for a smoke operation.
When considering the use of chemical and biological agents, additional weather
characteristics play a role. These characteristics are:
* Temperature Gradient is the difference in air temperature at two levels. For
chemical operations, it is determined by subtracting the air temperature one-
half meter above the ground from the air temperature about 4 meters above the
ground.
Vertical variations in temperature effect air stability, which in
turn effects the vertical air currents. The more stable the air layer, the
more effective a chemical attack will be.
* Temperature Gradient Inversion is an increase in air temperature with an
increase in height. In this condition, the air lies essentially in unbroken
layers with little or no vertical movement between layers.
This condition
usually exists on a clear or partially clear night when clouds cover less
than 30 percent of the sky and during early morning hours about 1 hour after
sunrise, when the wind speed is less than 5 knots.
These are ideal
conditions for the use of chemical agents.
* Temperature Gradient Lapse is a decrease in air temperature with an increase
in height. The same conditions exist for an inversion temperature gradient,
except the lapse temperature gradient is characterized by turbulence and
thermal air currents.
These conditions are least favorable for use of
chemical agents.
* Temperature Gradient Neutral is the condition that exists between temperature
gradients lapse and inversion. It usually exists on heavily overcast days or
nights one to two hours before sunset and after sunrise, when clouds cover 30
percent of the sky. It may also exist when speed is greater than 5 knots and
during periods of precipitation.
* Direction.
Casualties from downwind chemical cloud travel are a bonus for
most weapon systems.
Wind direction is determined by the direction from
which the wind is blowing.
IT 0462
3-4
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