employment of chemical agents.
Over large bodies
of
water,
weak
inversion conditions tend to prevail during the day.
2.
Lapse Temperature Gradient
A decrease in air temperature with an increase in height is known as a
lapse (unstable) condition. This condition normally exists on a clear or
partially clear day when the middle and low clouds cover less than 30% of
the sky and when the wind speed is less than 5 knots.
It is
characterized by turbulence and thermal air currents and is the least
favorable condition for the employment of chemical agents. Over large
bodies of water, weak lapse conditions tend to prevail at night.
3.
Neutral Temperature Gradient
A condition intermediate between lapse and inversion is known as a
neutral condition. It usually exists on heavily overcast days or nights
at 1 to 2 hours before sunset, or 1 to 2 hours after sunrise, when the
middle and low clouds cover more than 30% 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 knots.
Additionally, periods of
precipitation are normally accompanied by a neutral condition.
Changes in surface temperatures, where land meets large bodies of water,
affect the flow of air carrying a chemical agent cloud.
As shown is
Figure 3-1, air flow will be seaward at night and landward during the
warmth of day.
Temperatures can be extremely important in the
persistency of chemical agents.
In warm temperatures, the rate of
evaporation will shorten the time that a liquid chemical agent is
effective, while at cooler temperatures a chemical agent will remain for
a much longer period of time. Some liquid chemical agents will freeze at
low temperatures. When exposed to heat such as in a warm-up tent, these
agents thaw and again become casualty producers.
CM7114
3-8
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