Unlike nuclear fallout. the dispersal of chemical
contamination is normally confined to a far smaller
area, with much of the effects of the chemical
depending on the local weather conditions.
Considering the limited area effected by chemical
weapons, any plot of chemical contamination is
normally done only on the smaller scale charts, such as
In amphibious battle situations supported by U.S.
Naval ships and Marine Corps forces, you must prepare
for the possibility that the opposing force will employ
chemical weapons either against the ships or against the
ground forces supported by your ship. The attack may
come from bombs, rockets. or aerial spray. Accurate
calculations for the area contaminated by the chemical
agent are only possible after the agent has been
identified. These calculations are preformed by
forecasters or NBC evaluators following procedures
detailed in ATP-45. Evaluated information is passed as
a message following the NBC 3 CHEM format. As a
Aerographer, you should be able to interpret a NAV
NBC 3 CHEM or an NBC 3 CHEM reportthe
Chemical Downwind Message, and to plot the possible
contamination area reported in the message.
Later, as you study to become a forecaster, you will
learn about the format of a Chemical Downwind
Forecast (CDF)a product of basic forecast
meteorological parameters used to evaluate how far
downwind a particular chemical agent remains
hazardous. CDFs are normally used by non-
meteorological personnel at NBC centers to evaluate
the duration and extent of chemical contamination for a
specific chemical agent.
CHEMICAL WARFARE TERMS
Chemical agents may be spread by many means,
such as bombs, mortar shells, artillery shells, rockets,
missiles. mines. generator fog, or aircraft spray. The
area where the chemical agent is released is called the
attack area. The attack area includes a minimum radius
of 1/2 nautical milc (at sea) or 1 kilometer (ashore)
around the release site. This area is considered to be
immediately contaminated because of the explosives
distributing the agent or by the initial chemical spray.
Chemical agents are spread outward from the attack
area by diffusion through the air and by mixing caused
by the winds. The area contaminated by airborne
chemical agents outside the attack area is called the
hazard area. In situations involving calm or light winds
(5 knots or less), the contamination is considered to
spread outward from the attack area in all directions.
When the winds are greater than 5 knots, describing
the hazard area is not as simple. Generally, the hazard
area will form a triangular pattern. The contamination
on the upwind side of the attack area is prevented
because the contamination is carried with the wind
toward the downwind side of the attack area. The
information needed to describe the triangular pattern
and the orientation of the hazard triangle is the chemical
downwind direction, the downwind hazard distance,
and the radial angles.
The direction the contamination is carried by the
wind is called the chemical downwind direction (CDD).
The CDD is 180 degrees opposite the surface-wind
The distance the contamination is carried by the
wind, outside the attack area, in high enough
concentration to cause casualties, is called the
downwind hazard distance (DHD), as shown in figure
5-9. The DHD depends on the type of agent, the means
of delivery, the terrain, and the effects of the
meteorological elements on the agent. The DHD is a
minimum of 5 nautical miles (10 kilometers) and may
extend up to 27 nautical miles (SO kilometers) from the
center of the attack area.
Radials extend tangent from the edge of the attack
area 30 degrees either side of the CDD ashore. At sea,
35° radial angles are used when the winds are between 5
and 10 knots, but only 20° radial angles are used when
the winds are 10 knots or greater. The angles used to
draw the radials account for variations in wind direction
and horizontal diffusion of the chemical agent. These
angles are referred to as half-sector angles (HSA).
The chemical down windspeed (CDS) is considered
the same as the prevailing surface wind speed.
However, calculations involving the time the leading
edge of a chemical cloud arrives at a specific downwind
location use a maximum speed of the CDS multiplied by
1.5 in order to account for variability in wind speed or
One basic characteristic of a chemical agent that
must be considered is the persistency of the agentthe
tendency of the agent to vaporize to form a hazardous
gas. Non-persistent (NP) agents are generally
hazardous gases that remain in the target area for
minutes or, in exceptional cases. for several hours after
the attack. Persistent (P) agents are generally
hazardous liquids which usually emit a hazardous vapor
that remains in the target area for hours or days or, in
exceptional cases, weeks after the attack.