Wind direction and speed (degrees true and
knots)
Current air temperature and daily mean surface
air temperature (°C)
Relative humidity
Total cloud cover (clear, scattered, broken, or
overcast)
Air stability (ashore)
Air-sea temperature difference or sea-surface
temperature (sea)
Presence and type of any precipitation
Presence of any temperature-inversion layers
aloft with a base less than 1,500 feet
The elevation angle of the sun above the horizon is
also an important value you may be asked to provide.
However, the Quartermasters on the bridge may be able
to determine this value faster and more accurately by
measurement with a sextant.
Ashore, the air stability is determined by a near
surface temperature gradient—difference between the
surface air temperature and the air temperature at 100
meters (330 feet) AGL. The stability should be
determined by your forecaster, as should the presence of
temperature inversion layers aloft. The evaluated
stability that the NBC evaluator needs is a statement of
“stable” (any temperature increase or a decrease <l°C
from surface to 100 meters), "neutral" (1°C decrease
from surface to 100 meters), or “unstable” (>1°C
decrease in the first 100 meters). As for elevated
temperature inversions, the evaluator only needs to
know if an elevated inversion is present below 1,500
feet.
At sea, a slightly different evaluation procedure is
used. The difference between the air temperature and
the sea-surface temperature (not necessarily the sea-
water injection temperature) is used by the evaluator,
along with the wind speed, to determine stability. The
value required is Tair - T
sea surface
. Negative values
generally indicate unstable conditions, values near zero
indicate neutral conditions, and positive values indicate
stable conditions. Since this value is critical, the
evaluator may request the sea-surface temperature,
instead of or in addition to the air-sea temperature
difference, to preform or check the calculation.
As long as your ship or unit remains in or near a
hazard area, you must also monitor the winds. You
must immediately report any significant changes in
wind speed or direction to the forecaster or NBC
evaluator. Significant changes include the following:
Any change in the 2-minute wind speed of 5
knots or greater.
Any change in the 2-minute wind speed
decreasing to less than or increasing to more than
5 knots.
Any change in the 2-minute wind direction of
more than 20 degrees.
Any change in the stability category, that is from
stable to unstable or vice versa.
At sea or ashore, your activity may be tasked to
maintain a chart of chemically contaminated areas.
Unlike a standard weather chart, a single chart is used.
It is updated by plotting new attack and hazard areas as
attacks occur and by erasing or deleting old hazard areas
or attack areas as the contamination decreases to safe
levels. The duration of the hazardous contamination
may be different within the attack area and the hazard
area. This information. along with other critical
evaluated information, may be provided by your
forecaster or received from an NBC evaluation center-as
a chemical downwind message.
Q16.
Q17.
Q18.
Q19.
Q20.
Q21.
REVIEW QUESTIONS
What is the minimum radius of a chemical
warfare attack area from ground zero?
What is meant by the term "DHD"?
Given a wind speed of 16 knots, what would be
the approximate downwind speed of the leading
edge of the chemical cloud?
What is the assumed downwind hazard distance
of a ground contaminating agent used against
Naval Forces?
What meteorological parameters are required
for a chemical hazard evaluation?
What would an air/sea temperature stability
value of -5 indicate?
CHEMICAL DOWNWIND MESSAGE
The NBC 3 CHEM message, known as a chemical
downwind message, is used to report evaluated infor-
mation on the chemical agent employed in an attack and
the area contaminated ashore (fig. 5-10). A NAV NBC
3 CHEM message is used to report similar information
to Naval Forces.
5-12
