Restricted visibility weakens ultraviolet radiation, decreasing the decay rate of
some biological agents. It helps achieve surprise in the attack by making
observation of chemical and biological clouds more difficult. It reduces the need
for screening smokes.
* Clouds affect aviation activities by reducing visibility.
Natural
illumination is decreased by clouds, although cloud cover may increase the
effects of certain types of illumination devices such as searchlights.
Cloud cover reduces ultraviolet radiation and decreases the decay rate of
some agents.
Heavy clouds increase surface temperatures which is favorable
for daytime use of chemical dispersal agents. It is during heavy cloud cover
and a moderately stable atmosphere that production of smoke is most
favorable.
Heavy snow or rain affects mobility, personnel, and
equipment.
Precipitation reduces the effectiveness of chemical agents.
Heavy rain washes liquid contaminants into low areas and steambeds,
presenting a lingering hazard. Restricted visibility produced by heavy rain
or snow makes the use of smoke unnecessary.
* Temperature/Humidity.
Extremes of temperature adversely affect tactical
operations. Wet biological agents in aerosol form survive best in moderately
cool temperature.
Aerosols have high decay rates in low humidity; moisture
slows decay rates.
Blister agents (HD) are highly effective in hot, humid
weather because the body is more sensitive to vapor and liquid under these
conditions.
High temperatures increase evaporation rates of agents and
decrease the duration of effectiveness.
The reverse is true at low
temperatures.
The temperature has no significant effect on smoke screens generated by
hexachlorethane mixture (HC) smoke pots.
Humidity has only a slight effect
on screens produced by smoke generators.
In high humidity, HC smoke pots
generate more effective smoke screens.
Humidity has the least significant
effect on the duration of most liquid chemical agents.
* Wind speed affects the degree of cold-windchill factor.
For example,
personnel can be reasonably comfortable at temperatures down to 0F; however,
a 15-mph wind at 5 or a 30-mph wind at 15F can cause flesh to freeze. Wind
may affect visibility by creating dense screens of blowing sand, dust, or
snow.
Wind is important in airmobile and airborne operations.
It also
affects the accuracy of indirect-fire weapons.
Biological agents are effective at various speeds.
Downwind area coverage
increases with increased wind speed.
3-3
IT 0462
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