Index

METOC Effects Smart Book

Notice:  Provided as information only.  FM 34-81/AFM 105-4 (Weather Support For
Army Operations) was used as the source reference.  Consult with your local operators/customers before use
.

Table of Contents

Chapter 1 - Weather Effects on Armor
Chapter 2 - Weather Effects on Artillery
Chapter 3 - Weather Effects on Aviation
Chapter 4 - Weather Effects on Nuclear, Biological, and Chemical Ops
Chapter 5 - Weather Effects on Enemy/Threat Systems
Chapter 6 - Weather Effects on Engineer Ops
Chapter 7 - Weather Effects on Intelligence and Electronic Warfare
Chapter 8 - Weather Effects on Light Infantry
Chapter 9 - Weather Effects on Mechanized Infantry
Chapter 10 - Weather Effects on Electro-Optical Systems
Appendix 1 Light Data Samples
Appendix 2 Temperature, Heat Index, Windchill Charts
Appendix 3 Brightness Value Chart, NVG Data Example
Appendix 4 Glossary

Chapter 1

WEATHER EFFECTS ON ARMOR

Armor operations are influenced primarily by those weather elements that degrade trafficability and visibility. Although the effect may be more pronounced for armor operations, weather elements have generally similar types of impacts on other units.

BAROMETRIC PRESSURE.  The weight of the air affects gunnery computations and ballistic performance.

CLOUDS AND SKY COVER.  Low overcast clouds limit the effectiveness of aerial illumination devices. Overcast clouds tend to limit heating of inactive targets and lower target detection ranges for thermal sights. NVG are limited by clouds blocking natural light from the moon and stars. Close air support (CAS) and aerial resupply missions are degraded by low clouds.

HUMIDITY.  Coupled with high temperatures, high humidity decreases crew effectiveness in closed vehicles.

ILLUMINATION.  Best use of most NVG requires about a quarter (23 percent) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 5 degrees below the horizon. Detailed products dealing with the use of E-O devices are available from the SWO.

PRECIPITATION.  Rain and snow degrade trafficability and limit visibility. They also degrade target acquisition and NVG.

REFRACTIVE INDEX.  This index affects radar, laser, and infrared distance measurements.

SURFACE WIND.  Trajectory projections and first round hit capability affected by high crosswinds.

STATE-OF-THE-GROUND.  Frozen ground improves mobility and significantly increases the time available to prepare fighting positions. Deep snow slows movement of tracked vehicles. Frozen ground and mud affects munitions, sensors, and indirect fire.

TEMPERATURE.  Temperatures influence the type of lubricants to be used, engine warm-up periods, and sustained rates of fire for weapons. High temperatures decrease the time personnel can remain in vehicles. High temperatures cause gun tube droop, shimmering, mirages, and vehicle exteriors to be too hot to touch. Extremely high temperatures increase water consumption. Low temperatures degrade the ballistics of main guns and require frequent starting of engines and may increase maintenance problems and possible detection by the enemy. Extremely low temperatures reduce personnel effectiveness and decrease the availability of water because of freezing. Temperatures changing from above to below freezing can freeze stationary tracks into the mud.

VISIBILITY.  Visibility affects visual acquisition, degrades laser range finding and target acquisition systems.

WINDCHILL.  Winds affect the apparent temperature in which soldiers must operate. The windchill table must be consulted to determine the actual effective temperature.

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Chapter 2

WEATHER EFFECTS ON ARTILLERY

Artillery in the field is heavily weather dependent. Not only must you contend with those weather effects common to all other units but you must also compensate for a number of special effects in the area of target acquisition and aiming.

BAROMETRIC PRESSURE.  Air pressure affects projectile trajectory, barofuzing, and fire control calculations.

CLOUDS AND SKY COVER.  Low ceilings affect target acquisition systems and terminally guided munitions. Low overcast clouds will limit the effectiveness of aerial illumination devices.

DENSITY.  The thickness of the atmosphere (heavy air) affects fire control. The greater (heavier) the density, the shorter the range.

HUMIDITY PROFILE.  This scale is used to compute virtual temperatures for ballistic firing data.

ILLUMINATION.  The best use of most NVG require about a quarter (23 percent) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 5 degrees below the horizon. Additional weather products dealing with the use of E-O devices are available from your SWO.

PRESSURE PROFILE.  Barometric pressure profiles are essential in both baroarming and barofuzing. They are required for calculating densities for ballistic firing data.

REFRACTIVE INDEX.  This index affects radar, laser, and infrared distance measurements.

SURFACE WINDS.  Trajectory data and first round hit capability are degraded by high crosswinds. Winds affect the accuracy of rocket fire and Firefinder radar trajectory computations.

SURFACE TEMPERATURE.  Frozen ground increases the time a crew has to stabilize their weapon. Extreme cold affects gun accuracy and fuse functioning. High temperature affects stability of ammunition such as white phosphorus (WP). It also reduces rate of fire greatly because of crew heat fatigue.

TEMPERATURE PROFILE.  This is another condition that affects calculations of ballistic artillery firing. The profile is used to compute virtual temperatures for artillery firing. Extreme cold affects gun accuracy and fuse functioning.

THUNDERSTORMS AND LIGHTNING.  Electrical storms restrict the use of some munitions and fuse types.

VISIBILITY.  This affects visual target acquisition, fire adjustment, and E-O target designation. Reduced visibility affects the placement of forward observers (FO) and fire support teams.

WINDS ALOFT.  Strong winds aloft impact all ballistic projectile aiming calculations. Accurate and timely meteorological data can compensate for the problem.

WIND PROFILE.  Wind profiles play a major role in ballistic wind compensations for artillery firing.

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Chapter 3

WEATHER EFFECTS ON AVIATION

Army aviation operates over the length and breadth of the battlefield. Missions are varied and include both fixed-wing and rotary-wing aircraft. Battlefield aviation assets play a role in a host of missions including mobility, countermobility, survivability, C2 , fire support, maneuver, air defense, IEW, combat service, and combat service support. Weather support is critical and impacts both planning and execution.

Aviation planners must include weather conditions in takeoff areas, employment routes, engagement zones, PZs, and LZs to list just a few. Aviation commanders must consider all weather conditions both favorable and unfavorable. Accurate forecasts in the AO and AI are more critical to air operations than most land operations. This is especially true for the deep battle missions.

CLOUDS AND SKY COVER.  Clouds are always a major consideration for aviation operations. Low overcast clouds will limit the effectiveness of aerial illumination devices. Overcasts tend to limit heating of inactive targets and lower target detection range for thermal sights. NVG are limited by clouds blocking natural illumination from the moon or the stars. CAS and aerial resupply missions are degraded by low clouds.

DENSITY ALTITUDE.  This is a critical measurement that determines if an aircraft has enough lift capabilities and performance to get off the ground. Too much density altitude limits fuel, weapons, and passenger loads.

DEWPOINT.  The dewpoint serves as a warning of possible fog formation or icing conditions. It is a key measurement in computing density altitude (see above).

ICING.  Ice on lifting surfaces affects the aerodynamics of the aircraft. Even a little ice is a big problem.

ILLUMINATION.  NVG are most efficient with about a quarter (23 percent) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 5 degrees below the horizon. Detailed products dealing with the use of E-O devices are available from the SWO.

INFRARED (THERMAL) CROSSOVER.  This is a temperature condition that affects target acquisition. Essentialy, this means that the target and its background are the same temperature, thereby, making the target invisible with thermal imagery systems.

PRECIPITATION.  Rain and snow affect visibility and the safety of both crew and airframe. In some instances, precipitation may cause predetonation of munitions.

PRESSURE ALTITUDE.  This computed figure affects all aircraft engine performance.

SNOW DEPTH.  Snow compounds ground handling problems. Light, powdery snow may interfere with helicopter hover operations.

STATE-OF-THE-GROUND.  Ground conditions impact on the effectiveness of serially delivered munitions.

SURFACE WINDS.  Strong winds, especially cross-winds, affect aircraft control near the ground during take-off and landings. They also affect ground speed for low-level flights.

TEMPERATURE.  High temperatures reduce lift capability. Cold temperatures increase maintenance requirements and the time needed to accomplish each task. The number of personnel that can be carried on a flight is reduced due to the weight of cold-weather gear.

THUNDERSTORMS AND LIGHTNING.  Extreme weather that includes thunderstorms and lightning is very hazardous to inflight operations, refueling, and rearming operations.

TURBULENCE.  Severe weather and clear air turbulence is a critical condition affecting all aviation assets and missions. It may cause aircraft structural damage or even crashes during take-offs and landings. Severe turbulence may cancel all operations.

VISIBILITY.  The lack of good visibility affects landings and take-offs, visual reconnaissance, target acquisition, E-O target designation, terminally guided munitions, and the ability to distribute scatterable mines.

WINDS ALOFT.  Winds at flight altitudes always affects navigation and fuel consumption.

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Chapter 4

WEATHER EFFECTS ON NUCLEAR, BIOLOGICAL, AND CHEMICAL OPERATIONS

NBC operations are extremely sensitive to environmental conditions that affect the transport and diffusion of CB agents. Humidity, air temperature, ground temperature, wind direction and speed, low-level temperature gradient, precipitation, cloud cover, and sunlight are a few of the critical elements to consider when planning NBC operations. The degree of impact depends upon the synoptic situation and the local influence of topography, vegetation, and state-of-the-ground.

The low-level stability of the atmosphere is an important factor in determining whether there will be a good horizontal transport of radioactive or CB clouds. However, stability is not measured directly but is calculated by considering the above weather elements. The WETM or SWO will assist you in making a stability determination. Listed below are weather effects for NBC operations that are not contained in the WTDA tables.

CLOUDS AND SKY COVER.  Persistent overcast low clouds usually indicate a neutral (favorable) condition, while broken low clouds indicate an unstable (unfavorable) condition during the day and a moderately stable (favorable) condition at night.

HUMIDITY.  Humidity has little effect on most chemical agents; however, high humidity destroys some chemical agents such as lewisite and phosgene because of rapid hydrolysis. High humidity increases the effectiveness of HC and phosphorous smokes, some chemical agents, and both wet and dry forms of biological agents. High humidity improves the effectiveness of wet aerosols by reducing evaporation while low humidity assists agent aerosols. High humidity, combined with high temperatures, reduces time in which troops in MOPP gear are effective.

PRECIPITATION.  Rain and snow will effect the persistence of chemical agents and may produce radioactive rainout and hot spots. Snow may cover and neutralize certain liquid agents. Rain may even work as a decontaminate. On the other hand, some agents may be very persistent on snow.

STATE-OF-THE-GROUND.  Soil conditions impact the effectiveness of chemical agents. Bare, hard ground favors short-term effectiveness and high-vapor concentrations. If the surface is porous, such as sand, the liquid agent quickly soaks in. Vegetative cover reduces exposure to ultraviolet light and favors the survival of wet aerosols. Wet soil degrades the effectiveness of smoke munitions.

SUNLIGHT.  A bright sun will shorten the lifespan of biological agents. Sunlight also plays a role in temperature gradients, winds, and temperature (stability).

TEMPERATURE.  Some agents are more persistent at low temperatures. Vaporization may be a problem with higher temperatures. Normal atmospheric temperatures have little direct effect on a biological agent aerosol. Sub-freezing temperatures make water-based decontamination methods ineffective.

THUNDERSTORMS AND LIGHTNING.  Severe electrical storms will restrict munitions handling because of safety.

WINDS.  Winds play a significant role in CB agent dispersion, chemical agent persistence, and aerial delivery methods. Very light and strong winds degrade effectiveness of smoke and NBC operations. Wind direction is considered for fallout pattern determination.

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Chapter 5

WEATHER EFFECTS ON ENEMY SYSTEMS

MILITARY ASPECTS OF WEATHER

Current weather conditions and weather forecasts for the AO and AI are analyzed to determine the effects on friendly and enemy operations. This is especially significant when threat forces have the capability to employ NBC weapon systems.

CRITICAL WEATHER FACTORS

The following are major critical weather factors that can enhance or degrade combat operations, systems, and personnel.

LOW VISIBILITY.  Low visibility (less than 3 km) can be beneficial to both threat and friendly forces. It conceals the center of gravity and maneuver of offensive forces and increases the possibility of achieving surprise. Some disadvantages of low visibility are that it hinders C2 and reduces the effectiveness of R&S and target acquisition--especially during the defense.

SURFACE WINDS.  Strong winds can reduce the effectiveness of downwind forces by blowing dust, smoke, sand, rain, or snow on them. The upwind force generally has better visibility and can advance faster and easier. Strong winds also limit airborne and aviation (primarily helicopters) operations. Winds in excess of 35 knots can cause personal injury, damage materiel and structures, create false radar returns, and reduce visibility because of blowing sand, dust, and other battlefield debris.

PRECIPITATION.  Precipitation is significant because it affects trafficability, visibility, personnel effectiveness, and a wide variety of tracked and wheeled military equipment. Heavy rains can make some unsurfaced, low-lying, and off-road areas impassable. In addition, both rain and snow can drastically reduce personnel effectiveness by limiting visibility, causing discomfort, increasing fatigue, and creating other physical and psychological problems.

CLOUD COVER.  The type and amount of cloud cover and the altitude of cloud bases and tops influence aviation operations. CAS employing fixed-wing aircraft would like a ceiling of at least 2,500 feet (762 m), but can be employed with ceilings as low as 500 feet. Threat CAS rotary-wing aircraft and aerial resupply missions require a minimum ceiling of 300 feet (100 m). Cloud cover affects ground operations by reducing illumination and visibility, or, in some instances, by enhancing the effects of artificial light.

TEMPERATURE AND HUMIDITY.  Together, these elements have a direct impact on personnel and vehicle performance. Excessively high temperatures cause heat-related injuries to personnel and vehicle engine wear that leads to equipment failure. Very low temperatures increase cold weather injuries, cause damage to vehicle cooling systems and engines, decrease the effectiveness of vehicle lubrication, and create excessive logistics requirements.

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Chapter 6

WEATHER EFFECTS ON ENGINEERS

Engineer operations are influenced by past, current and future weather conditions. The interaction of weather with terrain produces a greater impact on engineer operations than previously understood. A source of help in identifying weather and terrain impacts is the terrain analyst team at division. Below are some weather effects on engineer operations that are not contained in the WTDA tables.

CLOUDS AND SKY COVER.  Low clouds can limit the effectiveness of aerial illumination devices.

FREEZE AND THAW DEPTH.  The frost line impacts site selection, construction, excavation, and trafficability.

HUMIDITY.  Extreme humidities affect handling, storage, and use of building materials. When coupled with high temperatures, humidity affects personnel and significantly increases the time to perform physical work.

ILLUMINATION.  Optimum use of most NVG requires about a quarter (23 percent) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 15 degrees below the horizon. See Chapter 10 for further information on E-O devices.

PRECIPITATION.  High rainfall rates influence river currents, water depth, and bridging operations. It complicates other construction or maintenance jobs, affects flooding, rivercrossings, soil bearing strength, and explosives.

SNOW DEPTH.  Snow affects site selection, construction, and flood prediction.

STATE-OF-THE-GROUND.  Ground conditions impact mining operations, trenching, and any excavation job. Snow cover can impact the emplacement of scatterable mines.

SURFACE WINDS.  Ground level winds affect river crossings, port management, and all watercraft. Construction projects in chronic wind areas may need to recalculate structural strength figures.

TEMPERATURE.  High temperatures impact trafficability, influence flood control, and dictate the use of certain construction materials. Cold weather influences ice thickness and river crossings, Ice flow problems affect bridges. For example, armored vehicle launched bridges (AVLBs) are affected by warming if they were set up on frozen ground. Alternating freezing and thawing (frost heaves) may destroy the effectiveness of emplaced mines.

THUNDERSTORM AND LIGHTNING.  Electrical storms, and the associated rain and wind, affect electronic systems in general and antennas, shelters, and mobility in particular.

SEA STATE.  This condition affects site selection and the operations of port and beach facilities.

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Chapter 7

WEATHER EFFECTS ON INTELLIGENCE AND ELECTRONIC WARFARE

Intelligence operations, primarily sensors, are influenced by weather. Collection and dissemination may be hindered by weather. All-source processing requires evaluation of all weather conditions as they impact enemy and friendly operations and systems. Listed below are other weather effects for IEW operations that are not listed in the WTDA tables.

CLOUDS AND SKY COVER.  Overcast skies with low cloud bases reduce the effectiveness of infrared and photographic collection systems, and may restrict the use of UAVs.

ICING.  In addition to icing conditions associated with aircraft, ice is also a problem with electronic systems that depend on ice-free antennas for optimum operation.

ILLUMINATION.  NVG require about a quarter (23 percent) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 5 degrees below the horizon.

PRECIPITATION.  Even moderate amounts of rain and snow will obstruct vision and degrade photographic and infrared collection systems. Heavier amounts of rain can generate background electronic noise that reduces the efficiency of GSR.

SURFACE WIND.  Strong winds may damage or prevent erection of system antennas.

TEMPERATURE.  Frozen soil increases the difficulty of grounding equipment. At extreme cold temperatures cables snap and wire is unmanageable. Extreme cold also shortens battery life and may put systems requiring a good source of battery power out of service.

VISIBILITY.  Low visibility decreases the effectiveness of visual, photographic, infrared, and E-O collection systems. However, LRSU's may benefit from restricted visibility and increase their infiltration success. This condition may affect visual, laser range finding, and target acquisition systems.

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Chapter 8

WEATHER EFFECTS ON LIGHT INFANTRY

Infantry operations are influenced primarily by those weather elements that degrade trafficability and visibility. The effect may be significant for infantry operations, but weather elements have similar impacts on other units. The following contains other weather effects that are not contained in the WTDA tables.

CLOUDS AND SKY COVER.  Low overcast clouds limit the effectiveness of aerial illumination devices. Overcast clouds limit heating of inactive targets and lower target detection range for thermal sights. NVG are limited by clouds blocking natural moonlight or starlight. CAS and aerial resupply missions are hampered by low clouds.

HUMIDITY.  When coupled with high temperatures, humidity decreases the effectiveness of crews in closed vehicles.

ILLUMINATION.  The use of most NVGs requires about a quarter (23 percent) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 5 degrees below the horizon. See Appendix F for weather products dealing with E-O devices.

PRECIPITATION.  Rain and snow degrade trafficability, limit visibility, and affect certain target acquisition and NVG.

STATE-OF-THE-GROUND.  Ground state affects trafficability and movement rates. Frozen ground improves mobility and will increase the time available for preparing fighting positions.

SURFACE WIND.  Trajectory data and first round hit capability are degraded by high crosswinds.

TEMPERATURE.  High and low temperatures influence the type of lubricants used, engine warm-up periods, and sustained rates of fire for weapons. High temperatures decrease the time soldiers can remain in vehicles and increase water consumption. Low temperatures degrade the ballistics of main guns, require frequent starting of engines, increase maintenance problems, and increase possible detection by the enemy. Extreme low temperatures reduce personnel effectiveness, and decrease availability of water due to freezing.

VISIBILITY.  Poor visibility affects visual sighting, laser range finding, and E-O target acquisition systems. Poor visibility increases the survivability of infantry units.

WINDCHILL.

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Chapter 9

WEATHER EFFECTS ON MECHANIZED INFANTRY

Mechanized infantry operations are also influenced by those weather elements that degrade trafficability and visibility. Indeed, the weather effects impacting mechanized infantry units will include most of the conditions that play a role in both armor and regular infantry. Although the effect may be more pronounced for mechanized infantry, armor, and cavalry operations, weather elements have generally similar impacts on other units. Listed below are weather effects for mechanized infantry that are not contained in the WTDA tables.

CLOUDS AND SKY COVER.  Low overcast clouds will limit the effectiveness of aerial illumination devices. Overcast clouds tend to limit heating of inactive targets and, therefore, lower target detection range for thermal sights. NVG are limited by clouds blocking natural light from the moon and stars. CAS and aerial resupply missions are hindered by low clouds.

HUMIDITY.  When coupled with high temperatures, humidity decreases effectiveness of crews in closed vehicles.

ILLUMINATION.  NVG require about a quarter (23 degrees) of the moon, 30 degrees above the horizon, scattered clouds, and the sun more than 5 degrees below the horizon. Detailed products dealing with the use of E-O devices are discussed in Chapter 10.

PRECIPITATION.  Rain and snow degrade trafficability, limit visibility, and degrade the effectiveness of certain target acquisition and NVG.

STATE-OF-THE-GROUND.  Wet grounds play an important role in the effectiveness of chemical agents and smoke munitions. They can also affect trafficability and movement rates. Frozen ground improves mobility and significantly increases time available for preparing fighting positions. Deep snow slows movement of tracked vehicles. Frozen ground affects systems such as mines, sensors, and indirect fire.

SURFACE WIND.  Trajectory data and first round hit capability are degraded by high crosswinds. Wind, or in some cases the lack of it, affects smoke and indirect fire illumination missions and increases the number of indirect fire rounds used.

TEMPERATURE.  Too cold or too hot conditions dictate the type of lubricants to be used, engine warm-up periods, and sustained rate of fire for weapons. High temperatures decrease the time personnel can remain in vehicles. Extremely high temperatures increase water consumption. Low temperatures degrade the ballistics of main guns. Extreme low temperatures reduce personnel effectiveness, and decrease the availability of water because of freezing. Temperatures changing from above to below freezing can freeze stationary tracks into the mud. High temperatures cause gun tube "droop," shimmering, mirages, and vehicle exteriors to be too hot to touch.

VISIBILITY.  Poor visibility affects visual, laser range finding, and target acquisition systems. Poor visibility increases the survivability of infantry units.

WINDCHILL

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Chapter 10

WEATHER EFFECTS ON ELECTRO-OPTICAL SYSTEMS

Army battlefield capabilities have improved significantly in recent years due, in part, to the development and deployment of E-O systems. A great variety of these systems are in the Army's inventory and more are being programmed. Basically, E-O systems enhance the Army's battlefield reconnaissance, target acquisition, and target destruction capabilities.

They enable ground and aviation units to see better and accurately strike the enemy day or night even during limiting weather conditions. Since the weather parameters affecting E-O devices are numerous and the use of E-O systems is so widespread, they deserve special attention as a separate category of tactical systems.

CLASSIFICATION OF ELECTRO-OPTICAL SYSTEMS

E-O systems are classified as active (overt) or passive (covert). Active systems emit a detectable wavelength signal, while a passive system senses emitted or radiated energy. E-O systems include image intensifiers, infrared imagers, laser designators, and low-level-light (LLL) television (TV). To fully understand how weather impacts these systems, we need to know their basic operating principles.

NVG are an example of passive image intensifiers. They use extremely LLL sources (starlight), and amplify that light so that objectives are visible. These systems operate in the visual and near-infrared wavelengths.

LLL TV is a passive system and is capable of picking up targets at light levels below those usable to the human eye. This TV electronically enhances the video signal and makes it visible to the operator. Night sights, heat seeker munitions, and infrared detector munitions are examples of passive infrared imaging systems.

These systems are characterized as near infrared (short wavelength) or far infrared (long wavelength). For infrared imagers to function properly, there must be a temperature or thermal contrast between target and background area (signature). They can tell the difference between target "hot spots" and the rest of the target itself, like a warm engine in a cold truck.

Laser designators are active systems. They are used with smart munitions capable of receiving reflected laser light. The designator "pings" a target with a laser beam at a specified wavelength. The receiver in the munitions recognizes the reflected beam and homes in on the designated target. These designators do not emit light in the visible spectrum and, therefore, cannot be easily seen or detected.

When combined with E-O guided munitions, the transmitted laser beam reflecting off the target greatly enhances the delivery accuracy over conventional delivery techniques. However, the E-O guided munitions must receive the reflected beam in time to make course changes prior to hitting the target. If the beam is not reflected off the target or transmitted, or reception is disrupted, a miss will probably result.

ENVIRONMENTAL IMPACT

The performance of E-O systems depends on three basic factors:

1) E-O characteristics of the target and its background on the battlefield. 2) The atmosphere between the E-O system and the target and its background. 3) The sensitivity of the E-O detector system (to include human operator performance).

Weather conditions affect the first two factors, both directly and indirectly and are summarized on page 52.

The most fundamental environmental conditions inhibiting E-O signals are--

• Attenuation or reduction of the signal by atmospheric moisture such as clouds, precipitation, fog, and high humidity.

• Temperature affecting atmospheric refraction near the surface.

• Temperature contrast between the surrounding environment and the target.

• Winds kicking up dust and sand.

•LLL (Low Level Light).

In addition to TV and binoculars, visible light systems include the TOW day sight and image intensifiers such as the individual-served weapon sight and the tank with gunner's periscope. Systems that use available light are the easiest to defeat obscurants such as haze, smoke, dust, and precipitation because visible light has a short wavelength and can be more easily attenuated.

As the wavelength of the light spectrum used by an E-O system increases, the less it is affected by obscurants. However, the long wavelength E-O system provides less target resolution. Near infrared systems like the handheld thermal viewer can penetrate some light fog oil and diesel oil smokes. Far-infrared systems, such as thermal sights for the Apache helicopter, Ml tank, TOW, and DRAGON, use longer wavelengths of the spectrum and can penetrate low densities of WP smoke and other obscurants that defeat both visible and near infrared.

In addition to attenuation from an obscurant, E-O devices are also affected by atmospheric refraction. Basically, the sun's heating of the surface air creates sufficient vertical motion or turbulence to cause this effect. A mirage is caused by this heating and can make a building appear to move or even cause a target to disappear altogether. Such apparent displacements can lead to target misses.

Although these refractive conditions are associated with periods of high heat, this condition has been observed over a snow cover when the air temperature was 25 degrees below zero Fahrenheit. The higher you are from the surface, the less likely you will encounter mirages.

Other parameters that impact on many E-O systems are weather conditions affecting the level of illumination. Although the level of light affects all devices operating in the visible spectrum, image intensifiers are influenced the most. Too much or too little light adversely affects the use of NVG.

On relatively clear nights with a near full moon, you can normally operate without the aid of NVG. With less than a full moon, there may still be too much light.

Too much light, when amplified by NVG, saturates the viewing area as seen through the device and makes the device unusable because light and dark contrasts are no longer possible. When illumination is limited, NVG must be used. For partial or heavy overcast skies with little moonlight, even these light levels may be too low to use NVG.

Additionally, terrain influences on available illumination must be considered. Even though illumination may be adequate to support the use of NVG, flying in a valley with shaded areas may end disastrously.

CLOUDS

"Smart" weapons such as COPPERHEAD and the air-launched HELLFIRE have critical cloud ceiling values. If these weapons pass into a cloud, they will lose "lock on" and miss their laser-designated target. Another system affected by low clouds is SADARM. Using E-O detectors, SADARM searches in a circular pattern at a fixed angle. As these munitions descend, the area they see becomes smaller. Low cloud ceilings drastically reduce their target search areas and time.

THERMAL CONTRAST

Millimeter wave and infrared E-O devices require a temperature difference between the target and its background. Bad weather can limit system performance. As the contrast diminishes, a condition is reached where the target is no longer discernible from the background and target acquisition becomes a problem.

Thermal imagers produce images of targets in scenes, somewhat similar to those seen on ordinary TV. The major difference is that, instead of observing light (visible energy) in the scene, thermal imagers observe heat (infrared energy) emitted, reflected, or generated by the objects in the scene.

The amount of infrared energy is determined principally by the object's temperature, its surface reflectivity, and its structural properties. Natural infrared energy is produced when objects absorb sunlight. Winds can change the image contrast by making target and background closer to the same temperature.

Man-made energy, particularly in vehicles, results from the heat of fuel combustion and the friction of moving parts. Infrared energy is not as greatly diminished at night as is visible energy. Thermal imagers tend to function as well or better during nighttime than during the day. For this reason, they are often used as night sights.

A target may be acquired with a thermal imager only if the amount of infrared energy of the target is sufficiently different from that of the background. This difference, called thermal contrast, is the difference between the temperatures of the target and its background.

Wind, rain, snow, humidity, and clouds reduce the temperature contrast between target and its background and even cause thermal reversal where instead of a "hot" target against a "cold" background, you find a cold target against a hot background. This can occur in the early morning and late afternoon when a thermal sight encounters a condition where some inactive targets without an internal heat source will warm up or cool off to the same temperature as the background.

In this instance, thermal devices will not be able to see targets because the difference in temperature is not enough to be detected. The sensitivity of the thermal device to the difference in temperature and the rate at which a target is heating or cooling will determine how long this "thermal crossover" will occur.

Most metal targets heat up or cool off faster than the ground and vegetation in the background. At night a metal target appears to be colder than the background, but with the sun shining on it, the target appears to be hot compared to the relative coolness of a background of trees or bushes.

In bright sunshine, the thermal crossover period may be just a few seconds. On cloudy days, however, the thermal crossover period may be a number of minutes. When this happens, optical sightings must then replace the infrared devices. Listed below are weather elements effecting thermal contrast at crossover time.

CLOUDS.  Clouds will reduce the thermal contrast. Lower and thicker clouds have a stronger influence than higher or thinner clouds.

SURFACE WIND.  Wind causes the temperatures of both the target and background to become closer to the air temperature, and as a result, closer to each other.

HUMIDITY. Moist air does not enhance the rate of cooling as much as dry air. With high humidity and a moist background, the thermal contrast would be minimal between target and background. If the air was dry, the cooling influence on the moist background would cause a greater thermal contrast.

PRECIPITATION.  Falling rain and snow have cooling effects that bring target and background temperatures closer together. In the case of operating vehicles, the temperature contrast may be increased since the precipitation will have little effect on the heat generated in the engine compartment and the exhaust.

The relationship between weather effects and E-O systems is very complex since the result is a function of the precipitation particle size and the wavelength of the E-O system.

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Appendix 1, Light Data Sample

DECEMBER
1997 LIGHT DATA
Gray AAF, Fort Lewis, WA
Lat N 47 deg 5 min Long W 122 deg 35 min
Times Are Local Standard Time (GMT - 8 hrs 0 mins)

DAY

BMNT

BMCT

SR

SS

EECT

EENT

MR

SNVG

ENVG

MS

ILL

FMT

FMI

LMI

LMT

DD

HM

HNM

1

0623

0701

0737

1622

1657

1735

0950

****

****

1918

2

1835

4

5

1918

1250

0043

1207

2

0625

0702

0738

1622

1657

1735

1043

****

****

2016

6

1835

10

10

2016

1251

0141

1110

3

0626

0704

0739

1621

1657

1735

1132

****

****

2119

12

1835

17

18

2119

1252

0244

1008

4

0627

0705

0740

1621

1656

1735

1214

****

****

2227

19

1835

26

28

2227

1253

0352

0901

5

0628

0706

0741

1621

1656

1734

1251

1749

1922

2337

29

1834

36

39

2337

1255

0503

0752

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BMNT-Beginning Morning Nautical Twilight FMI-Percent Illumination at FMT
EENT-Ending Evening Nautical Twilight HM-Hours of Moonlight 
BMCT-Beginning Morning Civil Twilight DD-Duration of Solar Darkness
EECT-Ending Evening Civil Twilight FMT-First Moonlight at or after EENT 
LMT-Last Moonlight at or before BMNT HNM-Hours with no Moon (Total Darkness) 

 
NVG-Night Vision Goggle For NVG - Lunar Alt >30 deg, Solar Alt <-5 deg, and Lunar Illum >23%

Table of Contents


Appendix 2, Temperature Conversions, Heat Indices, Windchills

Temperature Conversions
Enter the celsius temperture and press the equal sign.

Degrees Celsius  Degrees Fahrenheit


  HEAT INDEX CHARTS
Using Temperature Dew Point
Using Temperature Relative Humidity (%)

WIND CHILL CHART

Table of Contents


Appendix 3, Brightness Chart

Table of Contents


Appendix 4, Glossary

absolute humidity - A ratio of the quantity of water vapor present per unit volume of air, usually expressed as grams per cubic meter or grams per cubic foot.

atmospheric pressure - The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the column of air laying directly above any point.

barometric pressure - The measure of atmospheric pressure by a mercurial or aneroid barometer. Changes in pressure are significant in weather forecasting. The normal pressure at sea level is 29.92 inches of mercury or 1013.3 millibars. Rising pressures usually indicated improving weather conditions; falling pressures may reflect impending inclement weather. Barometric pressure is used in aircraft altimeter settings to tell pilots how far above ground level the aircraft is. This is critical for landing during obscured conditions and Nap of the Earth (NOE) flying.

battlefield environment - A phrase used to describe the combination of weather, terrain, BIC, illumination, and background signatures that occur on a battlefield. 

Belt Weather Kit - A small kit with simple equipment, used originally by the US Forest Service, and now employed by S2 personnel at maneuver brigades. The BWK equipment measures temperature, dewpoint, pressure, and wind speed and direction. It is slow and does not meet the accuracy standards necessary to support the Army's battlefield weapon systems.

ceiling (cloud) - The height above the earth's surface of the lowest layer of clouds. This layer is reported as broken, overcast, or obscured and not classified as thin or partial. See cloud cover for details. When 1/2 or more of the sky has cloud cover, a ceiling exists.

Celsius - A temperature scale (formerly called centigrade). The temperature interval between waters ice and steam points is divided into 100 parts (or degrees) with 0 Celsius at the freezing or ice point and 100 at the boiling or steam point. Used commonly worldwide.

climatology - The historical records of weather conditions measured or observed at a specific location is knows as climatology. Some data go back over 100 but generally a 10- to 25-year history is more common. Climatology is useful in planning operations beyond 5 to 7 days. It usually describes the average (or mean) conditions such as high and low temperatures and extremes.

cloud cover - The amount of clouds over, or at, a given location. Cloud conditions are expressed as cloud bases or ceiling, the amount of cover stated in eighths--1/8 to 4/8 described as scattered; 5/8 to 7/8, broken; and 8/8, overcast), and cloud tops. Several layers of scattered clouds added together may result in a broken or overcast condition. Low clouds impact many battlefield operations, especially the use of smart weapons.

density altitude - The height above MSL at which the existing density of the atmosphere would be duplicated in the standard atmosphere; atmospheric density expressed as height according to a standard scale. Extremely important in flight operations.

dewpoint - The temperature to which a given weight of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur. When this temperature is below 0C, (32 F) it is sometimes called the frost point.

dry-bulb temperature - The temperature measured by the dry bulb of a psychrometer; ambient air temperature.

effective illumination - The level of light available for night operations.

effective wind speed - The combined effect of actual (meteorological) wind and other motion caused by the moving of an object or a person through air. Also caused by moving equipment such as aircraft propellers or rotors.

FALOP (Forward Area Limited Observing Program) - A program implemented in the 1970's to supplement battlefield observations taken by USAF AWS WETMS. FALOP is employed by S2 personnel at maneuver brigades and battalions. The BWK is used by the S2 to make limited measurements of weather conditions. Additionally, the S2 provides estimates of other weather and environmental conditions he observes. His report is encoded and forwarded to the closest WETM.

gust - Rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls.

humidity - A measure of the water vapor content of air.

icing - In general, any deposit or coating of ice on an object; a mass or sheet of ice formed on the ground surface during the winter by successive freezing of sheets of water that may seep from the ground, a river, or a spring.

instrument flight rules (IFR) - An aircraft operational term indicating that the weather conditions have deteriorated to the point that navigational instruments on board the plane must be used in flying from one place to another.

inversion (stable) condition - An increase in air temperature with an increase in height. The condition is called stable because there is usually little vertical movement of air.

lapse (unstable) condition - A decrease in air temperature with an increase in height. The condition is called unstable because it is accompanied by vertical air movement.

light and illumination data - Battle planning requires accurate timing based on available light. Light tables have been computed for any location that tell sunrise, sunset, moonrise, moonset, and moon phase. Illumination is a measure of sunlight, moonlight, starlight, and air glow. Illumination is a critical factor in the considered of NVG, cloud cover, and terrain masking.

mesoscale - Smaller scale weather features that exist for minutes or hours. Examples of mesoscale atmospheric phenomena are thunderstorms, tornadoes, and land-sea breezes.

moderate weather impact - A subjective measure of weather conditions impacting a system or operations that require alternative actions to be considered. Moderate impacts limit the effectiveness of the system or operations from 25 to 75 percent.

refractive index - A measure of the amount of "refraction," or bending, of an energy wave (visual light, infrared, radio and others) passing from one density to another in a medium such as air or water. The apparent bending of a stick when placed in a pool of water is an example.

relative humidity - Ratio, usually expressed as a percentage of air's water vapor content, to its water vapor capacity at a given temperature and pressure.

sea state - Also state-of-the-sea. Describes wind-generated waves on the surface of the sea.

slant-range visibility - The distance a pilot can distinguish objects that are both forward and beneath his aircraft. For example, looking down at an angle as he approaches a target or a runway.

soil trafficability - The capacity of a soil to withstand traffic, especially the traffic of military vehicles.

specific humidity - The ratio of the mass of water vapor to the total mass of air (including water vapor).

state-of-the-ground - A standardized surface observation that describes the condition of the ground surface. Basically, state-of-the-ground is characterized as dry, moist, wet, frozen, and ice or snow covered.

Staff Weather Officer (SWO) - A USAF officer, qualified in meteorology, that usually commands a WETM (Weather Team).  The SWO may be a second lieutenant to a colonel depending on the Army unit he is attached to.  Some SWOs, assigned to Army commands, serve without a WETM.  The SWO, a member of the Army commander's special staff, works under the direction of the G2 or S2.

steering wind - Winds that are winds measured at 50 feet (16 m), although they may extend as high as 2,500 feet. They are used in smoke operations where the moving air is measured far enough above the ground to be free of disturbances caused by local terrain variations. They establish the speed and direction of a smoke cloud. Steering winds also play a role in the direction that weather systems move.

surface observations - Weather and environmental observations measured or estimated on the land or water surface, and usually reflecting surface conditions. Cloud cover is an exception.

surface winds - Wind speed, direction, and gust speeds measured over the land or water. Technically measured at 10 meters above the surface.

synoptic scale - Large-scale atmospheric features that exist for days or weeks. An example of synoptic scale is the analysis of weather fronts, highs, and lows over the continental us.

temperature gradient - The change in temperature per unit of distance between two points.

temperature-humidity index - An indicator of the effect of temperature and humidity upon individuals. Sometimes called the misery index by television weatherpersons. An example is the WBGT (Wet Bulb Globe Temperature) index.

thermal/IR crossover - Thermal crossover is said to occur when the temperature of a target is the same temperature as its background. This would cause the target to appear invisible to IR sensors. Depending on cloud ceiling, this usually occurs soon after BMNT and soon after EENT.

tidal current - The alternating horizontal movement of water associated with the rise and fall of the tide. In relatively open locations, the direction of tidal currents rotates continuously through 360 degrees diurnally or semidiurnally. In coastal regions, the nature of tidal currents is determined by local topography as well.

tide - The periodic rising and falling of the oceans, large lakes, and the atmosphere. [t results from the tide-producing forces of the moon and sun acting upon the rotating earth. This disturbance actually propagates as a wave through the atmosphere and through the surface layer of the oceans.

turbulence - A condition of the atmosphere in which air currents vary greatly over short distances. Turbulence may occur at any altitude, and the intensity may vary rapidly over short distances. See wind shear.

twilight - The periods of incomplete darkness following sunset (evening twilight) or preceding sunrise (morning twilight). Twilight is designated as civil, nautical, or astronomical, as the center of the sun travels 6, 12, or 18 degrees below the celestial horizon, respectively. In general, civil twilight precedes nautical twilight by 2 hours.

visual flight rules (VFR) - In aviation a set of regulations that must be adhered to when piloting in calm, clear weather where the pilot can move from one point to another using ground features for navigational aids.

visibility - The greatest distance that prominent objects can be seen and identified by the unaided, normal eye. When NVG or other infrared devices are used to increase visual distance, "seeability" is used instead of visibility.

weather tactical decision aids (WTDA) - These refer to the manual lookup tables and matrices in this manual, or are computer-driven algorithms by which such a product is generated.  These tables provide the critical thresholds that adversely affect operations, systems, and personnel.  The tables, together with a current forecast, are used to brief the commander and staff.

wet bulb globe temperature index - A measure of heat stress potential.  It is calculated by using a formula which considers relative humidity, radiant heat, air temperature, and air movement.

windchill factors - These factors are revised temperature values based on the effect of wind and temperature combined on exposed skin.  This windchill temperature is the effective temperature for troops.  The effect of windchill differs individually because of body chemistry, but is an acceptable operating standard.

winds aloft - The flow of air, measured in speed and direction above the surface.  There is no distinct demarcation between winds aloft and surface winds, although winds above 100 meters are usually referred to as winds aloft.

wind shear - The rate of change of wind velocity (speed or direction)with distance.  Eddies and gusts form in areas of wind shear, thus producing turbulent flying conditions.  Wind shear may occur in either the vertical or horizontal plane.

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Page Last Updated: 24 Sep 1999