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FM 34-81: Weather Support for Army Operations



This appendix describes common weather and general weather effects on operations as they affect specific types of units and selected operations.


Although environmental elements tend to have different effects on different types of units and operations, many can be identified as having similar effects on a majority of combat elements and operations.

Many of the common effects can be derived for planning purposes from the climate of the theater of operations. Special attention must be given to those elements of weather which may limit operations or preclude them altogether. For instance, operations in the tropics must be planned to consider the recurring cycle of the monsoon season. In continental Europe, strategy must be considered with severe winters and with the annual autumn and spring thaws affecting trafficability and cross-country movement.

Very early in the planning process, planners must relate the possible courses of action to weather expectancies derived from c1imatological studies. There must be an acceptable likelihood that the weather conditions required for any proposed course of action will occur. It is imperative that an operation be feasible meteorologically at the operational level of warfare, and that planning for seasonal weather changes be considered early in the campaign process.

When considering the effects of environmental conditions, the impact weather and terrain have on each other must be considered. Weather and terrain are so interrelated they must be considered together when planning ground and air operations. Weather elements can drastically alter terrain features and trafficability. Conversely, terrain features may exert considerable influence on local weather. The relationship between weather and terrain must be carefully correlated in terrain studies to produce accurate terrain intelligence. This planning is an integral part of the IPB process.


Terrain features affect weather, climate, and weather elements such as--

  • Visibility.
  • Temperature.
  • Humidity.
  • Precipitation.
  • Winds.
  • Clouds.

These specific elements vary with the geographical area, time, and season. A description of the climate of a large area considers terrain influences only in general terms; whereas a description of a small area such as a single valley can be specific. FM 30-10 contains further information on terrain.

It is important that commanders and staffs understand and consider weather in their tactical planning. They must recognize the tactical significance of weather effects on intended operations and the risks or opportunities they present. The effects of weather are integrated with enemy and terrain analysis through IPB. Factors that must be considered include--

  • Visibility.
  • Wind.
  • Clouds.
  • Temperature and humidity.
  • Severe weather.
  • Illumination and obstructions to vision.


Low visibility is beneficial to offensive and retrograde operations and detrimental to defensive operations. In the offense, it conceals the concentration of maneuver or friendly forces, thus enhancing the possibility of achieving the element of surprise. Low visibility hinders the defense because cohesion and control become difficult to maintain, reconnaissance and surveillance are impeded, and target acquisition is less accurate.

These disadvantages may be offset partially by extensive use of illuminatives, radar, sound detection, thermal, and infrared devices; however, infrared devices are degraded in range by any moisture source, precipitation, or moisture-absorbing smoke. Smoke and obscurant aerosols can be expected on medium-intensity to high-intensity battlefields and may be used locally to reduce visibility. In all operations, obscurants limit the use of aircraft and aerial optical and infrared surveillance devices.


Wind speed and direction, both on the surface and aloft, usually favor the upwind force in the use of NBC weapons. Winds of sufficient speed can reduce the combat effectiveness of a force downwind as the result of blowing dust, smoke, sand, rain, or snow on personnel and equipment. The force located upwind has better visibility and can, therefore, advance and maneuver faster. Strong winds limit airborne, air assault, and aviation operations.

Strong surface winds and gusts can--

  • Injure personnel.
  • Damage material and structures.
  • Give false radar returns.
  • Restrict visibility due to blowing sand, dust, and other material.

Generally, winds above 20 nautical mph create such effects. Smoke operations are usually ineffective at wind speeds greater than 7 nautical mph. As surface wind speed (SFC) increases, either naturally or enhanced by vehicle movement, the windchill becomes a critical factor. The windchill factor adversely affects improperly clothed personnel and impedes activity in unsheltered areas. Wind speed also affects the distance that sound will travel. Wind may prove beneficial by aiding in drying soil. A windchill index chart developed by US Army Research Institute of Environmental Medicine is shown at Figure B-1.


The primary significance of precipitation is its effect on soils, visibility, personnel effectiveness, and the functioning of ground maneuver systems, aviation, and E-O and infrared systems. State-of-the-ground affects trafficability; heavy rain can make some unsurfaced roads and off-road areas impassable. Rain and snow can greatly reduce--

  • Personnel effectiveness by limiting visibility, increasing fatigue, and causing discomfort and other physical and psychological problems.
  • The persistence of chemical agents or create NBC hot spots.
  • The range of lasers, night vision devices, and thermal tank sights.
  • The effectiveness of aircraft.

Precipitation also adversely degrades the quality of supplies in storage. Snow accumulation of greater than 1 inch degrades trafficability and reduces the impact of mines and the blast effects of point munitions. Generally, precipitation in excess of .10 inches per hour or 2 inches in a 12-hour period is considered critical for tactical operations. Snowfall exceeding 18 inches reduces tracked vehicle speed; movement on foot is very difficult without snowshoes or skis.


The type and amount of cloud cover, as well as the height of cloud bases and tops, influence friendly and enemy aviation operations. Extensive cloud cover reduces the effectiveness of air support. This effect becomes more pronounced as cloud cover increases, as cloud bases lower, and as conditions associated with clouds (such as icing, turbulence, and poor visibility aloft) increase. In a relatively unstable airmass, clouds are associated with strong vertical currents, turbulence, and restricted visibility aloft. Generally, TACAIR CAS missions and MAC aerial resupply missions require a ceiling of at least 1,000 feet.

Clouds affect ground operations by limiting illumination and the solar heating of targets for infrared systems. Clouds limit the use of infrared-guided artillery by decreasing the envelope in which it can seek and lock on to laser-designated targets. Cloud-free line of sight (LOS) is required for delivery of precision-guided munitions from aircraft.


Temperature and humidity affect air density. Air density decreases as the temperature or humidity increases; thus, the efficiency of aircraft propulsion is reduced in areas of high temperature or high humidity. Although temperature and humidity may not directly affect a particular tactical operation, extremes will reduce personnel and equipment capabilities and may necessitate a reduction of aircraft payloads (for example, fuel, weapons, and personnel).

Tactics effective in one climate may be ineffective when applied in another. The high temperatures and humidity in the tropics are conducive to the growth of dense foliage which greatly affects tactical operations. Desert climates can range from extremely hot in the daytime to very cold at night, requiring added protective measures. In arctic climates, cold weather periods--

  • Create an almost constant need for heated shelters.
  • Cause difficulty in constructing fortifications.
  • Increase the dependence on logistical support.
  • Necessitate special clothing, equipment, and survival training.

Windchill factors are produced by a combination of temperature and wind speed. A windchill factor of -26 F (-32 C) is considered the critical value for equipment and personnel operating in cold weather. The opposite extreme, 120 F (49 C), is the critical value for personnel operating in hot weather. The critical wet-bulb-globe-temperature (WBGT) value for personnel operating in hot weather is 90 F. Similar restrictions occur in desert terrain, where the difference in temperature from day to night may vary as much as 100 F (37 C).

Temperatures of targets and objects on the battlefield at night are important for the use of thermal sights and forward looking infrared (FLIR) devices. A difference in temperature or thermal contrast is required for these devices to "see" a target. Normally, heating and cooling are at a different rate for the target and background. Twice a day, in the morning and evening, targets without internal heating come to relatively the same temperature as the background.

At this point thermal crossover occurs and the thermal device does not have the capability to "see" the target. Time of thermal crossover may be only a few seconds when the morning sun strikes a target, or for several minutes on cloudy adverse weather days; this depends on the threshold temperature's contrast required by the thermal device. Tactical decision aids can be used to predict these temperature differences for planners and estimate length of thermal crossover periods.


Severe weather affects most operations by presenting a threat of injury to personnel, damaging equipment and structures, limiting ground and air mobility and air operations, and threatening troop morale. Electrical storms often accompany severe weather conditions and add the hazard of lightning strikes at munitions storage areas and fueling points. Lightning also may interrupt landline communications and both communication and noncommnunication use of the electromagnetic spectrum.


Illumination and obstructions to visions impact on the visibility required for various operations. They affect the overall planning for--

  • Security.
  • Concealment.
  • Target acquisition by visual, electronic, or E-O means.


Meteorological products are categorized as either primary products or tactical weather products.

Primary products are used by the SWO in preparing tactical weather products. They are usually received in the form of weather observations, forecasts, and climatological studies. Primary products are received from AWS, indigenous sources, other WETMs, the Navy, Army, and in-flight aircraft in wartime, and from NWS and FAA in peacetime. While some primary weather products are passed directly to the Army user, many need to be modified or updated to reflect local observation, local terrain, and mission requirements.

Weather observations contain information on existing weather conditions and specific weather elements at specific locations and times. The basic types of observations are surface and upper air.


Surface observations are taken hourly or as required by the WETMs, ARTYMET sections, and selected Army units using the FALOP. Observations include--

  • Surface winds.
  • Prevailing visibility.
  • Precipitation type and intensity.
  • Obstructions to vision.
  • Clouds.
  • Temperature.
  • Dewpoint temperature.
  • Surface atmospheric pressure.
  • Altimeter setting.
  • Remarks.

Additional elements may include--

  • Snow depth.
  • Precipitation amounts.
  • State-of-the-ground.
  • Maximum and minimum temperatures.

Other products such as windchill, pressure altitude, and density altitude can be derived from the surface observation. The WBGT is obtained from medical units and provides information on heat casualty potential. Freeze-thaw depth, ice thickness, current water depth, river stages, and trafficability are obtained from engineer and cavalry units.


Upper-air observations are taken by ARTYMET sections at established time intervals. They measure temperature, pressure, relative humidity, and wind speed and direction. From these observations, fallout winds, ballistic, and computer meteorological messages are prepared.


This paragraph describes weather planning factors peculiar to specific units or selected operations.


Air defense operations require environmental information for both deployment and employment. Deployment requires climatological data, trafficability, and severe weather forecasts. Environmental elements affecting employment vary according to the type of weapon systems used. When missile systems require radar surveillance, elements such as refractive index and precipitation must be known. Other systems require visual target acquisition. Figure B-2 shows the effects of weather on air defense operations.


Weather effects on amphibious operations may be beneficial and detrimental. Certain weather condition may help conceal landing operations. Other conditions may hinder beaching and unloading, task force movement, and essential air support operations. Figure B-3 shows effects of weather on amphibious operations.


Armor and infantry operations are influenced primarily by those weather elements which degrade trafficability and visibility. Figure B-4 shows the effects of weather on armor or infantry operations.


Artillery operations are heavily weather-dependent. Not only must artillery contend with those weather effects common to all units but also must compensate for a number of special effects pertinent to their operations. Figure 6-15 provides a complete description of artillery requirements. Figure B-5 shows the effects of weather on artillery operations.


Army aviation is involved in multifaceted operations over the length and breadth of the battlefield. These operations include aerial weapons, reconnaissance and surveillance, and routine logistic support. Missions are varied and require the operation of aviation, both fixed-wing and rotary-wing assets in a variety of flight modes and altitudes. Figure B-6 shows the effects of weather on aviation operations.


Communications-Electronics (C-E) operations are affected by a number of weather elements. Virtually all of the special weather conditions that apply to C-E operations affect electromagnetic propagation. Figure B-7 shows the effects of weather on C-E operations.


Engineer operations are influenced by current environmental conditions, forecasted conditions, and climatology. Figure B-8 shows the effects of weather on engineer operations.


Many intelligence operations are dependent on weather. Collection and dissemination may be hindered by certain weather conditions. All-source processing requires evaluation of all weather conditions, current and forecast, as they impact on enemy and friendly operations. Figure B-9 shows the effects of weather on intelligence operations.


Logistical operations include supply, maintenance, and transportation required to support the combat force. Numerous weather factors affect the planning and the activities required for each operation. Those weather factors which influence logistic operations subsequently affect the supported combat force. If logistic units are prevented from supporting forward combat elements, the success of the combat mission may be jeopardized. Figure B-10 shows the effects of weather on logistical operations.


The extensive use of air ambulances requires the same weather support as other aviation elements. Besides aviation operations, weather influences are considered in establishing field hospitals and anticipating prestockage and workloads. The requirements for weather support for ground evacuation of casualties are the same as land transportation, including considering patient comfort under extreme weather conditions. Figure B-11 shows the effects of weather on medical support operations.


Military police are involved in weather-sensitive operations such as--

  • Route and area reconnaissance.
  • Security.
  • Traffic and movement control.
  • Rear area protection.
  • Refugee control.
  • Enemy prisoner of war (EPW) control.
  • Civil disturbance control operations.

Acoustical propagation can affect significantly the use of loudspeakers in civil disturbance control operations. Acoustical propagation is a function of attenuation and refraction, which in turn is influenced by temperature gradient, density, wind, and sky cover. Figure B-12 shows the effects of weather on military police operations.


NBC operations are extremely sensitive to environmental conditions that affect the transport and diffusion of chemical or biological (CB) fallout. Figure B-13 shows the effects of weather on NBC operations. A few of the critical elements to consider when planning NBC operations are--

  • Humidity.
  • Air temperature.
  • Ground temperature.
  • Wind speed and direction.
  • Low-level temperature gradient.
  • Precipitation.
  • Cloud cover.
  • Sunlight.


Tactical PSYOP are influenced primarily by those weather elements which degrade audibility of loudspeaker broadcasts and affect the distribution of leaflets. Figure B-14 shows the effects of weather on PSYOP.

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