If a chemical cloud is to be placed directly on an occupied area, the best possible weather conditions are calm winds with a strong, stable temperature gradient. Under these conditions, the cloud diffuses over the target with minimum dilution and does not move away. Such conditions are most apt to occur on a calm, clear night. If a small amount of air movement is required to spread the cloud evenly over the target area, a low wind speed and stable or neutral conditions are most favorable. These conditions most often occur on a clear night, a cloudy night, or a cloudy day.

When the desired effect is for the chemical cloud to travel, the most favorable conditions are stable or neutral conditions with a low to medium wind speed of 3 to 7 knots. These conditions may be present on a clear night, a cloudy night, or a cloudy day. The presence of low to medium wind speeds keeps the cloud traveling over the area without too much diffusion, and the stable or neutral conditions keep the agent concentration high and the cloud close to the ground.

Favorable terrain conditions for a chemical cloud are smooth or gently rolling contours or wooded areas. Unfavorable conditions for chemical clouds (usually found on clear days) are extreme or marked turbulence, wind speeds above 10 knots, an unstable dispersion category, rain, and rough terrain.

High wind speeds cause rapid dispersion of vapors or aerosols, thereby decreasing effective coverage of the target area and time of exposure to the agent. In high winds, larger quantities of munitions are required to ensure effective concentrations. Agent clouds are most effective when wind speeds are less than 4 knots and steady in direction. The clouds move with the prevailing wind as altered by terrain and vegetation. Steady, low wind speeds of 3 to 7 knots enhance area coverage unless an unstable condition exists. With high winds, chemical agents cannot be economically employed to achieve casualties.

Unstable conditions (such as many rising and falling air currents and great turbulence) quickly disperse chemical agents. Unstable is the least favorable condition for chemical agent use because it results in a lower concentration, thereby reducing the area affected by the agent. Many more munitions are required to attain the commander's objectives under unstable conditions than under stable or neutral conditions.

Stable conditions (such as low wind speeds and slight turbulence) produce the highest concentrations. Chemical agents remain near the ground and may travel for long distances before being dissipated. Stable conditions encourage the agent cloud to remain intact, thus allowing it to cover extremely large areas without diffusion. However, the direction and extent of cloud travel under stable conditions are not predictable if there are no dependable local wind data. A very stable condition is the most favorable condition for achieving a high concentration from a chemical cloud being dispersed.

Neutral conditions are moderately favorable. With low wind speed and smooth terrain, large areas may be effectively covered. The neutral condition occurs at dawn and sunset and generally is the most predictable. For this reason, a neutral dispersion category is often best from a military standpoint.

Temperature

There will be increased vaporization with higher temperatures. Also, the rate of evaporation of any remaining liquid agent from an exploding munition can vary with temperature. Generally, the rate of evaporation increases as the temperature increases. See FM 3-9/AFR 355-7 for specific information on chemical agents, such as their boiling and freezing points and vapor density.

Humidity is the measure of the water vapor content of the air. Hydrolysis is a process in which compounds react with water resulting in a chemical change. Chemical agents with high hydrolysis rates are less effective under conditions of high humidity.

Humidity has little effect on most chemical agent clouds. Some agents (phosgene and lewisite) hydrolyze quite readily. Hydrolysis causes these chemical agents to break down and change their chemical characteristics. If the relative humidity exceeds 70 percent, phosgene and lewisite can not be employed effectively except for a surprise time-on-target (TOT) attack because of rapid hydrolysis. Lewisite hydrolysis by-products are not dangerous to the skin; however, they are toxic if taken internally because of the arsenic content. Riot control agent CS  also hydrolyzes, although slowly, in high humidities. High humidity combined with high temperatures may increase the effectiveness of some agents because of body perspiration that will absorb the agents and allow for better transfer.

Precipitation

The overall effect of precipitation is unfavorable because it is extremely effective in washing chemical vapors and aerosols from the air, vegetation, and material. Weather forecasts or observations indicating the presence of or potential for precipitation present an unfavorable environment for employment of chemical agents.

Terrain contours influence the flow of chemical clouds the same as they influence airflow. Chemical clouds tend to flow over low rolling terrain and down valleys and settle in hollows and depressions and on low ground. Local winds coming down valleys at night or up valleys during the day may deflect the cloud or reverse its flow. On the other hand, they may produce conditions favorable for chemical cloud travel when general area forecasts predict a calm.

A chemical cloud released in a narrow valley subjected to a mountain breeze retains a high concentration of agent as it flows down the valley. This is because of minimal lateral spread. Hence, high dosages are obtained in narrow valleys or depressions. High dosages are difficult to obtain on crests or the sides of ridges or hills. After a heavy rain, the formation of local mountain or valley winds is sharply reduced. In areas of adjacent land and water, daytime breezes from the water and nighttime breezes from the land control chemical cloud travel.

Ground covered with tall grass or brush retards flow. Obstacles, such as buildings or trees, set up eddies that tend to break up the cloud and cause it to dissipate more rapidly. However, street canyons or spaces between buildings may have pockets of high concentrations. Flat country (during a neutral or inversion condition) or open water promotes an even, steady cloud flow.

The amount and type of vegetation in the area of the chemical operation also influence the travel of a chemical cloud. Vegetation, as it relates to meteorology or diffusion, is called vegetative canopy or just canopy. The effects of canopies are considered below.

Woods are considered to be trees in full leaf (coniferous or deciduous forests). The term "heavily wooded canopy" denotes jungles or forests with canopies of sufficient density to shade more than 90 percent of the ground surface beneath. For chemical operations, areas containing scattered trees or clumps of bushes are considered to be open terrain although drag is somewhat increased. In wooded areas where trees are not in full leaf or where foliage has been destroyed by previous attack so that sunlight strikes the ground, the diffusion (stability) category will be similar to those in the open.

When bombs are dropped into a wooded area, some may be expected to burst in the treetops. Although the released aerosol and vapor settle toward the ground, some of the agent is lost, depending upon the thickness and height of the foliage. The initial burst and pancake areas of chemical clouds released within woods or jungles are smaller than those released in the open. However, concentrations within the initial clouds are higher in wooded areas, sometimes three times that of bursts in the open. The magnitude of concentration from ground bursts depends upon the density of undergrowth and trees.

Generally, when conditions in the open are most favorable for the use of chemical agents, conditions also are favorable in heavily wooded areas if dispersion occurs below the canopy. Low wind speeds under the canopies spread agent clouds slowly in a downwind and downslope direction. Areas of dense vegetation also increase the potential surface area for the deposition of chemical agents. If there are gullies and stream beds within the woods, clouds tend to follow these features. This flow may be halted or diverted by upslope winds.

Vegetation absorbs some agents. However, for an attack against troops poorly trained in NBC defense (where lethal dosages may be obtained in 30 seconds or less), the amount of agent absorbed by foliage will have little or no effect on the success of the attack. High concentrations of chemical agents may destroy vegetation, since the leaves absorb some of the agent. In some instances, the absorbed agent may be released or desorbed when the vegetation is disturbed or crushed, creating a secondary toxic hazard.