# The Night Flight Environment The night flight environment is highly variable. The level and direction of solar and lunar illumination change constantly, as do the effect of environmental conditions. Additionally, night vision systems can only use energy they can see. The limited field of view means that a system's effectiveness can change dramatically with the viewing angle. These immediate changes are compounded by the slower cyclic changes in the light level due to lunar position and weather. As a result, the flying environment changes throughout the duration of the flight. Aviators must recognize these changes and adjust their flight profile accordingly. *** ## Ambient Light Sources During the day, the sun provides all required light and the level of illumination generally does not vary enough to cause or require any significant changes in flight profiles. During night flight, ambient light is provided by natural sources such as the moon and stars, and artificial sources such as aircraft lighting and cultural lighting. Illumination levels during the night can vary greatly and require significant changes in mission planning and aircraft employment. ### Natural Moonlight is the greatest source of natural illumination at night. The sun also has a significant impact on illumination for approximately the first hour after sunset and the first hour before sunrise. Starlight provides the illumination used by NVG when moonlight is insufficient. Much of the illumination provided by the starlight falls into the near infrared range and is not visible to the unaided eye. Lunar The moon is usually the primary source of natural illumination for NVG operations. The amount of light provided by the moon is highly variable and is primarily influenced by the lunar cycle and the moon angle. The primary lunar illumination factor is lunar cycle or phase (such as new, full, quarter) (figure 4-26). Each moon phase provides different levels of illumination. A lunar month is approximately 29.5 days. Moon phases are influenced by the time of year and global position (latitude and longitude) {% hint style="info" %} The moon's illumination is only available between moonrise and moonset. A full moon that rises at 0230 provides 0 percent illumination at 2130. {% endhint %}
### Solar Depending upon the azimuth and relationship to flight path, the sun can provide a significant amount of light. The time between sunset (when the sun is fully below the horizon) and astronomical twilight (when the sun is 12-18 degrees below the horizon) is a potentially dangerous time for aviators flying near the Earth's surface. It is too dark to fly unaided but still too bright to safely fly using NVG (due to potential activation of the NVG's automatic gain control circuitry). This time is referred to as end of evening nautical twilight. It occurs when the sun is less than 12 degrees below the horizon. For NVG operations, aircrew should plan to fly after end of evening nautical twilight whenever possible. The same effect is encountered during the last hour prior to sunrise, known as before morning nautical twilight. Although the sun may provide useful illumination if aircrew are flying away from it, a sun that is well below the horizon can continue to adversely impact NVG performance if flying toward it, especially in mountainous terrain. This effect is called sky glow. It can present a significant problem for aviators operating close to the ground, particularly when low-visibility hazards, like antennas, towers, wires or telephone poles are in the area. ### Starlight Starlight provides the illumination used by NVG when moonlight is insufficient (about 1/10 the illumination of a half-moon) (figure 4-27). Much of the illumination provided by starlight falls into the near infrared range and is not visible to the unaided eye. This night sky near IR energy matches the peak sensitivity of the NVG. It is possible to fly effectively with NVG under these conditions with a good training program and proper pre-flight mission planning. On a moonless night, about forty percent of the illumination is provided by emissions from atoms and molecules in the upper atmosphere known as air glow.
Sky Glow Sky glow refers to the effect the sun has on NVG when it is just below the horizon (figure 4-28). This normally happens during the first hour after sunset and the first hour before sunrise. Other energy sources such as a low-angle moon, large fires, or a city just beyond the visible horizon can also create this effect. The NVG gain down, resulting in minimal terrain detail and hazard detection, becomes very limited.
*** ## Artificial Light Sources There are many sources of artificial light in the night flying environment. Aircraft lighting systems provide local illumination in support of specific flight activities. Flares, laser pointers/illuminators, and weapons effects are all part of the tactical environment (figure 4-29).
*** ## Weather/Meteorology Some sensors provide increased visibility through light obscurations. This can lead aircrews to continue operations in conditions they normally would not due to deteriorating weather. A secondary scan and common sensor ques should be utilized to avoid hazardous weather conditions. ### Clouds Water vapor exists at all temperatures. Because the amount of water vapor a cloud formation can hold increases with temperature, summer clouds generally have higher liquid water content than winter clouds. These water particles are normally between 0.5 to 80 microns in diameter and are generally opaque to visible and near IR energy (figure 4-30). For this reason, thick, dense clouds can be easily seen with NVG, especially when silhouetted against the night sky. This also means thick clouds can reduce the amount of illumination that strikes the ground, thereby reducing the available luminance to the NVG. Thin and wispy clouds have greater space between particles. Therefore, a greater amount of the near IR radiation is passed without scattering. Near IR wavelengths have a greater chance of passing through these clouds without being scattered than do the shorter visible wavelengths. It is possible for thin, wispy clouds to be seen by the naked eye (visible light or shorter wavelengths) but remain invisible when viewed through NVG. This potential "invisibility" of thin clouds, fog or marine layers is possible when the clouds are thin and wispy, at least on the edges, particularly when the clouds are low level and seen against the terrain.
The presence of thin clouds that progress into thicker ones can hide terrain features and create a severe hazard for NVG operations. In that regard, a common question occurs; "If the cloud is invisible, why can't the aircrew see the terrain behind it?" The answer is predictably complex. First, the cloud reduces visual and near IR contrasts and detail. This produces a false perception of distance, resulting in aircrew either not seeing the terrain, or thinking it is much farther away. Second, the cloud may get progressively thicker, allowing the pilot to progress through the cloud without initially perceiving a "cloud wall." If a cloud is detected, the perception may be that it is off at a distance. Clouds reduce illumination to an extent dependent on the amount of cloudcoverage and cloud density or thickness. For example, a thick, overcast layer of clouds reduces the ambient light to a much greater degree than a thin, broken layer of clouds. If the NVG image becomes grainy and begins to scintillate (sparkle), this is an indication that weather may be causing a low ambient light condition, reducing visual acuity. The aircrew must be alert for a gradual reduction in light level and notice the obstruction of the moon and the stars. The less visible the moon and stars, the heavier the cloud coverage. Also, shadows caused by broken or scattered cloud layers blocking the moon's illumination can be seen on the terrain. Although clouds can decrease illumination and resulting luminance from the moon and stars, they can reflect enough cultural lighting to help offset the loss of lunar illumination. This only occurs in areas with significant cultural lighting and is only helpful if it is clear beneath the overcast layer. IIMC is a real possibility under these conditions, and aircrew members must be prepared to respond accordingly. ### Fog The effects of fog are similar to those of clouds. Generally, fog is distinguishable from clouds only in regard to distance from the ground. Particle size varies from 2 to 20 microns, which is very similar to a cloud. Typically, fog has fewer particles and a smaller range of particle sizes than clouds. Fall is the most likely season, and early morning the most likely time to encounter fog. Urban areas tend to have less fog (probably due to urban heat islands) than rural areas. Mountainous areas tend to have more fog than sites nearer sea level. Chances of fog increase as temperatures decrease and the dew point spread approaches zero. Since fog tends to stay close to the ground it is more a navigation hazard to RW aircraft than to FW aircraft. Fog can mask or partially mask ridgelines and other navigational features making it more difficult to navigate. One way to note an increase in the moisture content of the air while utilizing NVG is to observe a decrease in the intensity of ground lights. This is especially obvious when flying at an altitude high enough to compare ground lights in the immediate area to ground lights beyond the area of increased moisture content. Also, the halo effect noted around lights when viewed directly with NVG may become slightly larger and more diffuse in an area of increased moisture. The enhanced contrast in an area illuminated by ground lights is also lessened or absent. The discussion on NVG impact in the previous paragraph is also applicable to thermal sensors with some additional considerations. Fog droplets can produce almost 100 percent scattering for the FLIR. Larger particles such as raindrops or snow absorb as well as scatter far IR energy. In both cases, the end result is an attenuation of the FLIR image. Even with these adverse effects, the FLIR can "see" through this atmospheric obscurant better than the unaided eye. Under the right conditions, it can also still identify "hot spots" such as fires or operating factories. This information may help in detecting targets; but, due to the lack of detail surrounding these hot spots, the information may be of little help for navigating. ### Rain As with clouds, the performance of NVG and thermal systems in rain is difficult to predict as droplet size and densities are variable. All previous discussions on water vapor, clouds, fog, absorption, and scattering are applicable. Due to small droplet size and low density, light rains or mists cannot be readily seen with NVG. However, contrast, distance estimation, and depth perception are affected due to light scattering and the resulting reduction in light level. Heavier rains are more discernible due to luminance blocking and more obvious signs such as rain on the windscreen. The effect of rain on the FLIR is similar to that of fog. Since the droplet size is larger than fog there are relatively more absorption. Like fog, there can be some information gained through a light rain or mist; however, with a heavier rain there is significant attenuation. Also, constant rain over a period of time cools surfaces to a more uniform temperature and thus decreases the thermal contrast and ultimate scene discrimination. ### Snow The density of the snowflakes determines how much illumination and luminance is blocked and how much degradation occurs to the NVG image. Snow can reflect available light and thus enhance luminance when on the ground. Also, snow can add a slightly different texture that may aid in contrast discrimination. Due tothe excellent reflectivity of snow, less illumination is required to give the same luminance for the subject without snow. Thus the NVG can see the terrain under lower light level conditions. As with other forms of moisture, the effect on the FLIR depends on the flake size and density (figure 4-31). Due to the general size of snowflakes, scattering of the thermal energy causes most of the attenuation. Therefore, density of the flakes is of primary concern. For snow on the ground, the degree of attenuation depends on the duration of the snow cover. Snow can cool surfaces to a reasonably uniform temperature and thus attenuates the FLIR image. However, a fresh blanket of snow on the ground may be "invisible" to the FLIR, making it the sensor of choice if there is little texture/contrast for the NVG to work
### Sand, Dust, and Other Obscurants The impact of battlefield obscurants on NVG performance depends on particle size and density. NVG visibility "inside" or through these obscurants is usually poor (figure 4-32). Hovering in a dusty environment can be very dangerous. Visual references are easily lost and disorientation follows rapidly due to the swirling dust. Use of aircraft systems such as the HUD or HDU is strongly recommended. Dust operations are normally trained at the unit. Aviators should use high contrast references closer to the aircraft. Aircraft position and anti- collision lights can interfere with the ability to see outside the aircraft to the point of jeopardizing the safety of the aircraft. The pilot in command should consider turning off aircraft lights according to regulations and local SOP. Even small amounts of dust with light winds can obscure the horizon.
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