MCRP 12-10B.1, page 37

areas, can cause wind tunneling. This increases the effects of the wind along streets that parallel the wind direction, while cross-streets remain relatively well protected. Wind tunneling can have a negative impact on aviation support within the urbanized area by creating dangerous wind effects down streets and between buildings. (f) Day/Night Differences. Night and periods of reduced visibility favor surprise, which in turn, may facilitate infiltration, detailed reconnaissance, attacks across open areas, seizure of defended strongpoints, and reduction of defended obstacles. Because of the difficulties of night navigation in restrictive terrain (usually without reference points and in close proximity to the enemy) forces may consider using simple maneuver plans with easily recognizable objectives. Rotary-wing aircraft are best utilized at night when operating on urbanized terrain. The most effective method of protecting helicopters from a successful engagement by hostile ground forces is to remain unseen. During night flights, the use of night vision goggles (NVGs) should be weighed carefully. Artificial lights can render NVGs partially ineffective and can increase the possibility of mid-air collisions by degrading the goggles to the point where nearby aircraft and aerial obstructions are not seen. (g) Aviation Weather Considerations. Weather conditions can be a critical factor in determining the amount of aviation support the aviation combat element can provide in an urbanized area. Aviation forces should consider the following: Presence or absence of fog, industrial haze, low clouds, heavy rain, and other factors that limit visibility for pilots Illumination and moon phase/angle during night vision goggle operations Ice, sleet, and freezing rain that degrade aerodynamic efficiency Updrafts and downdrafts in the urban canyon that may present an unpredictable hazard High temperatures and/or high-density altitudes that degrade aircraft engine performance and lift capability High winds (large gust spreads) and crosswinds, to include the tunneling effect, that may create localized and unpredictable hazards to aviation Weather conditions that create hazards on pick-up zones and LZs, such as blowing dust, sand, or snow. MCWP 3-35.3 2-6