In outdoor wireless deployments, foliage --- particularly dense trees --- represents the most significant and variable RF obstruction that engineers must account for during site survey and design. Trees are problematic for multiple compounding reasons: the high water content of living tissue causes signal absorption consistent with the principle that water is an effective absorber of 2.4 GHz and 5 GHz RF energy; the irregular branching structure causes multi-path scattering; and foliage density changes seasonally, meaning signal propagation characteristics measured during winter may differ substantially from summer readings when leaves are fully developed. A fully leafed deciduous tree can attenuate a 5 GHz signal by 6--15 dB depending on density and depth. Wind (Option A) causes only momentary mechanical movement of foliage and is not an obstruction itself. Rain (Option B) causes some absorption at higher frequencies but its effect at 2.4 GHz and 5 GHz in typical conditions is marginal compared to foliage. Poles (Option D) are thin structures with minimal RF impact. Outdoor surveys for university campuses must specifically account for tree locations, canopy density, and seasonal variation. Reference: WLSD Study Guide --- Outdoor Wireless Design, Environmental RF Attenuation Factors, Outdoor Site Survey Considerations.