Which material has the lowest radar reflectivity in the given order?

• A. Earth
• B. Aluminum
• C. Wood
• D. Water

Answer: (D)

Radar reflectivity is about how much energy a surface returns to the radar. Two big factors influence this: how rough the surface is at the radar’s wavelength, and the material’s electromagnetic properties.

A highly conductive metal like aluminum presents a strong impedance difference from air, so most of the incident radar energy is reflected back, giving high reflectivity.

Earthy ground surfaces can reflect a fair amount too, especially when they are rough or moist, producing a noticeable backscatter.

Wood, being a dielectric with moderate contrast to air, scatters energy but not as strongly as metal or rough earth; its backscatter is comparatively lower but still present.

Water surfaces, particularly when calm, behave as very smooth, specular reflectors. Most of the radar energy is reflected away from the radar toward the same angle on the opposite side, so little energy returns to the sensor. Water can also absorb some energy, further reducing backscatter. Under typical calm conditions, this combination yields the lowest radar backscatter among the materials listed.

So water ends up with the least radar reflectivity in this set.