What describes a surface duct in atmospheric propagation?

What describes a surface duct in atmospheric propagation?

• A. A region where waves are amplified by a wind shear.
• B. A region where waves are amplified by rain.
• C. A layer of air trapped between two temperatures, forming ducting.
• D. A channel for electromagnetic signals created by man-made structures.

Answer: (C)

A surface duct is a shallow atmospheric layer that acts like a waveguide for radio waves, trapping them near the ground. This happens when the air near the surface has a temperature (and humidity) profile that creates two distinct boundaries: air at the bottom and air above with a different temperature. The refractive index changes with height in just the right way so that rays bend downward and reflect within that near-surface layer, allowing signals to travel longer distances with less loss. So the description of a layer of air trapped between two temperatures, forming ducting, captures this idea of a surface-based waveguide formed by temperature boundaries. It isn’t caused by wind shear, rain, or man-made structures.

A surface duct is a shallow atmospheric layer that acts like a waveguide for radio waves, trapping them near the ground. This happens when the air near the surface has a temperature (and humidity) profile that creates two distinct boundaries: air at the bottom and air above with a different temperature. The refractive index changes with height in just the right way so that rays bend downward and reflect within that near-surface layer, allowing signals to travel longer distances with less loss. So the description of a layer of air trapped between two temperatures, forming ducting, captures this idea of a surface-based waveguide formed by temperature boundaries. It isn’t caused by wind shear, rain, or man-made structures.