Atmospheric duct

Lore & Background

Atmospheric ducting occurs when the reduced refractive index due to lower densities at higher altitudes bends signals back toward Earth. Signals in a higher refractive index layer tend to remain in that layer because of reflection and refraction at the boundary with a lower refractive index material. In certain weather conditions, such as inversion layers, density changes so rapidly that waves are guided around the curvature of the Earth at constant altitude. Related atmospheric optics phenomena include the green flash, Fata Morgana, superior mirage, mock mirage of astronomical objects, and the Novaya Zemlya effect.

Reader's Guide

Atmospheric ducting is significant as a mode of propagation that allows electromagnetic radiation to travel far beyond normal radar and radio ranges. In over-the-horizon radar, ducting causes part of the radiated and target-reflection energy to be guided over distances far greater than the normal radar range, enabling detection beyond the horizon. It also permits long-distance propagation of radio signals in bands that would normally be limited to line of sight, contrasting with the usual behavior of ground waves that propagate as creeping waves diffracted around the Earth's curvature. This phenomenon is distinct from ionospheric reflection of HF waves and is exploited in systems such as the Russian Predel-E OTH radar system. The duct's ability to reduce attenuation and confine wavefronts horizontally makes it a key factor in anomalous propagation and tropospheric ducting, with implications for communication and radar systems.

Did You Know?

• Atmospheric ducting can cause radio signals to follow the curvature of the Earth at constant altitude.
• The Fata Morgana of the Farallon Islands is a visible example of atmospheric ducting.
• In over-the-horizon radar, ducting guides radiated and target-reflection energy far beyond normal radar range.
• Related optical phenomena include the green flash, superior mirage, and the Novaya Zemlya effect.