What type of propagation is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?

Tropospheric ducting occurs in the lower atmosphere (the troposphere) when a temperature inversion forms, trapping a layer of cool air beneath a layer of warm air. This sudden change in air density creates a "duct" that can trap VHF and UHF radio waves, bending them along the curvature of the Earth rather than letting them escape into space. This phenomenon allows for reliable over-the-horizon communications at distances of up to 300 miles, and sometimes much further, on bands like 2 meters and 70 centimeters. It is especially common during stable weather conditions, such as early mornings, late evenings, or over large bodies of water. Ionospheric layers like the D or F2 regions are primarily responsible for HF propagation and absorption, not the regular over-the-horizon extension of VHF and UHF signals.