What causes tropospheric ducting?

Tropospheric ducting occurs when there is a temperature inversion in the lower atmosphere. Normally, air temperature decreases with altitude, but during an inversion, a layer of warm air sits on top of a layer of cooler air. Because cooler air is denser, this abrupt transition creates a boundary that drastically changes the atmosphere's refractive index. When VHF and UHF radio waves hit this boundary layer, they are bent back down toward the Earth rather than escaping into space, allowing them to travel hundreds of miles. This effect has nothing to do with space weather like sunspots or solar flares, nor is it caused by localized violent weather like hurricanes, tornadoes, or lightning storms, which are completely different atmospheric phenomena.