What causes tropospheric ducting of radio waves?

Tropospheric ducting is caused by a temperature inversion in the lower atmosphere. Normally, air temperature decreases with altitude. However, during a temperature inversion, a layer of warm air traps cooler air beneath it, creating distinct atmospheric layers with different refractive indices. When VHF and UHF radio waves enter this boundary, they get trapped inside the "duct" created by these temperature layers. The signals bounce between the inversion layer and the ground, traveling over the curvature of the Earth for hundreds or even thousands of kilometers. Other weather phenomena like low-pressure areas or lightning do not create the stable, stratified atmospheric conditions required to form a radio duct. Auroras cause a completely different type of propagation known as auroral scatter.