What type of VHF/UHF propagation depends upon small variations in density and water-vapour content?

The troposphere is the lowest layer of the Earth's atmosphere, where all of our weather occurs. Small, continuous variations in atmospheric density and water-vapour content in this layer can scatter VHF and UHF radio waves, a mode known as tropospheric scatter. This scattering allows a tiny fraction of the transmitted signal to be received just over the visual horizon. It is a highly reliable, albeit weak, mode for over-the-horizon VHF/UHF communications. Tropospheric ducting is a different phenomenon that requires strong, distinct temperature inversion layers to physically trap the signal, rather than just small random variations. Sporadic-E and ionospheric scatter happen much higher in the atmosphere (the ionosphere) and are unrelated to tropospheric weather conditions.