Why Tropospheric ducting happens in the lowest part of the atmosphere, where a temperature inversion (warm air sitting above cooler, moister air) creates a sharp change in refractive index. VHF and UHF signals entering that layer at a shallow angle are bent back down and travel inside it like a waveguide, often giving contacts of several hundred miles on 144 MHz and above. It is a weather-driven, not sun-driven, mode, and is common along coastlines and after fair-weather high-pressure systems settle in.
Watch out Sunspots and solar flares drive ionospheric effects such as F region skip and auroral propagation, which happen far above the troposphere and mostly affect HF, not the low-altitude bending described here.
Tropo = troposphere = weather. Warm air over cool air makes a duct, like a pipe for VHF/UHF.