Tropospheric Scatter On 144 MHz
Tropospheric propagation is radio propagation that occurs in the lowest layer of the Earth's atmosphere - the troposphere. It is 100% weather-related. A Tropo DX mode is any abnormal condition that scatters, reflects or refracts VHF, UHF and/or microwave signals in the troposphere causing changes to their normal coverage. There are six main tropospheric DX modes.
Detail Here : https://www.dxinfocentre.com/propagation/tr-modes.htm