For anyone with an interest in the low VHF bands, Sporadic-E is the main mode of propagation during the summer months.
During the RSGB conventions of 2017 and 2018, there were presentations on the subject which are shown below. One of the take away points from the videos is the importance of the Jet Stream and how it can trigger Sporadic-E over certain points.
2018... In this presentation, Jim Bacon G3YLA gives an overview of where we are with current understanding of Sporadic E.
2017... In this presentation, Jim Bacon, G3YLA gives an outline of Sporadic-E while Chris Deacon, G4IFK talks about the polarisation of signals.