First off! what is meant by a band opening up? Simply that the atmospheric conditions (solar flares, meteor scatter, tropospheric ducts, sporadic E layer density changes) and their effect have formed in a way that is conducive to bouncing and bending radio waves for extremely long distances. Thus opening up good communication between distant stations.
Tropospheric Ducting: As per Google.com AI "Ducting occurs when the refractive index of the atmosphere rapidly decreases with height, which leads to ray trajectories bending inward towards the surface of the Earth, effectively creating a waveguide and enabling long-distance communication. "Temperature and it's changes seem to be key in taking advantage of Tropospheric Ducting. Click this link to view the Tropospheric Ducting Forecaster. Additionally stated "Tropospheric ducting is when the atmosphere allows radio waves, including those used for high frequency transmission, to travel long distances due to temperature and humidity variations. It can extend your signal range but may also cause signal fluctuations and interference, depending on atmospheric conditions"
Sporadic E: As per Google.com AI and Wikipedia.com "Sporadic E propagation reflects signals off relatively small ionization patches in the lower E region located at altitudes of about 95~120 km (50~75 miles)." and "Sporadic E Layer refers to a mesospheric phenomenon characterized by the occurrence of temporary enhancements in the density of the E layer of the Earth's ionosphere. These enhancements are observed to last from a few minutes to several hours and are associated with specific latitudes and times of day."
How do we know or predict these conditions? HF Propagation and Terrestrial Data Website offers extensive data on RF conditions. The data is available as a widget for Windows or your website. You can look up band availability anytime. Was weeks before I realized one could simply look up the propagation conditions as opposed to trial and error.
Please see banner below. That is the real time status and condition of band propagation data.
