Get that DX QSL even during the solar minimum.
Tropospheric ducting, or tropo, openings occur during stable weather periods of high pressure. While tropo can occur any time of year, it is most common during the summer and autumn. Typical conditions are warm, cloudless days with little or no wind.
Methodology:   If a large layer of cold air encounters a large body of warm air, a temperature inversion, or duct, can be created.   Ducting relies upon inserting a layer of warm air between two layers of cool dry air.   The more defined the boundaries are the better.   Such temperature inversions are not caused by local "weather" or terrain,   but rather by climatic conditions such as frontal boundaries.   The technician license test states that tropospheric ducting is the mode responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis.   The extra class license test "states warm and cold front are the weather related structures related to tropospheric propagation of microwave signals".   Another question on the test states "temperature inversion" is the type of "atmospheric structure that can create a path for microwave propagation".   The thickness of the inversion layer affects the LUF (Lowest usable frequency) - thinner layers only propagate higher frequencies such as microwaves,   thicker layers can be used into lower frequency regions of the spectrum.
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Utilization:   High mountainous areas and undulating terrain between the transmitter and receiver can form an effective barrier to tropospheric signals.   Ideally, a relatively flat land path between the transmitter and receiver is ideal for tropospheric ducting.   Sea paths also tend to produce superior results.   Because signals are refracted, reflected, and scattered expect much fading.   If it occurs on a larger scale, 2 meter signals can travel hundreds of miles.   Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles.   The extra class license specifies that "100 to 300 miles" is the "typical range for tropospheric propagation of microwave signals".   Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak.   Higher frequencies above 90 MHz are generally more favorably propagated.   In extreme cases, tropo openings can extend all the way up to 1.2 GHz.
Prediction:   Since we have a better understanding of tropo than some of the other phenomena that we have discussed we can to a limited degree predict it.   See William Hepburns maps at http://dxinfocentre.com/tropo.html for predictions.   This resource is of such importance that it is found on the extra class license exam which states that the probability of tropospheric propagation is predicted by Hepburn maps.