Today, a lot of references can be use like HAP Charts, Solar-terrestrial data & etc..all these can be find at internet..but how it works..the data's was done by old timer professional person which is have a long long time to be done the experiments & experiences..the data was base on the main element in the world..like water, wind, fire (solar), earth..then become some humidity..all these element have been experimented & can be use for the Prediction of Propagation..
The question is 'how far this data can be use on hams activities?' very hard to answer right..if we back by year 50's - 70's, there have a book supply for malitiry to calculate what freq can be use at that day & time..quite long calculation needed to get the actual freq..the book called S-10..every months the book will be change..because the value are diffrents..& that was by that time but today no need..just switch on the transciever, put some power, tuned antenna & transmit..to test the propagation..sometimes we heard seem like the propagation is good..but try to calling CQ DX..no body pick up..so, assumption is propagation not so good..if propagation not there..just turn off the radio & sleep..try again at another days & times..
**The best time for DX are after SUNSET**
all the best my dear friends..
73/88 de 9M2ODY