There are hundreds of books published and web sites that give very detailed information on radio propagation. This page provides links to some sources for learning about propagation as well as applications showing current propagation conditions.
Radio propagation is effected by many different things such as the time of day, season of the year and weather conditions both where you are at and where the DX station is located. Below are some sites and applications specifically for propagation conditions.
N0HR: Understanding HF Radio Propagation Forecasts
AE4TM: The Basics of Radio Wave Propagation
Australian Government Bureau of Meteorology
The largest factor determining propagation conditions on Earth is Space Weather. Solar conditions happening on the sun will either make or break radio propagation and DX conditions on Earth. As sun spots and solar conditions go through an 11 year cycle, conditions change literally minute by minute. Below are some sites and applications specifically for space weather.
Solar Terrestrial Dispatch
Radio Propagation : Space Weather : Sunspot Cycle Information
Space Weather: News and information about the Sun-Earth environment
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