AT&T is experiencing a major outage of service in major regions of the western US.
Until it is resolved, customers may encounter difficulties calling our toll free number.
We apologize for any inconvenience this may cause and appreciate your patience.
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If you listen to AM radio stations by night that are just impossible to pick up by day, chances are you're benefiting from sky-wave propagation. Propagation is just the technical word for how radio signals travel through the air. Sky-wave propagation is the specific name given to radio waves that travel through the sky. Sky-wave propagation takes place between sunset and sunrise. It's the flip side to the groundwave propagation used to transmit during the day.
During the day, groundwave propagation is preferable because the radiation from the sun causes so much ionization that radio signals sent into the air are absorbed into the atmosphere. When atoms in the D region of the ionosphere are ionized, you end up with free electrons and ions floating around in the air. It's kind of like trying to walk through a room filled with dancing couples. When they're in pairs, there's more room to walk through, but when they're not paired, it's easier to get stuck in a conversation with someone. That's kind of how the ions and electrons in the D region work. When they're not combined, they look for something else to combine with, and that's how they absorb radio waves. At night, however, once the sun begins to set, the electrons and ions in the D region recombine rapidly - leaving more room for the radio waves to travel a little farther up. Above the D region, the F1 and F2 regions are also recombining, but much more slowly than the D region. When the radio waves hit those regions of the ionosphere, they have a chance to be reflected or bent (some prefer refracted) back toward the earth.