Medium-wave and short-wave radio signals act differently during daytime and nighttime. During the day, AM signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred miles (or kilometers) from the signal transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling AM radio stations to be heard much farther from their point of origin than is normal during the day.
The ionosphere is a region of the upper atmosphere, where neutral air is ionized by solar photons and cosmic rays. When radio waves reach the ionosphere at a shallow angle, they are partly reflected by the surface. The ionosphere can also be similar to a prism refracting light; different frequencies are "bent" by different amounts.
Much as the surface of the ocean interacts with the wind, the condition of the ionosphere is constantly changing due to interaction with incoming radiation. When signals have "bounced" off this irregular surface, they may fade in and out and have the "phasing", "flanging" or "fluttery" character familiar to listeners of shortwave music broadcasts.