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Radio waves naturally travel in straight lines, so you would naturally expect (because of the curvature of the earth) that no radio station would transmit farther than 30 or 40 miles. And that is exactly the case for ground-based (as opposed to satellite) TV transmissions. The curvature of the earth prevents ground-based transmissions from going much further than 40 miles for TV. Certain radio stations, however, especially in the short-wave and AM bands, can travel much farther. Short-wave can circle the globe, and AM stations transmit hundreds of miles at night. The reason that is possible is because there is a layer of the atmosphere called the ionosphere. It is called the ionosphere because, when the sun's rays hit this layer, many of the atoms lose electrons and turn into ions. As it turns out, the ionosphere reflects certain frequencies of radio waves. So the waves bounce between the ground and the ionosphere and make their way around the planet. The composition of the ionosphere is different between night and day because of the presence or absence of the sun, and it turns out the reflection characteristics are better at night.
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