Why do you hear some radio stations better at night than in the day?
This extended transmission is possible is because of the ionosphere -- one of the layers of the atmosphere. It is called the ionosphere because when the sun's rays hit this layer, many of the atoms there 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 at night is different than during the day because of the presence or absence of the sun. You can pick up some radio stations better at night because the reflection characteristics of the ionoshphere are better at night.
(Source : http://www.howstuffworks.com)