Why do certain AM radio stations fade in and out at night?

Question

sometimes I get certain stations at only at night, but I can only listen for a few minutes then it fades away even if I try to retune the station.

Answer 1

AM radio signals are at a frequency where they can bounce off of the ionosphere. That means you can receive signals that are beyond the horizon, if conditions are right. But this can vary with the height of the ionosphere, so sometimes you can receive them, sometimes, when the ionosphere moves up or down, and you can't.

Wikipedia:

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. This phenomenon can be easily observed by scanning an AM radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. (Such stations are commonly referred to as daytimers.)

Answer 2

Any high power AM station never can reach more than about 100 miles,because the sun destroy  a layer of ionophere during the day time. AM wave contains ground wave and sky wave. During the day AM signal travels basicly only the ground  wave reaches far away. The sky wave goes through our sky towards to the outer space (alien can listen to our signal). At night as the sun down completely,ionosphere forms again that block the AM skywave and reflects back to the earth,the distance can be 10 times or more than the ground wave.  That is why short wave station can reach ten thousands miles away. Since  the thickness of ionosphere is changing all the time. As it is thick enough the signal reflects more,otherwise it goes through the ionospere and less singal being reflected. Therefore,at the other end of receiver,it produces faded result.

By cutting out the AGC control in the AM radio,we may get a better reception on long distance AM listening.