The [[D Layer]] is the lowest part of the [[Ionosphere|ionosphere]], and it matters a lot for AM radio.

## ELI5 version
Think of the sky as having invisible layers.

In the daytime, sunlight "turns on" the D layer and it acts like a sponge for AM radio waves, soaking up a lot of the signal. That is why AM stations are often more local during the day.

At night, the D layer weakens a lot, so it stops soaking up as much AM energy. Then more signal can reach higher layers and bounce back down to Earth, which is why you can sometimes hear far-away AM stations after dark.

## See also
- [[AM]]
- [[Electromagnetic Spectrum]]
- [[Ionosphere]]