The upper layers of the atmosphere can reflect radio waves. The weather and solar radiation effect the atmosphere's ability to reflect radio waves, so the effect is quite variable. However, as the sun goes down, it's common to observe a significant change in radio wave propagation, thus the fairly common 'nightly skip event'. ('Skip' as in radio waves skipping off of charged layers of the atmosphere.)
Last edited by GroundUrMast; 23-Aug-2012 at 7:09 PM.