Of course! Everyone has…! Here’s an example from my network…:
On our wireless links it occurs mainly in the evening or at dawn…!
It is called Thermal Fading http://en.wikipedia.org/wiki/Thermal_fade
Read this also: Atmospheric Ducting ( http://en.wikipedia.org/wiki/Atmospheric_duct )
Read this thread also http://forum.mikrotik.com/t/solving-20km-wireless-link-issues/30025/1 thoroughly…!
Cheers!
P.S.: To combat this fading try to increase the output power or use more directional antennas, e.g. dishes instead of grids… (if you’re allowed!)