Noctilucent, very high altitude, night time visible, clouds are becoming more common and at lower latitudes. This may be a result of climate change. One phenomenon associated with these is strong microwave radar echoes, raising the possibilty of very long distance DX by reflection from these on bands like 10GHz. This is an area ripe for experimentation but I am unaware of ANY groups anywhere investigating this exciting new propagation mode.
There is a link between noctilucent clouds and the solar cycle with NLC activity peaking just after solar minimum, possibly because low solar activity allows the upper atmosphere to cool, promoting the growth of ice crystals that make up the clouds. See http://spaceweather.com/