One has to be carefull when talking about angles of propagation and how the
antennas fit the mode and angles.

During the sunspot minima, the atmosphere shrinks, layers get more dense and
lower, compressed. Lower angles are favored. Also at the beginning of higer
bands openings low angles prevail, later on, in the middle of the opening to
particular area, the higher angles prevail. Having stacked antennas, or
combination of verticals and horizontals, one can observe the various modes
and how the antennas take advantage of them.

I have seen the effects and described them in my CQ article, see
http://members.aol.com/ve3bmv/bmvpropagation.htm

On low bands there are situations (mostly around sunrise) when high angle
prevail (low Inv Vee) and verticals (and beverages) with low angle pattern
can't even hear the station.

Also the ground conditions play big role in the performance especially of
verticals. Put them up at the salty beach, they are killers (loooow angles
enhanced). Put them up with lousy soil (clay) they don't work well.

73, Yuri