Visiblility Blocked by Stacked Hexes?

[MisterMark]

Seems a hex with a larger stack of units can inhibit or block visibility even when weather related visibility is greater. 

If that is truly the case, then how many units in a hex are needed to block the visibility?   And is it the amount of units or the cumulative value in men/vehicles of those units?  And by chance would that threshold happen to be the road stacking limit or the hex stacking limit?