I often see jet vapor trails as I sit eating grapes and
enjoying warmth of morning sun. At first the trail is very
narrow, but well behind the invisible plane, it sometimes becomes two parallel columns, and then one side only because broadened with tiny parallel barbs perpendicular to the one that stays a column, each gradually widening further
and fading, but looking like a mile long pinnate plume. I cannot imagine what the mechanism is that changes only one of the columns. Bean, where are you?