Erik Grosse at Ulm in Germany pictured this contrail chasing its shadow cast on to lower cloud. A UPS freight 747 flying from Dubai to Cologne at 34,000ft was responsible. Each of its four engines left a distinct trail of condensed water droplets.
Why are the contrail shadow in-line? Why does the shadow lead? The special geometry needed only happens when we see the aircraft or its contrail tip appear to cross the sun.
All contrail shadows are counter-intuitive. To the eye and in photos the shadow seems to be above the contrail. Cast upwards from a sun near the ground! But the sun is around 93 million miles away, its rays in our local sky are almost parallel and they travel downwards.
The contrail shadow is cast onto a lower level layer of translucent cloud. We see the darkened cloud from below.
Now comes the headachy bit. Imagine the sloping grey plane at left. It passes across the contrail line and the line of its shadow on the lower cloud. To have the contrail cross the sun and chase its shadow the grey plane must also intersect your eye.
To have the shadow leading the contrail, the aircraft must already have crossed the sun.