Stratospheric Anticrepuscular Rays & Shadows

Imaged by Chris Wright from the cockpit of a corporate aircraft flying at 45,000 feet heading south from Laredo Texas. The evening view is eastwards away from the sun. Relatively nearby clouds are sunlit. Shadows from westwards clouds streak under and over the high flying craft to apparently converge at the antisolar point.  
©Chris Wright, shown with permission.

Atmospheric
Optics
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The long shadows are parallel and only appear by perspective to converge towards the point directly opposite the sun. Roads and railway tracks similarly seem to converge in the distance.

The shadows darken as they near the antisolar point because the eye looks increasingly along the long corridors of shadowed air rather than across them.
These anticrepuscular (antisolar is a better name) rays and shadows are unusual. They cross the sky in the normally clean air of the stratosphere.

Rays and shadows need small suspended particles or droplets to scatter light and make them visible. Stratospheric rays are seen after volcanic eruptions have lofted fine ash and acid aerosol forming sulphur dioxide 10-15 miles or more upwards. Chris Wright suggests Mexico’s stratovolcano Popocatépetl might be responsible as there have been several recent small eruptions.