Letter: Is it a Bird? No! Is it a plane? No! Is it a Volumteric Shadow?

Published on: 29 May, 2020
Updated on: 29 May, 2020

Harry Eve’s view of an unusual optical effect

From Harry Eve

In April a strange phenomenon was visible from my garden. An aircraft was leaving a normal contrail across the sky but ahead of it there was a discernible dark line.

Could it be a top-secret military aircraft in disguise? Was someone testing some new sort of beam? Or was it caused by an alien visitor? Could it even be a flight of the legendary Guildford Dragon patrolling to scare off interlopers during lockdown?

The source of the contrail was an Air France Boeing 777 on its way from Paris to Mexico City. That was as far as the FlightRadar website took me in my attempt to explain what was going on.

And the real answer came from a relative who flies cargo aircraft. Apparently, it was a rarely-observed “volumetric shadow” (also known as a “contrail shadow” or “edge shadow”). It occurs when a contrail casts a needle-like shadow onto a thin, almost invisible layer of cloud.

At least it wasn’t making any noise or peeping into our gardens.