Italian Subsun ~ Imaged by Alessia Boscolo Nata near the Tirrenian coast between Liguria and France.  ©Alessia Boscolo Nata, shown with permission.

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".. When I saw the 'subsun' I was traveling on the sunward side of the airplane, above a cirrostratus layer, and this fact was important for me to realize that was not a reflection from the window or lakes on the surface."

Subsuns are direct reflections of the sun from the nearly horizontal hexagonal faces of plate-shaped ice crystals in clouds.

They can be blindingly bright. They are easily distinguished from surface water reflections because they drift across the landscape beneath with the motion of the aircraft. The large view of Alessia's subsun shows some much smaller and sharper reflections from a river and pools far below the cirrus layer.

Look at subsuns carefully - you might spot the rare Bottlinger's rings or they might show currently unexplained curved tails.