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Zero Order Glow

Frank Nieuwenhuys captured this scene at Delft in The Netherlands when the sun had already set at sea level (Delft is actually slightly below sea level).

"On August 26th) evening, between about 20:15 and 20:45 C.E.S.T. a line of heavy showers with strong wind gusts and a thunderstorm was raging over Delft.

At about 20:30 it was already pretty dark outdoors. Indoors all the lights were switched on. At about 20:40 something very surprising appeared in the sky An intense beautiful, bright but ominous-looking orange-red glow emerged, as if a giant inferno was going on in the distance.

Immediately, I understood the cause of the 'inferno-look-alike glow'. The setting sun in the west was shining on the rear of the downpour. I grabbed the camera and rushed outdoors. At 20:45 and 20:46, I made the attached pictures.

The sun was 0.3° below the horizon.

In an extraordinary way. I was lucky, I noticed the glow just in time, to rush outdoors and take the pics.

Images ©Frank Nieuwenhuys, shown with permission

At Delft the sun was already below the horizon at ground level. However, an already set sun lights higher altitude clouds and air to give a twilight glow.

But this intense light was very likely something different - A "Zero Order Glow".   It's an 'unofficial' term, I coined it more than a decade ago and it seems to have stuck!

Rain was likely falling in the west. When sunlight falls on a raindrop, most of it emerges from the other side. The curved surfaces refract the light and the leaving rays splay out over a wide angle. These rays - highly reddened as the sun is shining through much atmosphere - lit up the western sky with an intense diffuse glow.

A small fraction of rays entering a drop reflect instead off the far side and emerge towards the sun. These rays form the primary or 1st order rainbow.

An even smaller fraction reflect twice to form the 2nd order or secondary rainbow.

Why "Zero order glow" and not "Zero order rainbow"?   The zero order rays spread through 180° without concentrating in any direction. They do not gather or cross to form a rainbow. We see a diffuse glow in the sky and not an arc.

When you see a rainbow and the sun is low, look sunwards for the glow!