Tuesday, August 12, 2014

Q: Why is the Sky Brighter over the Sea when it's Windy?

Breezing Up, Winslow Homer
A: Ask a sailor. 

There was a time when reading the weather meant more than just clicking an app on your smartphone. Seafaring Greeks, Vikings or Moors weren't concerned about choosing appropriate footwear for the long walk to the car, or remembering to pack a sweater to brave frigid air conditioners in corporate HQ; they needed to learn to read natural signs of approaching weather because their lives depended on it.

Becalmed mariners have long known when a stiff wind is approaching because the sky over the sea gets brighter in that direction. Why? It was a mystery for years until Galileo Galilei came along and applied his massive brain to the problem. The famed mathematician and astronomer was known to take long walks on the beach while contemplating the heavens, and while sitting on a hill overlooking the ocean one day, it suddenly occurred to him.

He had been pondering the question of the texture of the moon: Since the moon appeared so bright in the night sky, did that mean that its surface was mirror-like? Was it so smooth that it reflected the sun's rays directly towards the eyes of this earth-bound observer? Or was it rough and rocky?

His Eureka moment came while witnessing the effect of approaching wind on the water. As the wind picked up, it would roughen the surface of the water. If the sun was shining at the time, and the air being dense with moisture, the now fractured surface of the water would bounce the reflection of the light source (the sun) in all directions, where it would be scattered by the humid atmosphere and appear bright:

"From such waves, as from many mirrors spread over a wide area, there would originate a much brighter reflection of the sun than would exist if the sea were calm. Then that part of the vapor-laden air may be made brighter by this new light and by the diffusion of that reflection.  
This air, being high, sends also some reflection of light to the eyes of the sailors, while they, being low and far off would not be able to receive the primary reflection from that part of the sea which is already being ruffled by the wind twenty or thirty miles away. And that is how they perceive and predict a wind from afar."
Galileo's theory that rougher surfaces appear brighter than smooth surfaces led him to claim that the surface of the moon must be rocky and not glass-like; a theory that would be proven correct many centuries later when we finally stood on it. I say "we": I had nothing to do with it. 

He proposed a thought experiment to illustrate his point: Imagine that the sun is shining on a bright white-washed stone wall. Now, imagine hanging a mirror on that wall and stepping across the courtyard. Which looks brighter; the mirror or the wall? Spoiler alert: it's the wall.

Man in Armour, Rembrandt van Rijn

What's this got to do with painting? 

Medieval illuminators knew to think of gold-leafed areas of their panels as dark compositional elements, even though symbolically they represented the effulgent light of the Lord. It was obvious to them that while the applied gold surface may have been reflective, it was not shiny: it was physically dark under normal lighting conditions

Basically my point is this: paint shiny surfaces dark except for their specular highlights.

You should also look at this post regarding the technique for painting reflective surfaces like gold.


  1. Loved it.

    * After the floods I approach weather reports as if my life depends on them... as was the case.

  2. Always thank you! your intelligence and humor are so refreshing, such a good things in these twitter days. (sorry for my english i'm Italian)

  3. Illuminating, as always. Thanks for the great lesson, Alan.

  4. This is great, especially because I just read this article on how some beetles managed to be be whiter than anything man has produced using a similar principle.

    "The elements are tightly packed, scattering light efficiently, but still able keep a degree of disorder in their shape. Dr Vignolini explained that, "these elements are elongated, but some of them are shorter, some of them are bigger". Disorder is important to keep the beetle white."


  5. A practical and an enjoyable read!

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