The green sky yesterday morning was a rare treat for us New Yorkers. If you read through the comments you noticed that the green sky is much more common in the Midwest, mainly because thunderstorm frequency is much higher in the middle of the country. Green sky conditions are often said to be an indicator of severe weather, especially hail.

Do you remember Roy G. Biv, the mnemonic for remembering the colors of the rainbow? Well old Roy is the key to understanding why the sky turned green as the storm approached yesterday. Roy is Roy because the colors the letters represent are listed in order of decreasing wavelength. Red has the longest wavelength and violet the shortest in the visible portion of the electromagnetic spectrum. When the sky turns green, the colors on either end of the spectrum are greatly diminished relative to yellow, green and blue.

What could cause the green to increase, or the other colors to dissipate? Gothamist hit the books to find out.

For a long time people had no idea what caused the sky to turn green. It was speculated that the color had something to do with odd reflections from green foliage. Another often heard explanation was that the green was an optical illusion of sorts –our minds get tricked into seeing green by the dark cumulonimbus clouds. A third hypothesis was that sunlight selectively passed through the thundercloud. When you see wildly different explanations of a natural phenomenon it usually means there's not enough data to support or reject explanations of the phenomenon.

It was only in 1995 that someone measured the spectrum of light in storms that produce the normal bluish-gray light and the more rare greenish-yellow sky. The measurements showed that the sky was indeed more green, quickly shooting down the optical illusion theory. The measurements also showed that the spectrum of colors in a green sky were sufficiently different from the green ground cover that reflections off vegetation were not sufficient to color the sky. Our experience yesterday with a green sky over our very non-green city also refutes the reflection explanation.

That leaves us with only one viable explanation: sunlight passing through a cloud gets selectively attenuated. The reds, oranges, blues, indigos and violets that enter a cloud don't make it out of the cloud, leaving us with a greenish-yellow sky. To a good approximation there are three things that affect how much light passes through a cloud: the thickness of the cloud, the size of the cloud drops, and the density of water in the cloud. Combine these three things and you come up with something called optical thickness. By doing a few calculations one of the meteorologists who first measured the green sky color spectrum found that thick clouds consisting of small drops and lots of water gave bluish-gray skies. This is the typical color seen in a severe thunderstorm. Thick clouds with large drops and a high water content produced greenish skies. In other words, the yellowish-green we saw yesterday happens when storm clouds are exceptionally thick with water.

We liked PR002i's sky photo so much we had to run it twice.