Why the sky turned purple at sunset as Hurricane Dorian pulled away

Northeastern Florida was treated to a rare purple sky at sunset

By Rebecca Barry - Meteorologist

At sunset the sky turned purple as Dorian was pulling away from our area

JACKSONVILLE, Fla. - As Hurricane Dorian slowly moved away from our area on Wednesday evening, Northeastern Florida was treated to a colorful sunset and the skies turned a vibrant shade of purple. Pictures and videos of the unusual sunset poured into social media and the newsroom. Here's some of the science behind why we got such a colorful end to a very dreary day. 

It starts with light and the light spectrum. The visible light spectrum that we see is actually a combination of rainbow colors. When that spectrum of light is bent or manipulated, it can spread out the spectrum to display the individual colors within the spectrum. The most familiar example of this occurring is a rainbow. Rainbows occur when sunlight passes through a shower, the light enters the water droplets and is bent, or spread out by passing through the water. The spread out rays of light will then be on display on the other side of the water droplets, displaying the red, orange, yellow, green, blue, and violet components of the light spectrum. 

The light spectrum colors are always in the same position, red is the longest wavelength within the spectrum, blue is the shortest. 

Typical sunsets are more colorful than when the sun is higher in the sky because the rays of light from the sunshine are passing through more atmosphere, which contains moisture, when it is at a low angle at sunset. The additional atmosphere and moisture spread the visible light spectrum out and we see the red, orange, and yellow wavelengths. 

Our sky looks blue because the shorter wavelengths on the spectrum, the blue, hit air particles and molecules and bounce around, spreading out and becoming visible as they do so. 

So why did our sky turn purple at sunset after Hurricane Dorian? Moisture. So much moisture. As the sun set at the low angle, the waves of light were passing through significantly more moisture than we typically see, from the rain in the rainbands, to the tropical clouds extending our from the eye of the hurricane. The spectrum of light was spread so the the violet wavelengths filtered through all of the moisture and turned our skies to purple. 

The scientific term for the light spectrum being spread out is called Rayliegh scattering. It is defined as the scattering of electromagnetic radiation (the ray of light) by a particle (moisture in this case,) much smaller than the radiation. 

The result was a beautiful, violet sky at sunset as Northeastern Florida enjoyed the tail end of the light spectrum Rayliegh scattered all over our skies. 

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