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Just before the hurricane Milton hit Florida as a Category 3 storm on Wednesday, with many people reporting the sky above them turning an ominous shade of purple. A sign of the apocalypse? Well, yes, actually: the climate catastrophe that we have brought on ourselves. But it is still a natural phenomenon with a scientific explanation.
Light and color
Visible light is a narrow band of the electromagnetic spectrum with wavelengths from 700 to 380 nm. (Nanometers are billionths of a meter.) Within this range, our eyes interpret different wavelengths as different colors: red, orange, yellow, green, blue, indigo and violet, in order from longest to shortest. (Also known as the rainbow).
In reality, we only have three color sensors in our eyes: one for red, one for green, and one for blue. The intensity of the light detected by each sensor and the combination of the three gives us all the other colors. If your eye detects equal amounts of all colors, you will perceive it as white. Violet is a unique wavelength close to 380 nm, at the limit of what our eyes can see.
Why is the sky colored?
If the sun produces white light, why do we see colors in the sky? The reason is that when an electromagnetic wave encounters tiny particles in the atmosphere, some of it is scattered. The precise effect depends on the size of the particles and the wavelength of the light. With very small things like oxygen and nitrogen molecules, shorter wavelengths (blue and violet) scatter more than longer wavelengths (like red and orange).
This means that when sunlight passes through the atmosphere, the reds and yellows will mostly pass through directly and the blues and violets will be scattered. If you are standing on the surface of the Earth and look up, you will see all that scattered blue and violet light. That’s why the sky on a clear day looks blue.
This also explains why the sun appears redder at sunset or sunrise. When the sun is lower in the sky, white light has to pass through more of the atmosphere, which disperses blue colors even more. That lets more red light through to produce that pretty red sunset.
Why isn’t the sky always purple?
Well, wait. I said that shorter wavelengths scatter more than longer wavelengths. But that means that purple colors disperse even more than blue. So why doesn’t the sky normally look purple? Very good point. Two reasons:
First, when the sun produces light, it does not have the same intensity for all the different colors. In reality, the sun produces higher light intensities at larger wavelengths (red and green) than at smaller wavelengths (blue and violet). So when sunlight hits the atmosphere, there is simply more blue light than violet.
The second factor has to do with human eyes. Since we only really perceive three colors (red, green, and blue), our eyes are not as sensitive to the smaller wavelengths of violet as they are to blue. So if the sky scatters both blue and purple wavelengths, our eyes prefer blue. In reality, heaven probably is More violet than you imagine.
Here is another important observation that you can check for yourself. The sky is not one color. Yes, there may be a Crayola color called Sky Blue, but the sky is actually a bunch of different colors mixed together. That’s what makes the sky so beautiful.
purple hurricanes
The hurricane is not violet; We all know it, but it’s still fun to say. But what is it about hurricanes that allows us to see this violet light? Well, first, this usually happens when the sun is low in the sky so that the light passes through more air. Pink hues of sunset or sunrise overlap scattered blue and violet light, creating a violet blend.
Plus, it’s not just fresh air. There are always many other things in the atmosphere that cause scattering, such as water vapor, dust, and debris. And there’s a lot more of that up there during a tropical cyclone. Finally, the cloud cover overhead can block the blue sky. All of these factors contribute to a wide variety of colors and yes, one of them is purple.
