When talking to people not interested in the weather or climate, I’ve often found they have trouble understanding basic topics taught in an introductory meteorology course. The one I hear most often relates to the question “Why is the sky blue?” Of the wrong answers, the most popular is that:

The sky is blue because of the blue water droplets that are suspended in the air. Let’s suppose the sky were blue due to the water droplets in the air. Why are clouds white? Aren’t they made of water droplets?

Maybe it’s due to water vapor! Alas, if that were the case, shouldn’t we see different shades of blue with changes in humidity levels?

It’s just a reflection off the blue ocean. Nope, the sky is still blue even when you’re far away from the ocean.

My grade school science teacher told me… Yeah, you should probably ignore all that too.

Okay knowitall, why is the sky blue then? It has to do with Rayleigh scattering “He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles. It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10.”

If that’s the case, why isn’t the sky purple? “The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That’s part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works. We have three types of colour receptors, or cones, in our retina. They are called red, blue and green because they respond most strongly to light at those wavelengths. As they are stimulated in different proportions, our visual system constructs the colours we see.” [ibid]

Whew, all of that just to get to this point.

If there were one simple science concept that you wished everyone in the world understood, what would it be?

My choice, if not obvious by now, is the above “Why is the sky blue?” This simple question gets to the core of the physics of scattering radiation. And to be able to understand the climate, and climate change issues, one needs to understand radiation. Not only is it necessary to understand the scattering of radiation, but also the aborption and emission of radiation as well. The fact that we don’t see the sky as purple leads into some really interesting biology questions as well.

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