I have to admit. Like many (I asked my roommates if they did, too), I never understood exactly why the sky is blue. It seems like the most basic question, and definitely one that a 20-year-old human being should be able to answer. Up until now, I just accepted blue skies as a part of reality, and never really probed further. We cherish a blue sky, and gaze up at it during the nice weather, but do any of us really know why it is blue or how it could be so? Alas, I finally found this out in my meteorology class this semester, and would be happy to relay the information to anyone else out there who wondered about this phenomenon like I did.
First of all, the way we all see colors in the world around us is because of the visible light spectrum. These are a set of electromagnetic waves that, when reflected off of objects, scatter certain waves that correspond to certain colors that our eyes can detect. In the visible light spectrum, the color blue has one of the shorter wavelengths, and the color red has one of the longest. If you can imagine it, that process is sort of exactly what happens in the sky when light comes down from the sun, but on a much grander scale.
As many may have noticed, during each day, the sun is in a path overhead through the sky until sunset. When the sun is so high in the sky, its rays travel almost directly downward towards the Earth’s surface. At this time of day, with the sunlight (which contains visible light) reaching the Earth at such a direct angle, the sun’s rays have to travel their shortest path in the 24-hour cycle through the atmosphere to reach our eyes and the environment around us. Because the sun’s rays have less atmosphere to travel through during the day, these rays also have to travel through less particles present in this atmosphere. (There is always stuff in the atmosphere, by the way: dust, smoke, water vapor, ozone, etc.) These particles in turn scatter the sun’s light as it travels through, and since these particles are “few and far between” because of the light’s direct path, only blue – a shortwave part of the visible light spectrum – is scattered, while the longer waves are absorbed. Thus the sun appears white during the day, blue being the only color scattered through the atmosphere and thus making the sky appear blue.
The same goes for sunsets. When the sun begins to sink in the sky, and therefore is not directly above us anymore, its rays must reach us on earth from a further angle and distance. Because of this, the sun’s rays must then travel through more atmosphere, and thus pass through more particles in the atmosphere; this, in turn, causes only the longwave part of the visible light spectrum to be scattered, which includes colors like red or orange. That is why the sun and the sky oftentimes appear red at sunset.