We all know that the sky is blue, but I have seen the color of the sky in the day (because during sunrise and sunset it is obviously different) it is not the same shade of blue across places. Why is that? There isn’t any fixed shade of blue which we can say is the color of the sky.
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The simplest answer is that the composition of the sky and angle of the sun affects the scattering of light, which in turn affects the color our eyes perceive when light reaches our eye. A more in-depth explanation as to “why” will follow:
First, I’ll start with *why* the sky is blue (and why it is different at sunrise/sunset).
**Visible Light** – The color of the sky stems from the physics of visible light. Visible light functions likes waves (and particles, but that’s a whole different conversation), so they have a wavelength. Our eyes can see light with wavelengths from about 380 nm [nanonmeters] to 750 nm, with the wavelength determining color (it follows the rainbow [ROYGBIV], with red as the longest wavelength and violet as the shortest).
**Why blue?** -The air particles in the atmosphere in the atmosphere scatter shorter wavelengths of light better, so violet and blue light gets scattered across the sky more so than other colors of light. Due to this, the light coming at our eyes from the sky is most direction has much more blue and violet than other colors (which then blend into some shade of “sky blue”). The exception is around the sun, where the longer wavelengths of light (other colors) come through in higher proportions, causing the Sun to appear as white (the sum of all colors of light). At sunrise and sunset, as you noted, the sky is red. The reason for that is that the angle of the sun means it goes the atmosphere a much longer distance, leading to the light scattering a lot more. As such, most blue light never makes it to our eyes (the light is instead being seen by someone west of you, where the Sun is still higher in the sky).
**What can affect the color of of the sky** – Now, all that is to say is that the color of the sky depends on how the atmosphere is scattering the light from the Sun. Depending on where you are, the Sun will actually be at a different angle in the sky, which will change how the light is scattered slightly. However, the other thing that can affect it is the composition of the atmosphere. Our atmosphere is mainly composed of nitrogen gas/N2 (78%) and oxygen gas/O2 (21%), both of which are small, diatomic (two atoms) particles. Because they are small, longer wavelengths of light are less likely to reflect off of them (which is why blue light, a shorter wavelength, is scattered more, as mentioned above). However, as the size of a particle grows, it is more likely to reflect and scatter longer wavelengths of light (it is, after all, a bigger target), meaning things like dust particles will scatter more light, the scattered light not being as heavily blue. Alternatively, if there’s a lot of water droplets in the air in a given place, light will end up getting scattered more evenly and equally.
**Putting it all together** – Having a more equal distribution of light wavelengths being scattered (for whatever reason) will alter the sky’s color, making it whiter/greyer. Having a less equal distribution will tend to make it bluer, as our eyes detect mainly blue and violet light coming from the sky and much less of other shades, which then is jointly perceived as a shade of blue. Because the angle of the Sun differs based on location and because the atmosphere/air can differ based on the weather, local terrain (dust), human activity (CO2), and more, the exact shade of blue will vary from place to place and day to day.
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