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.
In: 0
Not a scientist so I’m sorry if this explanation is lacking. The color you see is based on light coming through the atmosphere and bouncing on dust, smoke and water vapor as well as the angle the light is coming from which gives the orange red at dusk due to the longer time that light has to interact with things in the air.
It depends on where you are, where the sun is, and what’s in the sky.
The sky will scatter the light from the sun, that’s why you see the sky at all and not blackness with the other stars. It actually scatters starlight and moonlight too, but since they’re so much dimmer we don’t notice much. If you’re careful, you could notice a difference between the brightness of the sky in full moon vs new moon, but it’s tricky to do that comparison when you have to wait two weeks to do it.
The sky scatters different colors of light differently, with blue being much much more strongly scattered than red. This is really noticeable at sunrise and sunset, as you say, because the light goes through a lot of air. Looking at or near to the sun, you see the red light coming through straight and the blue is missing as it’s been scattered away before it got to you.
But this effect doesn’t just stop at other times of day. At midmorning, the sun might be up and to your left. If you look near it, the sky will be more whitish blue than the blue of the rest of the sky, and for the same reason. The direct path to you scatters more blue than red, so it looks a little less blue.
Now look up and to your right, so that the angle between the sun, you, and your new gaze is at 90 degrees to the sun. Now, *only* the light that scatters directly at 90 degrees hits you, and that will be almost all blue, so the most intense, bluest sky will be there, because it’s also a short path after it scatters once.
Polarized sunglasses will enhance this effect, as the scattering will polarize the light when it bounces.
This all ignores things like water vapor and clouds, which cause a different kind of scattering that is uniform in color and looks white. A thin wispy white in part of the sky will change its apparent color, even if you don’t identify it as being an obvious cloud.
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.
When you look towards the horizon, there’s a lot more air between you and space than when you look directly up. More air accounts for more of that blue hue.
Latitude also matters, because the closer to the poles you get, the lower in the sky the sun will be. That means its light needs to pass through more air to get to ou so the sky is bluer overall in high/low latitudes.
Latest Answers