Why does the sky appear white on the horizon?

[Eh]

The everyday sky is blue, though it can be noticed there are different hues depending on how high or low you look. Towards the horizon, the sky becomes much lighter, and looking low enough it appears white. What is the reason for this?

 

[Janitor]

As a pure guess, I would say that dust and maybe smog tends to hug the ground, and what you are seeing is scattered sunlight.

 

[Andre]

Exactly, some of us may remember from the old days, that the horizon tended to be a sharp line with the same contrasts as objex in the vicinity, the effect of really clean and dry air. The shattering of the light is caused by tiny particles, just like snow does.

 

[Eh]

Ahh, so we can blame it on pollution. What else is new?

On the topic of sky colors, that reminds me of something else. It seems the more cloudy it is, the lighter overall blue hue the sky has. On a perfectly clear day, the sky seems to be a much deeper blue. Why is this? I would guess the same conditions that determine the weather also determine the color, but I'm not exactly sure why.

 

[Janitor]

Good question. I had not picked up on that phenomenon. If it were just some psychological effect (like how the moon looks smaller when it is near zenith), I would have expected the opposite of what you state. The blue of the sky would look dark compared to the brilliant white clouds. But you are saying the opposite happens, so it must be a real effect. Maybe on cloudy days the humidity is high enough for there to be a thin haze layer that cannot be distinguished as a cloud per se, but nonetheless manages to whiten up the blue a bit.

 

[Eh]

I've read that the color depends on how much dust and what not in the atmosphere is present. I would guess this includes the state of molecules in the sky. At any rate, less blue light is scattered apparently. Do you suppose that is what is going on during a relatively humid day?

 

[holly]

A white sky means that there are many particles of mixed sizes present. The smoggy look of a muddy brown indicates many large particulates are present. I don't think humidity makes the sky appear white, because the water molecule is a very tiny one and would tend to scatter blue.

 

[Eh]

But what about the different hues of blue?

 

[Rader]

I live on the Mediterranean Sea. In the summer when there is a very high vaporation of seawater, the sky is a hazy white blue. Those are those real muggy sticky days. When the weather changes in the fall and into winter the sky is a deep blue again. The sky is the deepest blue when it is cooler less humidity and always after a good rain and the north winds bring dry air to the area. It seems to be combination of several factors here. We have 0 pollution, so that does not count. It is always humid here summer or winter, so maybe temperature is more of a factor. It seems that the spring and fall equinox is the moments when the changes start to be noticed, in the color of the sky.

One might ask, would the sky be a more deep blue, because of the higher angle of the sun in relation to the observer and horizon? From sunrise to sunset, this might be true but from winter to summer it is the opposite.

 

[sparkybubbles]

The reason is not all that simple. Also it would still be present with no smog or evaporate. The three types of scattering taking place here are Rayleigh Scattering, Mie Scattering, and Nonspecific Scattering. The occur where the ratio of light wavelength to the particle size is much larger than one, about equal to one, and much smaller than one respectively. Since there is no clear boundary between these, one can assume that some of all types of scattering is occurring at all times, one is simply dominant. In both Rayleigh and Mie scattering the scattering rises with the four power of the wavelength, which is why we tend to see only the shorter wavelengths (blue and purples) where these types of scatterings are dominant. In nonspecific scattering however, the particle is large compared to the light wavelength so that all lights are scattered nearly uniformly, resulting in white. This is the scattering we see present in clouds where the size of the water droplets is far larger than a few hundred nanometers across. Due to the Earths gravity, as well as a little common sense, particles found closer to Earth's surface are larger and thus more nonspecific scattering occurs and thus a more white hue. The same effect is seen in humid conditions for the same reason. Though it may be true that water molecules are much smaller than the wavelength of light, it is absurd to think that the water in humidity is segregated into its own separate molecules. Water is incredibly polar and most of the atmosphere is not. For this reason they tend to form droplets, that although small are large enough to shift the scattering into nonspecific regions.