The sky appears blue to the human eye as the short waves of blue light are scattered more than the other colours in the spectrum, making the blue light more visible. To understand why the sky is blue, we first need to understand a little bit about light.
As far as wavelengths go, Earth's sky really is a bluish violet. But because of our eyes we see it as pale blue.
The reason we see the sky as blue is because the molecules in the air scatter the light absorbing most wavelengths of light except for blue. In addition to this the sky is gray and overcast because of the water droplets in the atmosphere in the forms of clouds and humidity.
The sky during the day
At noon, when the Sun is overhead it appears white. This is because the light travels a shorter distance through the atmosphere to get to us; it's scattered very little, even the blue light. During the day the sky looks blue because it's the blue light that gets scattered the most.
While all colors are scattered by air molecules, violet and blue are scattered most. The sky looks blue, not violet, because our eyes are more sensitive to blue light (and the sun also emits more energy as blue light than as violet).
Light is made up of all different colours - that's why we get rainbows. Blue light can't travel very far so much of it 'scatters' out before it reaches us. But red light can, which is why the sky appears more red and pink than usual.
Actually, the sky was orange until about 2.5 billion years ago, but if you jumped back in time to see it, you'd double over in a coughing fit. Way back then, the air was a toxic fog of vicious vapors: carbon monoxide, carbon dioxide, nitrogen, cyanide, and methane.
there is no such thing as a grey sky. The sky is always blue. Sometimes, however, grey clouds come and cover the blue sky. We then think the sky is grey.
Blue Color
If all the colors were spread equally the sky would look white, since all of the colors of light combined make white light. But the atmosphere does not spread all colors equally in all parts of the sky, and that is why the sky is not a uniform color.
A: On hazy days, large particles in the air make the sky appear gray or even white, McRoberts explains. "These larger particles tend to scatter more wavelengths of light in the color spectrum," he says. "Hazy air has a lot of water molecules, and these molecules can scatter light of all wavelengths, not just blue.
Grey and gray are two different spellings of the same word. Gray is more common in the U.S., while grey is more common in other English-speaking countries. In proper names—like Earl Grey tea and the unit Gray, among others—the spelling stays the same, and they need to be memorized.
Sometimes, in Australia's outback the sky is very blue because it's so dry, but on other days when it's dusty or during bushfires, the sky will appear much redder. The blueness of the sky is also affected by other factors, such as the moisture and particulate matter in the air.
This scattering is called Rayleigh scattering. Blue and violet colors are scattered the strongest by air. As a result, when white light travels through air, it gives a slight bluish-white tint to the air.
On warm days, shorter wavelengths of blue light are scattered quickly, leaving the sky with vivid colors on the yellow-orange-red end of the spectrum. As a result, the same process that initiates brilliant colors at sunset makes the sky turn orange or yellow when a storm is brewing.
The most common colour for the sky on Earth is a white (or rather grey) sky. Clouds cover around 70% of the Earth, and the nature by which they are scattering shortwave radiation makes them appear white or grey.
The sunlight reaching our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, so we perceive the sky as being blue. Without an atmosphere the sky appears black, as evidenced by the lunar sky in pictures taken from the moon.
When you look up at the sky, any light that you see has been redirected toward your eyes—it has been scattered. Because you are seeing only scattered light, the sky appears blue. But violet light has an even shorter wavelength and a higher frequency than blue light, so by all accounts the sky light should be violet!
The particles which have larger size compared to the wavelength of the light scatter the incoming solar radiation and their scattering intensity is of the same extent for all the wavelengths of the white light. As the scattering intensity is nearly the same for all wavelengths, to human eyes it appears white.
The stunning colors result from particles in the atmosphere scattering light from the Sun. When there are more particles in the air, more light is scattered, and the colors are enhanced.
The particles and gases in the air scatter visible light all over the atmosphere, and since blue light has a shorter / smaller wave than most of the visible light spectrum, we see the sky as mostly blue during the day. During the evening however, the sky can change colors and become red, orange, yellow, etc.
In the living world beneath our red-ravenous atmosphere, blue is the rarest color: There is no naturally occurring true blue pigment in nature.
The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see. The ocean may also take on green, red, or other hues as light bounces off of floating sediments and particles in the water.
“Blue Marble” true-color image of Earth taken from a single remote-sensing device-NASA's Moderate Resolution Imaging Spectroradiometer, or MODIS.
But when it comes to nature, blue is very rare. Less than 1 in 10 plants have blue flowers and far fewer animals are blue. So why is that? Part of the reason is that there isn't really a true blue colour or pigment in nature and both plants and animals have to perform tricks of the light to appear blue.