- Education and Science»
- Geology & Atmospheric Science
Why is the Sky Blue?; Blue Skies
Colors of The Sky
The sky displays a variety of colors; looking at the sky can give anyone the thrill of their life, especially during sunrise or sunset, which is when the sky horizon changes into an array of different tinges, such as orange, red and even purple. In between sunrise and sunset, the sky may acquire a bluish color or bluish white, depending on the conditions of the atmosphere and the particles that interact with light. At some time during these hours of the day, the sky is blue.
During a clear day, the sky is blue, especially when the sun appears high above in the sky, while on the horizon, the sky may show shades of lighter blue or a whitish color. If we look at the Sun, although not directly, we can see that it looks yellow. This is the result of the colors that were not scattered, such as red, orange and yellow. The great variety of blue tinges that the sky attains throughout the day is the result of the interaction between the colors of the visible spectrum, especially the blue wavelengths of light, and the Earth's atmosphere.
The Visible Spectrum
Light waves are a form of electromagnetic radiation emitted by the Sun. Electromagnetic radiation travels through space in the form of waves. Shorter wavelengths correspond to higher frequency radiation, whereas, longer wavelengths correspond to lower frequency radiation. The visible part of the electromagnetic spectrum, which is the part that our eyes are able to see, contains the colors; red, orange, yellow, green, blue and violet and their wavelengths range from 380-720 nanometers, with violet as the most energetic and red as the less energetic color of the visible spectrum.
The sky is blue due to the scattering of portions of light colors from the visible spectrum. These colors have different wavelengths and frequencies. Red has the longest wavelength, lowest frequency and less energy. Violet, which is at the other end of the visible spectrum, has the shortest wavelength, higher frequency and higher energy. Light travels through space without being disturbed, but once it reaches the atmosphere of the Earth, light gets reflected, scattered or absorbed in the sky, usually turning the sky blue.
Why is the Sky Blue?
The sky is blue because the particles in the Earth's atmosphere, especially nitrogen and oxygen, scatter the blue wavelengths coming from the sun in all directions in the sky. When light enters the atmosphere, the shorter wavelengths of the visible spectrum, such as blue and green lights are scattered more than the longer wavelengths, such as orange or red, turning the sky blue. In this way, the atmosphere functions as a filter that allows the longer wavelengths pass through the atmosphere and scatters the shorter wavelengths to the side and this most commonly has to do with the thickness of the atmosphere.
At sunrise, when the atmosphere is thicker, the light from the sun has to go through a thicker atmosphere. As light enters the atmosphere at this time of the day, the shortest wavelengths, such as blue wavelengths of light, are scattered the most, followed by green, yellow, orange and red. Since the longer wavelengths, such as yellow, orange and red are not scattered as much by the atmosphere, these colors are able to reach our eyes and we perceive a reddish sun with an orange tinge around it. A bluish color can be perceived farther away in the sky, produced by the scattering of blue and green light.
Blue Wavelengths of Light
As the Sun begins to rise up in the sky, the amount of atmosphere becomes thinner and cleaner, so that the particles interacting with the light from the Sun are only nitrogen and oxygen, which are responsible for most of the scattering of blue wavelengths of light. If the sky is clear and free of pollution, the sky will become bluer and bluer, as the sun gradually rises up in the sky. When the sun has reached its highest position in the sky, the amount of atmosphere will be minimum, and the sky around the Sun will have attained its more intense hue, making the sky blue.
Diffuse Sky Radiation
Not all light reaching the Earth's surface comes directly from the Sun. Some light reaches the Earth's surface after having been scattered by air molecules or other particles suspended in the atmosphere. This rescattered of light in the presence of suspended particles produces different tinges in the sky, for example, white clouds, crepuscular rays in the Sun or moon or hazy skies. From approximately 25% of light scattered in the atmosphere, around two thirds of it reaches the surface of the Earth as diffused radiation, which is what produces the variations on the color of the sky.
Why are Clouds White?
Droplets that form in clouds scatter all wavelengths of light. Clouds are usually poor absorbers of light and even the smallest clouds are thick enough to allow sunlight to pass through them. Clouds appear white at the region where sunlight reaches them because water droplets inside clouds scatter all wavelengths of light in every direction.
As a cloud becomes denser, more sunlight is scattered in all directions, making it impossible for sunlight to reach the lower part of a cloud. This causes the cloud to attain a darker tinge at the bottom.