Why is the sky blue?

The answer begins with our solar system's ultimate source of light: the Sun. Sunlight seems white, yet it is composed of all the hues of the visible spectrum, from red to violet. Sunlight is absorbed, reflected, and transformed as it travels through the atmosphere by various elements, compounds, and particles. The color of the sky is principally determined by the wavelengths of incoming light, although air molecules (primarily nitrogen and oxygen) and dust particles also play crucial roles.

When the sun is directly overhead, the majority of its rays strike the atmosphere at almost vertical angles. Light with shorter wavelengths, such as violet and blue, is more easily absorbed by air molecules than light with longer wavelengths (that is, from red, orange, and yellow bands in the spectrum). After then, air molecules emit violet and blue light in diverse directions, saturating the sky. However, because our eyes are more sensitive to blue light than violet light, the noon sky looks blue rather than a blend of blue and violet.

When the sun is close to the horizon at dawn and dusk, its rays impact the atmosphere at more-oblique (slanted) angles, requiring them to travel a larger distance through the atmosphere than they would at midday. As a result, additional nitrogen and oxygen molecules, as well as other particles, can obstruct and scatter incoming sunlight. During this long journey, incoming radiation in the shorter blue and violet wavelengths is mainly filtered out, and its effect on sky color lessens. Longer wavelengths remain, and some of these rays impact dust and other particles near the horizon, as well as water droplets in clouds, to produce the red, orange, and yellow colors we see at dawn and sunset.