Sky

Sky is the atmosphere and space visible from Earth. The atmosphere, which extends hundreds of miles or kilometers above Earth, is composed mainly of nitrogen and oxygen. It also contains tiny water droplets and ice crystals in the form of clouds and precipitation. Smoke, dust particles, and chemical pollutants may also fill the sky over cities. Space begins above the atmosphere. When the atmosphere is clear, space can be viewed all the way to neighboring stars.

During the day, the colors of the sky result from the scattering of sunlight by the gas molecules and dust particles in the atmosphere. Sunlight consists of light waves of varying wavelengths, each of which is seen as a different color (see Light (The spectrum of light sources) ). The shortest light waves appear blue and the longest red. The blue light waves are scattered by gas molecules in the atmosphere, but the red light waves travel undisturbed unless they are struck by larger particles, such as dust or pollutants.

When the sky is clear, the waves of blue light are scattered much more than those of any other color. As a result, the sky appears blue. When the sky is full of dense clouds or smoke, the light waves of all colors are scattered, causing the sky to turn gray. At sunrise or sunset, sunlight must travel farther through the atmosphere than when the sun is overhead. Light waves of most colors are scattered. However, red light waves are undisturbed and give the sun and sky near the horizon a red or orange appearance.