Why the sky looks blue

The sky looks blue because molecules and very small particles in the air scatter short-wavelength light more than long-wavelength light. Sunlight is a mix. Your eye, looking away from the sun, mostly receives the scattered short end.

The sky is not a blue gas, and it is not the ocean reflected upward. On the Moon, where there is no air to scatter sunlight, the sky is black even in the daytime.

The mechanism is more particular than the slogan. This kind of scattering is stronger for shorter wavelengths. Violet is scattered even more than blue, but the Sun puts out less violet, and human eyes are less sensitive to it. The mix that reaches you from a clear sky is what you name blue.

Follow it one step further. The same physics makes the Sun look redder at sunset. The light then travels a longer path through the air. Much of the blue has been scattered out of the beam before it arrives, and the reds and oranges are what remain in the direct light.

The case worth keeping in mind is specific. After a rain, or on a very clear dry day, the sky can look a harder blue because there is less large dust and haze. Haze scatters all colors more evenly and washes the sky toward white.

That case has a boundary. Clouds are white for a different reason. The droplets are large compared with the wavelength of light, so they scatter all colors about equally. A blue sky and a white cloud are two scattering regimes, not one.