Why is the sky blue?
The sky appears blue to human observers because blue light from the sun is scattered in all directions by the gases and particles in the Earth's atmosphere. This process is called Rayleigh scattering.

When sunlight reaches the Earth's atmosphere, it is made up of many different colors of light, including red, orange, yellow, green, blue, indigo, and violet. These colors are all part of the visible light spectrum. Blue light has a shorter wavelength and higher frequency compared to other colors in the spectrum, making it more susceptible to scattering. When the sun's rays reach the atmosphere, they collide with atoms and molecules of air, which scatter the light in all directions. Blue light is scattered more than other colors because of its shorter wavelength, so it reaches our eyes from all directions, making the sky appear blue.

It's also worth noting that, at sunset, the sun is closer to the horizon and the sunlight has to travel through a lot more of the atmosphere than it would during the day, so more of the blue light gets scattered out before it reaches our eyes, that's why the sky is red, orange, yellow and pink at sunset.
Why do clouds form?