Planet earth is surrounded by an atmosphere made from gasses such as nitrogen, oxygen, argon, carbon dioxide, and other chemicals. As light travels through the atmosphere, it interacts with the gas molecules and changes its wavelength.
One reason why the sky looks blue on Earth is that the atmosphere absorbs almost all the light that reaches the surface. When sunlight passes through the atmosphere, some of its colors get absorbed by molecules of nitrogen and oxygen, leaving only the blue light (long wavelengths) behind.
Another reason why the sky appears blue is that clouds absorb more than 99 percent of the light that falls upon them. Clouds are made up of droplets of liquid water and other substances floating in the air. Water has a strong absorption band in the blue light part of the spectrum. As a result, the blue light wavelengths are scattered away from the path of the incoming sunlight. Because the scattering happens in all directions, the sky appears blue.