The sky appears blue due to a phenomenon called Rayleigh scattering. When sunlight travels through Earth’s atmosphere, it collides with gas molecules. These molecules scatter the light in all directions, but they scatter shorter wavelengths (blue light) more strongly than longer wavelengths (red light).
This selective scattering means that when we look at the sky, we see the blue light that has been scattered by the atmosphere towards our eyes. The process happens millions of times before the light reaches our eyes, creating the blue dome effect we see overhead. On very clear days, the sky appears a deeper blue because there are fewer particles in the air to scatter the light.