The phenomenon that explains why the sky appears blue during the day is known as Rayleigh scattering. As sunlight enters the Earth's atmosphere, it collides with gas molecules, scattering shorter wavelengths of light, particularly blue light, more than longer wavelengths like red light. This scattering causes the blue hue to be more prominent when observed from the surface. The intensity of the blue can vary depending on the angle of the sun, atmospheric conditions, and the presence of particles or pollutants.