The mixture of gases that surrounds and is retained by the gravity of a celestial body such as the Earth.The atmosphere is denser near the Earths surface, and becomes gradually thinner until it fades away into space. Particularly near the Earths surface, the physical matter in the atmosphere attenuates electromagnetic signals due to absorption, refraction and other phenomena. At the outer limits of the atmosphere are four layers of the ionosphere, which is useful for skywave radio propagation. See also attenuation, ionosphere, propagation, refraction, and skywave.