Light travels through a vacuum at 186,282 miles per second; through the Earth's atmosphere, it moves at 186,227 miles per second. The speed of light, in general, is affected by the medium through which it moves.
The speed at which light travels through a medium depends greatly on the material of the medium, as different materials have a different refractive index. This index indicates how quickly light or other radiation moves through them. The Earth's atmosphere has a refractive index of 1.000293, which slows light infinitesimally more than in a vacuum.
Two independent teams of scientists claim to have slowed a pulse of light almost to a standstill by using a super-cooled gas of the element rubidium.