The speed of light when traveling through air is roughly 670,398,880 miles per hour. This number can be determined by dividing the speed of light in a vacuum by the index of refraction of air.
The speed of light in a vacuum is equal to 670,600,000 miles per hour. The fastest medium in which light can travel is through no matter, which is a vacuum. Any matter slows light down, but the degree to which is slows light is determined by the medium itself. The index of refraction of air is 1.0003, which is substantially lower than solids or liquids.