The speed of light in air at 0 degrees Celsius is very close to the speed of light in a vacuum, which is approximately 299,792,458 meters per second (m/s).

Here's why:

* The speed of light is fundamentally constant: The speed of light in a vacuum (often denoted as "c") is a fundamental physical constant. It's the fastest speed anything can travel in the universe.

* Air slows light down slightly: Light travels slightly slower in air than in a vacuum because it interacts with the air molecules. However, this slowing effect is very small.

* Temperature and pressure influence: The exact speed of light in air does vary slightly with temperature and pressure. At 0 degrees Celsius and standard atmospheric pressure, the speed of light in air is only about 0.03% slower than in a vacuum.

For most practical purposes, we can consider the speed of light in air to be essentially the same as the speed of light in a vacuum.