The mean free path is the average distance a particle travels between collisions. It turns out that this is given by:
l = 1 / [2pd2 N/V]
where d is the diameter of a molecule.
The table below shows mean free path values for air molecules at different altitudes.
| Altitude | Mean Free Path |
|---|---|
| Sea Level | 0.1 mm |
| 100 km | 16 cm |
| 300 km | 20 km |