critical level of escape

critical level of escape

[′krid·ə·kəl ′lev·əl əv ə′skāp]
(geophysics)
That level, in the atmosphere, at which a particle moving rapidly upward will have a probability of 1/ e (e is base of natural logarithm) of colliding with another particle on its way out of the atmosphere.
The level at which the horizontal mean free path of an atmospheric particle equals the scale height of the atmosphere.