The level in the atmosphere at which a particle moving rapidly upward will have a probability of 1/e of colliding with another particle on its way out of the atmosphere. It is also the level at which the horizontal mean free path of an atmospheric particle equals the scale height of the atmosphere. The critical level of escape is the base of the exosphere. The height of the critical level, which can vary from 500 to 1000 km, is strongly dependent on the temperature of the upper thermosphere, and varies diurnally and with the level of solar activity. See cone of escape, fringe region.