Mean underStorm Level

Mean underStorm Level, or MSL, is the altitude where the Understorm and the habitable zone of Azimuth's atmosphere meet. It is typically very narrow, sometimes no more than 10 metres, with terrific windshear often exceeding 100 kph. The level is taken as a datum from which all altitude is derived: heights above the boundary are expressed as positive MSL (eg. 1000 MSL is 1000 metres above the transition); while below they are termed as 'Neg' altitudes (eg. Neg 500). Where notation is not given, positive altitude is assumed.