Mean underStorm Level
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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 30 ft, with terrific windshear often exceeding 50 knots. The level is taken as a datum from which all altitude is derived: heights above the boundary are expressed as Above Mean underStorm Level (eg. 1000 ft AMSL); while below they are termed as Negative altitudes (eg. Neg 500 ft). Where notation is not given, positive altitude is assumed.
