Compressibility and viscosity of air always play a role but are lesser factors at low altitude (high density). At high altitudes however (30kft+), compressibility of air becomes a dominant factor. Under low pressure conditions, shock waves form in the turbulent flow, i.e. in the portion of the wing where the flow stops being laminar. This leads to a reduction of CLmax, onset of buffet at higher IAS, and less AoA wiggle room to maintain unstalled lift conditions.
Last edited by physicus; 11th June 2017 at 00:30.