If we assume normal conditions; temperature increasing as we descend in the troposphere.

                                                    CAS

                                         EAS              TAS

                                                                   MN

                           Altitude

 

                                                                                                At constant CAS

                                                                                                As altitude decreases MN decreases

                                                            Speed                                      

Or:

CAS is a measure of dynamic pressure = ˝ MN decreases ρ V2

Where ρ = air density (increasing in descent) and V = TAS (decreasing to keep CAS constant)

LSS is increasing as temperature increases.

MN = TAS/LSS = Decreasing/Increasing = Decreasing