Below the tropopause temperature decreases with altitude.

                                                   CAS

                                         EAS               TAS

                                                                    MN

                           Altitude

 

                                                                                                At constant CAS

                                                                                                As altitude increases

                                                            Speed                           TAS & MN increase

Or:

CAS is a measure of dynamic pressure = ˝ ρV2

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

LSS is decreasing as temperature decreases.

MN = TAS/LSS = Increasing/Decreasing = Increasing