wiggy
Let’s get something clear. It doesn’t get warmer when you get above the trop, it just doesn’t get colder (I.e. lapse rate is 0 in the stratosphere). So whatever the temperature at the tropopause, then that’s the temp for the majority of the stratosphere, it does get warmer higher up, but most planes won’t get that high.
As you’re all well aware aircraft performance is calculated with a tropopause at ISA 36,000ft at -56 Celsius (15-[1.98 x 36]). And fundamentally for performance it’s delta ISA that’s important. The issue is that in colder climes where the tropopause can drop to below 30,000ft. and lapse rates are less, the tropopause “starts sooner” and the temperature stops dropping sooner e.g. at -42 degrees. So once you reach the low tropopause your delta ISA keeps increasing until 36,000ft.
In warmer climes where the tropopause can be 55,000ft.+ the temperature keeps dropping as you climb and once you reach 36,000ft. as the trop is still higher your delta ISA will keep decreasing and performance will improve the higher you go. For most modern airliners performance begins to be significantly impacted once delta ISA gets above 10 and you’ll start seeing a signinificant drop in Max Alt.
Sorry if this is obvious information to you guys.