Realistically, in a standard atmosphere contrails begin to for above about 32,000 ft and this will vary depending on SAT and humidity; however, if you see contrails at 20,000 it’s almost certainly not realistic.
Realistically, in a standard atmosphere contrails begin to for above about 32,000 ft and this will vary depending on SAT and humidity; however, if you see contrails at 20,000 it’s almost certainly not realistic.