DERG
Thanks for your reply. May I ask if you are a formal student of aviation?
No, I'm just an aviation enthusiast. Still I've always been fascinated by how things work.
Upper Air
The formula for Machnumber, as you know is (grab a calculator - its sssooooo easy)
the sqrt of 273 (+/- T) x 38.94
So stab in 273 add or subtract you outside temperatur (+/-T)
sq rt the result
then times it by 38.94
So at 273 K, you get 643.3944 kts, correct?