Quote: Rough math here...really rough. Std temp is 15C. Subtract 2C/ 1000ft. 90+15=105/ 2= 53,000ft roughly.
That's with out a calculator, METS, so forgive me if I'm way off.
Way Up there about 9-10 miles High!
By checking the Skew-T log P diagram of the San Andreas Islands you will see that the -90 level would correspond to 16.6 km in this case, 10.3 miles, around 96 mbar of air pressure. Not so rough after all