Aside from it being a place filler to match FL 0 it means -1000ft based on 1013. If your surface pressure is 980hPa then FL -90 is ground level.
However it's not counting as you'd normally think it where -0.1 is 0.1 less than zero. You start at -1 & and increment up, so it follows the form of the positive FLs.
Just like:
FL 90 is 9000ft higher than FL 0
FL 80 is 8000ft higher than FL 0
Then:
FL -90 is 9000ft higher than FL -100
FL -80 is 8000ft higher than FL -100
Clear as mud?
Last edited by le Pingouin; 27th Jul 2011 at 05:48.
Reason: FL 90 is 9000 not 900ft, etc :(