"Hilton" wrote in message
ink.net...
How is standard temperature and pressure defined below sea level?

Same as above sea level. Take the standards at sea level, apply a standard
adjustment, viola.

How does DA increase/decrease below sea level?

Same as above sea level, except in the opposite direction. Below sea level,
pressure altitude decreases, which decreases density altitude. Of course,
temperature has the same effect regardless of altitude; high temperature
increases density altitude, low temperature decreases density altitude.

I know that the gravitation force
starts decreasing in a *linear* fashion below sea level

As far as I know, the difference in gravity has to do with the amount of
mass under you, and is negligible with respect to temperature or pressure.
Same thing would be true of high-elevation terrain.

but how that affect the temp, pressure, and DA is beyond me.

I do not believe that the change in gravitation force is significant for the
purposes of piloting an aircraft.

Basically, nothing unusual happens just because you went below sea level.
"Mean sea level" is simply a convenient reference point. Just as you could
set your reference point to what we now call 4000' MSL, and then find that
density altitude simply decreases below that according to pressure altitude,
and temperature increases or decreases density altitude in the same way it
does at any altitude...the exact same thing happens below sea level.

I haven't seen any calls to adjust density altitude to account for higher
gravitational forces at higher altitudes, nor would I expect to see any call
to do so at extremely low altitudes. Note that we also don't compensate in
our calculations for changes in gravity due to changes in our flight
altitude above ground level.

As far as us pilots are concerned, gravity is exactly the same everywhere.

Pete