At high altitudes , atmospheric pressure decreases but we know that P = h *rho *g , why its so?

but g decrease much at high altitude

as the altitude is increased i.e. at high altitudes the value of "g" is less as compared to earth as the value of "g" is less at poles as compared to equater

and second reason is the value of rho decreases as the altitude increases as air becomes less dense resulting in less density. 

dude pressure is always measured frm top(thats y in case of liquid the pressure is zero on top and increases with depth...same applies for atmosphere also)...so h is less as u go top...

and plz dont think of sea level for pressure...sea level is just a reference...

:)