Applying rule of thumb: For every 1000 m, temperature drops by 2 °C. If the ground temperature is room temperature at 20 °C, then:
At 6694 m, temperature = 20 - (6694/1000*2) = 6.6°C
Now,
H= (30t+8000)*ln (C/P), where H = Altitude, t = temperature, C = ground atmospheric pressure, say 101325 Pa, P is the pressure at the altitude.
Therefore,
6694 =(30*6.6+8000) ln (C/P) => 0.8165 = ln (1013255/P) => e^0.8165 = 101325/P => 2.2627 = 101325/P => P = 101325/2.2627 = 44781.39 Pa
