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.