The air at sea level is compressed by the atmospheric air column above that level. At higher altitudes the air column above decreases. So the air will expand and its density decreases. Also at higher altitudes gravity is less because of which air molecules can spread freely. Hence density of air decreases with increasing altitude. So the density of air is less at a height of about 6 km from the ground than the density of air at sea level.

We know that hydrostatic pressure is directly proportional to density of the fluid and the height of the liquid column above that level.

P = ρgh

As both density of air and height of the air column 6 km above sea level are less compared to that at the sea level , Atmospheric pressure decreases to nearly half of its value at 6 lm above the sea level.