Final answer:
Atmospheric pressure decreases with altitude due to the thinning of the air. It is half of the sea level pressure at an elevation of roughly 5.5 kilometers.
Step-by-step explanation:
The student's question pertains to the change in atmospheric pressure with altitude. At sea level, atmospheric pressure is defined as 1 atmosphere (atm) or approximately 101 kPa. Due to gravity, the weight of the air above a surface creates this pressure. As one moves to higher altitudes, the density of the air and consequently the pressure decreases.
The elevation at which the barometric pressure is half of that at sea level can be determined using the barometric formula, which accounts for the decrease in pressure with height. According to this formula and empirical data, the barometric pressure drops to half its sea level value at approximately 5.5 km in elevation.