This is called atmospheric pressure . Any pressure above atmospheric pressure is referred to as gauge pressure . Pressures below are referred to as vacuum. 29.92 inches high. This concept can best be understood by comparing it to a teeter-totter.
Atmospheric pressure decreases at higher elevations.
Going above sea level, to the summit of Mt. Whitney, for example, eliminates some of the 600 miles of atmosphere and, consequently, some of the pressure. Atmospheric pressure, therefore, governs the boiling point of water. The lower the atmospheric pressure is, the lower the boiling point of water. This principle is illustrated in the chart below. Transport the system to a higher elevation where the ambient temperature is sufficient to boil water at the existing psi.