ABSTRACT
The atmosphere extends upwards in excess of 50 miles (80 km), becoming less dense with altitude. Just as the pointer on a set of kitchen scales will show more and more weight as we add coins to a stack in the scale pan, so atmospheric pressure increases if there is a greater height and therefore weight of air above us (Figure 3.1). Conversely if you were to measure the atmospheric pressure at the foot of a tall building and on the roof, the pressure (that is, the weight of air) on the roof would be less than at the base. So it is that atmospheric pressure falls with height. The density
of air will vary, being less dense at altitude, so the fall of pressure with altitude is rapid near sea level and less at high altitude. For example a climb from sea level to 1500 ft (455 m) results in a fall in pressure of 44 mmHg whereas an ascent from 46 000 ft to 47 500 ft (also a 1500 ft/455 m difference) results in only an 8 mmHg drop. At 18 000 ft (5486 m) half the weight of air is below and half above, the pressure is half that of sea level, at 33 500 ft (10 210 m) it is one quarter and at 53 000 ft (16 154 m) one tenth.
