Decrease of pressure and density with altitude

The rate at which the pressure decreases is much greater near the earth’s surface than at altitude. This is easily seen by reference to Fig. 2.2 (overleaf); between sea-level and 10 000 ft (3480 m) the pressure has been reduced from 1013 mb to 697 mb, a drop of 316 mb; whereas for the corresponding increase of 10 000 ft between 20 000 ft (6096 m) and 30 000 ft (9144 m), the decrease of pressure is from 466 mb to 301 mb, a drop of only 165 mb; and between 70 000 ft (21 336 m) and 80 000 ft (24 384 m) the drop is only 17 mb.

This is because air is compressible; the air near the earth’s surface is com­pressed by the air above it, and as we go higher the pressure becomes less, the air becomes less dense, so that if we could see a cross-section of the atmos­phere it would not appear homogeneous – i. e. of uniform density – but it would become thinner from the earth’s surface upwards, the final change from atmosphere to space being so gradual as to be indistinguishable. In this respect air differs from liquids such as water; in liquids there is a definite dividing line or surface at the top; and beneath the surface of a liquid the pressure increases in direct proportion to the depth because the liquid, being practically incom­pressible, remains of the same density at all depths.