Vertical Variation of Pressure

In the lower atmosphere, pressure does not stay constant as you go up — it falls off quickly with height. This is the single most important idea in this section: the higher you climb, the less air is pressing down on you, so the pressure drops. The drop is steep near the ground and becomes gentler as you go higher, because air is compressible and most of the atmosphere's mass sits in the lowest layers.

The textbook gives a handy rule of thumb: pressure decreases by about 1 mb for every 10 m increase in elevation. But it is careful to point out that this rate is not uniform — it does not always decrease at the same rate. The actual rate depends on conditions like temperature and density of the air column.

To show how pressure and temperature change with height, the book presents a standard atmosphere table. At sea level, pressure is 1,013.25 mb and temperature is 15.2°C. At 1 km, pressure falls to 898.76 mb and temperature to 8.7°C. By 5 km, pressure is 540.48 mb and temperature has dropped to –17.3°C. At 10 km, pressure is only 265.00 mb and temperature is –49.7°C. Notice how the pressure drop between sea level and 1 km is about 115 mb, but between 5 km and 10 km it is far smaller — this illustrates the rapid decrease near the surface and the slower decrease aloft.

Now comes a subtle but crucial point about forces. The vertical pressure gradient force — the force pushing air upward because pressure is higher below and lower above — is actually much larger than the horizontal pressure gradient force. If that were all that acted, we would feel strong upward winds all the time. But we do not. Why? Because this upward force is almost exactly balanced by the downward pull of gravity. The two forces are nearly equal and opposite, so they cancel each other out. The result is that the atmosphere stays roughly in hydrostatic balance, and we do not experience strong upward winds in everyday conditions. …