As air pressure depends on the amount of air above, pressing down under the influence of gravity, the highest average values occur in sunken valleys like that of the Dead Sea in Israel, which is 395 m below sea-level. In terms of equivalent sea-level pressure, the highest yet recorded (1,084 hPa) occurred during a winter in Siberia. At the other extreme, a value of only 870 hPa was encountered in the middle of a tropical cyclone in the Pacific ocean (Chapter 13). More typical values are around the global average of 1,013 hPa. For instance, half the measurements at Sydney lie within 1,004-1,014 hPa.
The latitudinal variation of average pressures (Figure 1.8) is connected with the pattern of global winds discussed in Chapter 12, e.g. the steep change of pressure between 35°S-65°S is related to the strong westerly winds between those latitudes. More locally, daily or hourly changes of pressure by only a few hectopascals are important in determining the winds and weather (Chapters 13-15).
The pressure at sea-level is normally about 1,013 hPa, but only about 700 hPa at 3 km elevation, where there is less air above. That is why your ears 'pop' when you are driving up or down a big hill. The barometric pressure at the top of Mount Everest (8,848 m), and hence the air density and oxygen concentration (per unit volume) are only about a third of what they are at sea-level (Note 1.G). Other effects of low pressures at high elevations are considered in Note 1.I.
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