The areas of low pressure are much larger in extent than those of high pressure and, as a rule, they are not so well defined. The area of lowest pressure is in the south polar region; it is inclosed by the sixtieth parallel; its mean pressure is 29.40 inches (996 mb). A low pressure area in the North Atlantic lies east of Greenland; a similar low pressure area in the North Pacific covers Bering Sea.
The zone of ascending air currents in equatorial regions is a region of low pressure. Its mean, summer and winter, does not vary much from 29.80 inches (1009 mb). Indeed, the changes in pressure from month to month throughout tropical regions are very slight. The ascending currents in equatorial regions and the descending currents near the tropics are assumed to exist. Their existence, established circumstantially rather than positively, explains satisfactorily the position of the constant highs and lows.
The great differences between summer temperature and winter temperature explain the apparent shifting of each summer high from a position over the ocean in summer to one over the nearby continent in winter. Cold air is heavier than warm air; and, in the latitude of the constant highs, the temperature of the air over the land in winter is much lower than over the ocean; in the summer, on the other hand, the temperature is lower over the ocean. In each case the high forms in the region of lower temperature.
Mean Pressure over the Earth.—It is customary to reduce all pressure observations used for comparison to a sea level basis and to a temperature of 32° F (0° C). The maps, pages 40 and 42, show the marked variations in pressure that are seasonal. From these pressures the mean pressure over the earth has been calculated by meteorologists to be between 29.90 and 29.85 inches. W. M. Davis has calculated the mean pressure over the northern and the southern hemispheres, for the summer and the winter months, deducing the following values: 29.99 inches (1016 mb) for January and 29.87 inches (1012 mb) for July in the northern hemisphere; 2991 inches (1013 mb) in July (midwinter) and 29.79 inches (1009 mb) in January (midsummer) in the southern hemisphere.
The determination of mean pressure over each of the two hemispheres is more important than that of the mean pressure