The various gases of the air together amount to a layer around the Earth which appears from the Moon to be no thicker than the skin on an apple. But this mass of air is attracted by the Earth's gravity and so has a considerable weight. The weight of air in a room 3 mx4 mx6 m at sea-level is approximately the weight of an adult. The result is that the air exerts what is called barometric pressure. At sea-level it is approximately 10 tonnes (i.e. 10,000 kg) on each square metre of surface. This means a pressure of about 105 kg/m.s2, i.e. 100,000 Pascals. Such a pressure is more usually referred to as 100 kilopascals, or as 1,000 hectopascals (written as 1,000 hPa) in the units used by meteorologists, equal to the older units of millibars (Note 1.F). The pressure due to the weight above is exerted
Figure 1.5(a) The simultaneous changes of ozone in Octobers over Halley Station in Antarctica (Chapter 16) and of the CFC content in the global atmosphere. A 'Dobson unit' is the thickness of a layer of ozone, assuming it has all been separated out and lowered to sea-level; one unit is equivalent to a layer 0.01 mm thick. The solid line shows the change of CFC concentration.
in all directions; an airtight box that is evacuated is pressed inwards on all sides, not just the top.
It is important to grasp the differences between pressure, mass, density and weight. They are discussed in Note 1.F. Pressure is important in meteorology, because its measurement tells us how much air there is above. A fall of pressure shows a net loss from the entire air column above, indicating a wind outwards. Differences between pressures at various places indicate the direction and strength of the wind (Chapter 12). There are also seasonal variations of surface pressures, and departures from the normal annual cycle imply unusual temperatures aloft, for instance, which are relevant to weather forecasting (Chapter 15).
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