Living in two dimensions under pressure

`How inappropriate to call this planet Earth, when clearly it is Ocean." Arthur C

`How inappropriate to call this planet Earth, when clearly it is Ocean." Arthur C. Clarke was drawing our attention to the relatively small areas of land which protrude above the vast surrounding oceans of our world. But we can add another twist.

Although we humans are not always conscious of the fact, we live our lives in yet another ocean: we are crab-like creatures, crawling along the bottom of a gaseous "sea" of air - the atmosphere. Air is a more flimsy medium than water, and we are unable to swim in our enveloping sea. While the birds that fly are the nearest thing it has to fish, humans are constrained by shape and weight to live their lives in two dimensions.

But like any fluid, the atmosphere exerts a pressure on anything immersed in it. You do not have to know what pressure is to have experienced it; its influence is obvious to anyone who dives into a swimming pool and feels the surrounding water squeezing tighter and tighter, the deeper he or she descends.

This kind of experience of pressure is as old as humankind, yet only comparatively recently did it become evident that our atmosphere behaves in the same way.

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We do not feel its pressure because, in normal circumstances, it is not registered by our senses; our remote ancestors evolved with it, and adapted to it, and consequently became unaware of its existence.

With the invention of the mercury barometer in the 17th century it became possible to measure atmospheric pressure, and shrewd observers began to notice that high pressure was associated with quiet, settled and rather pleasant weather.

Low values, conversely, often brought changeable, wet and windy conditions, and thus it has been in recent times. For weeks now the daily weather chart has been dominated by large Atlantic depressions, and as each has passed over us, or close nearby, it has brought strong winds and frequent spells of very heavy rain. The pressure has often been very low indeed.

But it can drop even lower. The lowest pressure ever experienced in Ireland was 927 millibars (or hectopascals) and was recorded in Belfast 114 years ago yesterday, on December 8th, 1886.

The lowest value in Britain was 925mbs, and occurred on January 26th, 1884, at a place called Ochtertyre, in Scotland, in the valley of the Tay river. And the all-time world record for low atmospheric pressure at sea level, 870 millibars, was measured by a "dropsonde" released from an American aircraft into the eye of Typhoon Tip as it passed near the island of Guam in the north Pacific on October 12th, 1979.