Like any fluid, our atmosphere exerts a pressure on anything immersed in it. This pressure is a direct consequence of the not inconsiderable weight of the air itself, and any part of the surface of the Earth may be thought of as supporting all the air that lies directly overhead. The share of this weight resting, for example, on a twopenny piece lying on the ground is roughly equivalent to a stone of potatoes. And since a volume of air near the bottom of the pile, and subjected to this pressure, responds by thrusting itself out equally in all directions, 14 lb or thereabouts is the pressure that each square inch of our bodies must resist.
Atmospheric pressure, however, varies in both space and time. Warm air is less dense, and therefore lighter, than cold air, so changes in temperature at various levels in the atmosphere result in changes in the pressure near the ground. The highest pressure reading obtained was the 1084 millibars recorded on December 31st, 1968, at a place called Agata in northern Siberia. The lowest value was 870 mb, obtained on October 12th, 1979, near the island of Guam in the North Pacific.
Coming closer to home, and looking at pressure measurements in Britain and Ireland, the range of values is much less impressive. The highest known value is a full 30 mbs lower than the world record at 1054 mb, this being noted in Aberdeen on January 31st, 1902. The "lowest" value on record in these parts is 925 mbs, which occurred on January 26th, 1884, at a place called Ochtertyre, also in Scotland, in the valley of the River Tay. Here in Ireland the highest and lowest values are 1052 mbs and 927 mbs, recorded respectively at Valentia Observatory, Co Kerry, on January 20th, 1905, and in Belfast on December 8th, 1886.
But of course, none of these figures is necessarily the "highest" or the "lowest" in the real sense. The atmospheric pressure is measured with barometers at thousands of places around the world, and we know the extreme values recorded at these points - but even greater departures from the norm may have occurred from time to time elsewhere, undocumented and unknown. As Thomas Gray put it:
Full many a flower is born to blush unseen,
And waste its sweetness on the desert air.
And the time factor is important, too. Organised instrumental records of pressure and temperature go back only 150 years or so; we have no way of recapturing the extreme values of these elements which may have occurred before that time.