SOME 350 years ago, Evangelista Torricelli, inventor of the barometer, made a profound remark about life on planet Earth Noi viviamo sommersi nel fondo d'un pelago d'aria elementare, he wrote - "We live our lives submerged at the bottom of an ocean of elementary air".
His observation was only slightly metaphorical. The air in which we live envelops us just like the liquid ocean surrounds the countless creatures of the deep, and provides us with essential oxygen in a very similar way. Air, however, is a flimsier medium than water, so we are unable to swim in our enveloping "sea"; but we have learned to use aeroplanes to achieve much the same effect, emulating the birds who are the nearest things in our airy world to fish. We ourselves resemble the crab like creatures of the ocean, constrained by our shape and weight to live our lives in two dimensions.
Our sea, the atmosphere, differs from the liquid ocean in two important ways: it is a gaseous rather than a liquid fluid; and it does not have a sharply defined upper boundary, corresponding to the surface of the sea. The atmosphere simply becomes thinner and thinner near its outer extremities, until eventually, for all practical purposes, it is not there at all.
It took the philosophers of old a very long time to come to grips with the essential properties of air. Aristotle, for instance, was one of the first to suspect that it might have weight. To prove his suspicion, he took a leather bag and weighed it when it was "empty" of air - when it was pressed flat. Next he weighed the bag when it was full of air, and to his great disappointment he found there was no difference, and thereby concluded that the air was weightless.
We, of course, know better. We know that like any fluid, the atmosphere exerts a pressure on objects immersed in it, and that this pressure may be thought of as the weight of the tall thin column of air resting on the object. The average weight of such a column one inch square in cross section and stretching upwards to the top of the atmosphere is about 14 lb - a surprisingly large value to those accustomed to thinking of air as an insubstantial medium.
Atmospheric pressure varies in both space and time. Warm air is less dense - and therefore lighter - than cold air, so changes in the temperature of the air in different layers of the atmosphere result in those variations in atmospheric pressure at ground level which meteorologists know to have such profound effects upon our weather.