Under pressure

Abstract

How heavy is the weight of air above your head? At first, this seems a ridiculous question. However, gases do actually have a weight and since the atmosphere consists of a mixture of gases (mainly nitrogen and oxygen), air has a weight. This weight is also described as pressure. In fact, the atmosphere is exerting about 5 tonnes of pressure on your head. A cubic metre of air typically weighs about 1.2 kilograms (kg) at sea-level. Thus, the weight of air in a car or a large tea chest is about 1 kg, which is approximately the weight of a large bag of sugar. Fortunately, we do not experience this weight or pressure because our internal pressure acts as a counter-balance. However, we do feel rapid changes in pressure. For instance, if you dive into a swimming pool, the sudden increase in pressure is expressed particularly by compression on the head, especially the eardrums. This is because water is about 1000 times denser than air. Furthermore, we also sense changes in pressure during air travel. Aircraft are normally pressurized to the equivalent pressure at about 2000 metres. Therefore, on ascent and descent we can experience changes in pressure, this is usually noticeable by a slight discomfort in your eardrums, with the drums tending to ‘pop’. Large animals have difficulty in adjusting rapidly to pressure changes. For instance, there have been reports of cows exploding when hit by tornadoes (extremely violent whirlwinds). The inner core or ‘vortex’ of tornadoes is the lowest pressure system on Earth. When these extremely low-pressure systems hit cattle, they cannot rapidly adjust their internal pressure and they can therefore explode. For the same reason, people in tornado-vulnerable areas (such as ‘tornado alley’ in the central USA) are advised to open windows in advance of possible tornadoes, as this diminishes the chances of the house exploding when hit by a tornado (i.e. very low pressure outside and relatively high pressure inside). Evangelista Torricelli was one of the first scientists to experiment with pressure. In 1644, Torricelli inverted a tube of mercury into a vessel containing mercury (a liquid metal). In the upper part of the tube a vacuum was created when the mercury fell. This meant Torricelli was able to show that air pressure was capable of supporting a column of mercury. When the air pressure rose, the height of mercury increased. When the air pressure decreased, the height of the mercury decreased. To illustrate air pressure to yourself, simply place a drinking glass under the water level in a bowl. Raise the inverted full glass above the surface of the water, keeping the rim below the water level. Air pressure keeps the water in the glass, even though it is above water level.