Science Puzzle
The Unseen Weight
People often say that air is weightless. Yet a barometer measures atmospheric pressure, and a vacuum pump can lift water up a pipe ten metres high just by removing the air above it.
If air truly had no weight, how could it press down on anything? What is atmospheric pressure actually measuring?
The Answer
Air is not weightless. The atmosphere is a layer of gas roughly 100 kilometres deep, and all that gas has mass, and therefore weight. At sea level, the column of air above every single square centimetre of the Earth's surface weighs about 1 kilogram.
Atmospheric pressure is simply the weight of that air column pressing down per unit area. At sea level this is about 101,325 newtons per square metre, or roughly 10 tonnes pressing on every square metre of your body, balanced from all sides so you do not feel it as a crushing force.
A barometer works by measuring how high a column of liquid the air pressure can support. Standard atmospheric pressure can hold up a column of mercury 760 mm tall, or a column of water 10.3 metres tall. When atmospheric pressure changes, the height of the liquid column changes with it.
This is why altitude matters: the higher you go, the less air remains above you, so the weight of the air column above you is smaller, and the pressure is lower. At the top of Everest, atmospheric pressure is about a third of sea-level pressure.
The principle: Atmospheric pressure. Air has mass and therefore weight. Atmospheric pressure is the weight of the air column above a unit area. At sea level this is equivalent to about one kilogram per square centimetre.