Air pressure
The push of the air's weight on every surface, about 1013 hectopascals at sea level; differences in it make air flow.
Atmospheric pressure is the weight of the air above pressing on everything below it. We do not notice it because it pushes equally on all sides and our bodies push back, but it is substantial: standard pressure at sea level is 1013.25 hectopascals, about 14.7 pounds on every square inch, enough to hold up a column of mercury 760 millimetres tall. That is how a mercury barometer measures it.
Pressure falls with height, since there is less air above, and it rises and falls with the weather: low pressure tends to bring cloud and rain, high pressure more settled conditions.
What makes things happen is a difference in pressure, because air always flows from higher pressure to lower. Suck on a straw and you lower the pressure inside it; the atmosphere then pushes the drink up. A powerful fan can push a close-fitting car along a tube like a piston, and fast air moving across the top of an opening lowers the pressure there and draws air up through it.
Medicine uses the same idea. An iron lung lowered the pressure around a patient's chest, negative pressure, so the chest expanded and air flowed in through the mouth; a modern ventilator instead pushes air in under positive pressure.
A common misconception is that suction is a pulling force. It is the higher pressure on the other side doing the pushing.
See it in action
Coming up in future stories
- Nazca's Spiral Wells Breathed Water From the Desert c. 200 BC – 600 AD · Nazca region, Peru
- The Andean Scientist Who Measured Height by Boiling Water 1801–1816 · Popayán and Bogotá, New Granada (Colombia)
- The Bang Heard 4,800 Kilometres Away 1883 · Krakatoa, Sunda Strait, Indonesia
- The Formula One Car That Sucked Itself to the Road 17 Jun 1978 · Anderstorp, Swedish Grand Prix
- The Hungarian Pen That Took Off in Argentina 1938–1945 · Budapest, Buenos Aires and RAF bases
- The Spy Who Mapped Tibet With Prayer Beads 1865–1866 · Kumaon (India) to Lhasa, Tibet