Atmospheric Pressure And Barometer

Air feels weightless, yet the atmosphere presses on every square centimetre of your body with a force of about 10 newtons — roughly the weight of a litre of water on each fingertip-sized patch. This atmospheric pressure is simply the weight of the kilometres of air stacked above us.

It matters more than you might think: it lets us breathe, pushes water up straws, and its ups and downs steer the weather. Measuring it accurately — with a barometer — was one of the great experiments of early science.

Key Concepts

• Atmospheric pressure: the pressure exerted by Earth's atmosphere, about 1.013 × 10⁵ Pa (101.3 kPa) at sea level.
• The atmosphere's weight: that pressure equals roughly 10,300 kg pressing on every square metre — about ten tonnes.
• Torricelli's experiment (1643): filling a tube with mercury and inverting it into a dish left a 76 cm column held up by nothing but air pressure — the first barometer.
• Mercury barometer: the height of the mercury column directly measures atmospheric pressure.
• Aneroid barometer: a sealed, partially evacuated metal box that flexes as air pressure changes; what most home barometers use.

Formulas

The mercury column balances the atmosphere: P = ρgh, where ρ = 13,600 kg/m³ is mercury's density, g = 9.8 m/s², and h = 0.76 m. Multiplying out: 13,600 × 9.8 × 0.76 = 101,293 Pa ≈ 1.013 × 10⁵ Pa. This defines the unit "76 cm of mercury" (760 mmHg). The formula assumes the space above the mercury is a vacuum and the mercury is at standard temperature.

Worked Example

Why use mercury instead of water in a barometer? Apply P = ρgh to water: h = P ÷ (ρg) = 101,300 ÷ (1000 × 9.8) = 101,300 ÷ 9800 ≈ 10.34 m. A water barometer would need a glass tube more than ten metres tall — taller than a three-storey building. Mercury is 13.6 times denser than water, so its column is 13.6 times shorter: 10.34 ÷ 13.6 = 0.76 m, a convenient 76 cm. That single calculation explains the classic barometer's design: extreme density makes the instrument desk-sized instead of building-sized.

Common Misconceptions

• "We don't feel atmospheric pressure, so it must be tiny." It is enormous — about ten tonnes per square metre. We don't feel it because air pushes equally from inside our bodies and lungs, balancing it out.
• "A straw works by sucking." Sucking only removes air; it is the atmosphere pushing down on the drink's surface that drives liquid up the straw.
• "A barometer measures weather directly." It measures pressure only. Falling pressure often signals approaching storms and rising pressure fair weather, but the link is an inference, not a measurement.

Real-World Applications

• Weather forecasting: maps of high- and low-pressure systems are the backbone of every weather report.
• Aircraft altimeters: planes measure altitude from the drop in air pressure as they climb.
• Vacuum packaging: removing air lets atmospheric pressure press packaging tightly onto food.
• Siphons: transferring liquid over a hump works because atmospheric pressure pushes liquid up the short arm.
• Tyre pressure gauges: calibrated against atmospheric pressure to report the useful gauge pressure.

The air above you weighs about ten tonnes per square metre, and a 76 cm column of mercury proves it. From Torricelli's tube to the altimeter in a jet, the barometer turns that invisible weight into a number we can read — and a forecast we can use.