Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of Earth's atmosphere above us. At sea level it equals about 101 325 Pa (1 atm).

What Is Atmospheric Pressure?

The air around us has mass. Gravity pulls it down, creating atmospheric pressure that pushes on every surface.

Standard Atmospheric Pressure

P_0 = 101\,325 \text{ Pa} = 1 \text{ atm} \approx 1013 \text{ hPa}

At sea level. Pressure decreases with altitude.

Why Don't We Feel It?

The same pressure pushes on us from all sides equally — and from inside our bodies too. The forces balance out.

Measuring Atmospheric Pressure

The barometer was invented by Evangelista Torricelli. It measures air pressure using a column of mercury:

$P_0 = \rho_{Hg} \cdot g \cdot h = 13\,600 \times 9.8 \times 0.76 \approx 101\,300 \text{ Pa}$

A column of 760 mm of mercury = 1 atm.

Atmospheric Pressure and Altitude

Altitude	Approx. Pressure
Sea level	101 325 Pa
1 000 m	~89 500 Pa
5 000 m	~54 000 Pa
10 000 m	~26 500 Pa

Note At high altitudes water boils at a lower temperature because the atmospheric pressure is lower.

Why do ears pop in an aeroplane?

As the plane rises, atmospheric pressure outside decreases. The pressure inside the ear canal, still at a higher value, pushes the eardrum outward until equilibrium is regained — causing the "pop."

Real world: Drinking through a straw

When you suck on a straw, you reduce the air pressure inside your mouth. The atmospheric pressure outside (~101 325 Pa) then pushes down on the surface of the liquid — which is greater than the pressure inside the straw — forcing the liquid up into your mouth. You are not "pulling" the liquid up; atmospheric pressure is pushing it up.

Real world: Suction cup on a window

Press a suction cup against a glass surface and squeeze out the air. The air pressure inside the cup drops close to zero, while atmospheric pressure (~101 325 Pa) pushes the cup firmly against the glass from the outside. A 10 cm diameter cup (area ≈ 0.008 m²) can hold a force of about 800 N — enough to support a heavy mirror.

Real world: Weather forecasting with a barometer

Meteorologists use barometers to predict weather. Falling air pressure (below ~1010 hPa) usually signals approaching storms and rain, as warm, moist air rises and reduces the weight of air above. Rising pressure (above ~1020 hPa) indicates fair, settled weather — the air is sinking and becoming denser.

Tip Where do you see this in real life? Straws, suction cups, vacuum bags, weather forecasts, aircraft cabin pressurisation, and even the "pop" when you open a jar — all involve atmospheric pressure!

Practice Problems

Convert 1 atm to hPa and Pa.

Show answer

1 atm = 1013 hPa = 101 325 Pa.

Why does atmospheric pressure decrease with altitude?

Show answer

There is less air (mass) above you at higher altitudes, so the weight of the atmosphere above is smaller.