Air Pressure

Air pressure is the weight of the atmosphere pressing down on whatever is below it. At sea level there is approximately 1 kilogram of air pressing on every square centimetre of your body: a force equivalent to a small bag of sugar resting on every fingernail-sized patch of skin. You do not feel it because the same pressure exists inside your body, pushing back out. Air pressure changes with altitude and with weather, and the changes drive most of what happens in the atmosphere.

  • Pressure at sea level101.3 kPaOr 1013 millibars / 1 atmosphere
  • Pressure on top of EverestApprox. 33 kPaAbout a third of sea level
  • Air weight per square cmApprox. 1 kgYou do not feel it because of internal pressure
  • High pressure meansSettled weatherClear skies, light winds
  • Low pressure meansStormy weatherClouds, rain, strong winds
  • Measured withBarometerInvented in 1643 by Torricelli

What causes air pressure

Air may seem light, but the entire atmosphere weighs around 5,000 trillion tonnes. Gravity pulls all that air down towards Earth, and the closer you are to the ground, the more air sits above you pressing down. So air pressure is at its highest at sea level and gets lower as you go up.

The standard air pressure at sea level is approximately 101.3 kilopascals (kPa), or 1,013 millibars (mb), or 1 atmosphere, or 760 mm of mercury depending which unit you use. By the time you reach the top of Mount Everest (8,849 m), the pressure has dropped to about 33 kPa: only a third of sea-level pressure. That is why climbers need extra oxygen high on big mountains, even though the percentage of oxygen in the air is still 21%.

How air pressure causes weather

The atmosphere is full of regions of high pressure and low pressure, constantly shifting around. The weather you experience depends largely on which one you are in.

  • High pressure (also called an anticyclone): air is sinking. As it sinks it warms up and any clouds evaporate. Result: clear skies, light winds, dry weather. Often warm in summer, cold and frosty in winter.
  • Low pressure (also called a depression): air is rising. As it rises it cools, water vapour condenses, and clouds form. Result: clouds, rain, sometimes storms. Most British weather comes from Atlantic low-pressure systems.

Wind flows from high pressure to low pressure, just like water flows downhill. The bigger the pressure difference between two places, the stronger the wind.

How we measure air pressure

The tool for measuring air pressure is the barometer. The first one was invented by Italian scientist Evangelista Torricelli in 1643. He filled a glass tube with mercury, sealed the bottom and put it upside down in a dish of mercury. The mercury inside the tube dropped to a particular height (about 76 cm in standard conditions), balanced by air pressure pushing down on the mercury in the dish. By measuring the height of mercury in the tube, you could measure the air pressure.

Modern barometers usually use a small flexible metal box (called an aneroid) that squashes or expands as air pressure changes. A needle attached to the box shows the pressure on a dial.

Fact Your ears popping on a plane or when going up a mountain happens because air pressure outside your ears changes faster than the air trapped behind your eardrums. The eardrum bulges in or out under the pressure difference, until a tiny tube (the Eustachian tube) opens and lets the air pressure equalise. Yawning, swallowing or chewing gum opens the tube and makes ears pop on demand.

Air pressure and altitude

As you climb higher, the air gets thinner. Some rough numbers:

  • Sea level: 100% pressure.
  • 1,000 m (top of Snowdon): 88% pressure.
  • 5,000 m (Everest base camp): 53% pressure.
  • 8,849 m (Everest summit): 33% pressure.
  • 11,000 m (cruising altitude of a passenger jet): 22% pressure.
  • 30,000 m (top of stratosphere): 1% pressure.

This is why aircraft cabins are pressurised: at cruising altitude, the air outside is so thin that humans cannot breathe it without help.

Did you know? Air pressure changes can affect how things taste. In the low pressure of an aircraft cabin (kept at the equivalent of about 2,000 m altitude), your sense of taste and smell are both reduced by approx. 30%. That is partly why airline food is famously bland: it has to be flavoured strongly to taste of anything at all in flight, even though it tastes too strong on the ground.
Deeper dive: how altitude sickness works

People who climb high mountains too quickly often get altitude sickness, also called acute mountain sickness (AMS). Symptoms include headache, nausea, dizziness, shortness of breath and trouble sleeping. They usually start above about 2,500 m and get steadily worse the higher you go.

The cause is low oxygen. The percentage of oxygen in the air stays at 21% at any altitude, but the lower air pressure means each breath delivers much less oxygen to your lungs. At 5,000 m, every breath has only about half the oxygen you would get at sea level. Your body has to work harder to deliver enough oxygen to your tissues, especially your brain.

The body adapts over days and weeks (a process called acclimatisation) by producing extra red blood cells, increasing breathing rate and several other changes. Climbers tackling Everest typically spend weeks at intermediate camps to acclimatise gradually.

Severe altitude sickness can lead to deadly conditions: pulmonary oedema (fluid in the lungs) or cerebral oedema (fluid in the brain). The only sure treatment is to descend immediately. Above about 8,000 m (the so-called death zone), the body cannot acclimatise at all, brain cells slowly die, and climbers can only survive for a few days even with bottled oxygen. This is why summits of the highest mountains are usually done in a single dash up and down.

For more, see layers of the atmosphere and what is weather.