Right now, at this moment, there is a column of air above your head extending up through the troposphere, the stratosphere, the mesosphere, and beyond. That column has weight — the total mass of all the air molecules above you, pulled downward by gravity. The weight per unit area at your location is what we call atmospheric pressure. At sea level, the average atmospheric pressure is about 1,013 millibars (or 1,013 hectopascals, or 14.7 pounds per square inch, or 760 millimetres of mercury — all equivalent measures of the same physical quantity). This pressure does not feel like much because it presses equally from all directions, including from inside your body. But the slight day-to-day variations in atmospheric pressure — typically only 1-3 percent of the average value — are what drive the wind, what determine the weather, and what create the rich variety of meteorological phenomena that shape life on the planet. Understanding atmospheric pressure is fundamental to understanding weather.
How Pressure Is Defined
In physics, pressure is force per unit area. Atmospheric pressure is the force exerted by the weight of the atmosphere on a surface, divided by the area of that surface. The standard atmospheric pressure at sea level (101,325 pascals, or about 14.7 pounds per square inch) is huge in absolute terms — it would crush most objects if applied unevenly, like a vacuum chamber. But because it presses equally from all directions, including from inside the bodies of living creatures and from inside everyday objects, we do not feel it as a crushing force. The first conclusive experimental demonstration of atmospheric pressure was by Otto von Guericke in 1654, who used two large hemispheres pressed together with the air evacuated from between them — 16 horses could not pull them apart, demonstrating the enormous force the atmosphere exerts.
The Decrease with Altitude
Atmospheric pressure decreases with altitude. The reason is simple: there is less atmosphere above any given altitude than below it, so the column of air above gets thinner and lighter as you go up. The decrease is roughly exponential — pressure halves with every 5.5 kilometres or so of altitude. At the top of Mount Everest (8,849 metres), the pressure is about a third of sea-level pressure. At the typical cruise altitude of commercial aircraft (10-12 kilometres), it is about a quarter. At 30 kilometres altitude, it is about 1 percent of sea-level pressure. At 100 kilometres (the Kármán line, the boundary of space), it is less than 0.001 percent. This relationship between pressure and altitude is the basis for the altimeter — the instrument that aircraft and high-altitude athletes use to measure altitude by measuring local pressure and comparing it to known pressure-altitude relationships.
Highs and Lows
Day-to-day variations in atmospheric pressure are what produce the patterns of weather we experience. "High pressure" systems (anticyclones) are regions where pressure is higher than the surrounding areas. "Low pressure" systems (cyclones) are regions where pressure is lower. The pressure differences are not large in absolute terms — perhaps 30-50 millibars between the centre of an exceptionally strong low and a neighbouring high — but they drive the winds and weather. The air moves from high pressure regions toward low pressure regions, with the motion modified by the Coriolis effect to flow around rather than directly toward the lows. The result is the characteristic cyclonic and anticyclonic circulation patterns that dominate mid-latitude weather.
What Highs and Lows Bring
The weather typical of highs and lows differs sharply. High pressure systems are typically associated with descending air, clear skies, fair weather, and stable atmospheric conditions. The air subsides slowly from above, warming as it sinks and inhibiting cloud formation. Continental anticyclones in winter can produce extended periods of cold, clear weather. Subtropical highs (like the Bermuda High and the Pacific High) produce the warm sunny weather that makes those latitudes desirable destinations. Low pressure systems are typically associated with rising air, cloudiness, and precipitation. The air converges and rises, cooling as it ascends and producing condensation. Mid-latitude cyclones (the dynamic low pressure systems that drive most mid-latitude weather) bring the active weather we associate with storms, including frontal precipitation, thunderstorms in summer, and snow in winter.
The Barometer
The instrument that measures atmospheric pressure is the barometer. The first mercury barometer was built by Evangelista Torricelli in 1643. A glass tube filled with mercury and inverted into a reservoir of mercury establishes a vacuum at the top of the tube; the height of the mercury column is held up by the atmospheric pressure pushing on the reservoir. At sea level, the column is about 760 millimetres — hence the unit "millimetres of mercury" or "torr" still used today. Aneroid barometers (developed later) use a sealed metal box whose expansion or contraction with pressure changes can be measured mechanically. Modern electronic barometers use various physical effects to measure pressure with high precision and digital output. Barometers are essential tools for weather forecasting, aviation, and altitude measurement.
The Tendency: Rising or Falling
The absolute value of atmospheric pressure at any moment is less informative than the trend over time. A "falling barometer" indicates an approaching low pressure system and typically deteriorating weather. A "rising barometer" indicates an approaching high pressure system and typically improving weather. The rate of pressure change is also informative: rapid falls are associated with intense storms; slow steady changes are associated with broader synoptic-scale weather. Sailors and farmers have used barometric trend forecasting for centuries — the phrase "the glass is falling" (referring to the mercury column) is an old phrase warning of approaching bad weather. Modern weather forecasting still uses pressure tendency as a key input, though it is now supplemented by satellite data, weather radar, and computer modelling.
Pressure Gradient and Wind
The "pressure gradient" — how rapidly pressure changes over distance — drives wind. The larger the pressure gradient between two locations, the stronger the wind between them. In synoptic weather charts, the lines of constant pressure (isobars) are drawn — where the isobars are closely packed, the pressure gradient is strong and the winds are typically strong. Where the isobars are widely spaced, the pressure gradient is weak and the winds are light. The relationship between pressure and wind is direct and quantitative; meteorologists can estimate wind speed from the pressure gradient on a weather chart. The Coriolis effect modifies the direction of wind — instead of flowing directly from high to low pressure, the wind flows along the isobars (with low pressure to its left in the Northern Hemisphere and to its right in the Southern Hemisphere) in the geostrophic approximation that applies to upper-level flow.
Cyclones and Hurricanes
The strongest pressure gradients on Earth are in tropical cyclones (hurricanes, typhoons). The strongest tropical cyclones have central pressures below 900 millibars — about 100 millibars below normal sea-level pressure. The pressure differential between the eye of a strong hurricane and the surrounding undisturbed atmosphere can be 100-150 millibars, producing the extreme winds (up to 350+ km/h in the most intense storms) and the characteristic spiral cloud patterns. The lowest sea-level pressure ever recorded was 870 millibars in Typhoon Tip in 1979 — about 14 percent below average. The most intense Atlantic hurricane on record was Hurricane Wilma in 2005, with a central pressure of 882 millibars. The relationship between low central pressure and storm intensity is well-established; the central pressure is a standard metric for ranking tropical cyclone strength.
Diurnal and Seasonal Variations
Beyond the synoptic-scale variations in pressure (typically 10-30 millibars between highs and lows), there are smaller-amplitude daily and seasonal variations. A daily pressure cycle — the "atmospheric tide" — produces small (1-2 millibar) variations on a roughly 12-hour cycle, driven by solar heating of the atmosphere. Seasonal variations are larger; continental interiors typically have higher pressure in winter (when the cold air is denser) and lower pressure in summer. The Siberian High and the Asiatic Monsoon are the most extreme expressions — the Siberian High in winter can have pressures over 1,050 millibars (high above the normal sea-level value), while the South Asian summer low can drop to around 990 millibars. These large-scale seasonal pressure patterns drive the monsoon circulations that bring rain to much of South and East Asia.
The Practical Significance
Atmospheric pressure has direct practical implications for human activities. Aviation depends on pressure for altimetry — pilots set their altimeters to local pressure values to ensure accurate altitude readings, and pressure varies by 10-30 millibars between different weather systems. Boiling temperatures depend on pressure — water boils at lower temperatures at high altitudes, requiring longer cooking times. Decompression effects on the human body — both at high altitudes (mountain sickness) and during scuba diving (the bends) — depend on pressure changes. Even the apparent "popping" of ears during elevation changes is a response to pressure differences. The pressure of the air around us is one of the most fundamental physical quantities of our environment, and the small variations in that pressure produce all the variety of weather we experience day to day.