Barometry is the science of measuring atmospheric pressure, the weight of the air pressing down on the Earth’s surface at any given point. That weight changes constantly based on weather patterns, altitude, and temperature, making barometry essential to weather forecasting, aviation, and medicine. At sea level, the atmosphere exerts a standard pressure of 29.92 inches of mercury, or 1013.25 hectopascals.
How Atmospheric Pressure Works
The air surrounding Earth has mass, and gravity pulls that mass downward. The result is a column of air stretching from the ground to the edge of the atmosphere, pressing on everything beneath it. Barometric pressure is simply the accumulated weight of that column measured at a specific location and time.
Several factors shift this weight around. Warm air is less dense than cold air, so it exerts less pressure. Moist air is actually lighter than dry air (water vapor molecules weigh less than nitrogen and oxygen molecules), which also lowers pressure readings. And altitude matters enormously: the higher you go, the less air sits above you, so pressure drops. A mountaintop at 1,500 meters experiences roughly 12% less atmospheric pressure than a valley below.
The First Barometer
In 1644, the Italian scientist Evangelista Torricelli described the first mercury barometer in a letter containing one of the most famous lines in the history of physics: “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.” The actual experiment was carried out by his colleague Vincenzo Viviani, who filled a glass tube roughly 110 to 120 centimeters long with mercury, placed a finger over the open end, and inverted it into a basin of mercury. The mercury column dropped partway but didn’t empty completely, held up by the weight of the atmosphere pushing down on the basin.
A few years later, in 1648, Blaise Pascal put the concept to a dramatic test. He asked his brother-in-law, Florin Perier, to carry a barometer up the Puy-de-Dôme, a volcanic peak in central France. At the summit, roughly 500 fathoms above the starting point, the mercury column read only about 625 millimeters compared to the roughly 710 millimeters at ground level. That 12% drop proved conclusively that atmospheric pressure decreases with altitude, confirming that air has finite, measurable weight.
Units of Measurement
Barometric pressure gets reported in different units depending on the field. Weather services in most countries use hectopascals (hPa), which are identical to millibars (mb). In the United States, weather reports often use inches of mercury (inHg), a direct holdover from Torricelli’s original design. Medical and laboratory settings sometimes use millimeters of mercury (mmHg), also called torr.
The conversions are straightforward. One inch of mercury equals 33.86 hectopascals and 25.4 millimeters of mercury. Standard sea-level pressure is 1013.25 hPa, 29.92 inHg, or 760 mmHg. The World Meteorological Organization sets the benchmark for precision: its ideal measurement uncertainty is just 0.5 hPa, with readings taken at least every hour for global climate monitoring.
Barometry in Weather Forecasting
Barometric readings became one of the earliest forecasting tools almost as soon as barometers were invented. The core principle is simple: falling pressure signals approaching stormy weather, and rising pressure signals clearing skies.
The reasons behind those shifts involve several overlapping processes. Pressure drops when a low-pressure trough approaches, when warm or moist air moves into an area, or when air rises near a weather front. All of these conditions reduce the weight of the air column overhead and tend to produce clouds, rain, or storms. Conversely, sinking cool, dry air adds mass to the column and raises pressure, which generally brings fair weather. Watching how fast pressure changes matters as much as the direction. A slow, steady decline over a day or two suggests a broad weather system moving in gradually, while a rapid plunge can signal a more intense storm.
How Pilots Use Barometric Pressure
Aviation depends heavily on barometry because altimeters in aircraft work by measuring atmospheric pressure and converting it to an altitude reading. Below 18,000 feet, pilots set their altimeters to the current local barometric pressure reported by stations along their route, within 100 nautical miles of the aircraft. This keeps altitude readings accurate relative to the terrain below.
At or above 18,000 feet, all pilots switch to a standardized setting of 29.92 inches of mercury, regardless of actual conditions on the ground. This ensures that every aircraft at high altitude is using the same reference point, preventing dangerous discrepancies between planes flying near each other. The margin for error is thin: a one-inch error in the altimeter setting translates to roughly 1,000 feet of altitude. If a pilot flies from a high-pressure area into a low-pressure area without updating the setting, the aircraft will actually be closer to the ground than the instruments show.
Effects of Pressure Changes on the Body
Between 25% and 40% of the general population reports physical symptoms tied to weather changes, and barometric pressure shifts are a leading suspect. The most common complaints include joint pain, headaches, back pain, and fatigue. Research from Japan has linked low barometric pressure to increased migraine frequency, and a Norwegian study found that low pressure raised both pain and stress levels in people with fibromyalgia. Multiple studies have also found associations between pressure changes and osteoarthritis pain, though the exact biological mechanism remains unclear.
The effects go beyond aches and pains. Rapid pressure changes cause barotrauma, physical damage to tissues when air trapped in enclosed body spaces (like the sinuses or middle ear) expands or contracts faster than the body can equalize. This is why your ears pop during a flight’s descent. At more extreme levels, divers ascending too quickly can develop decompression sickness as dissolved gases form bubbles in the blood and tissues.
Barometry in Medical Treatment
Hyperbaric oxygen therapy flips barometric pressure into a treatment tool. Patients breathe pure oxygen inside a sealed chamber pressurized to two to three times normal atmospheric pressure. At that pressure, oxygen dissolves into the blood at much higher concentrations than normal breathing allows, following a basic physics principle: the amount of gas that dissolves in a liquid increases in direct proportion to the pressure applied.
This supercharged oxygen delivery serves several purposes. It corrects oxygen-starved tissues, speeds wound healing by promoting new blood vessel growth and faster skin repair, and shrinks gas bubbles in the bloodstream. The therapy gained its first widespread clinical use treating divers with decompression sickness during World War II. Today it is also used for carbon monoxide poisoning, certain non-healing wounds like diabetic foot ulcers, and tissue damage from radiation therapy. The underlying principle in every case is the same: controlling barometric pressure to change how gases behave in the body.