Barometric pressure is considered “standard” at sea level when it reads 1013.25 millibars (also written as hectopascals) or 29.92 inches of mercury. Anything measurably above that baseline is high pressure, and anything below it is low pressure, though in practice meteorologists and weather-watchers tend to think of readings above roughly 1022 mb as distinctly high and readings below about 1009 mb as distinctly low. The numbers shift depending on your elevation, the season, and even the time of day, which is why the question is more layered than it first appears.
The Baseline That Defines Everything Else
Before you can call pressure “high” or “low,” you need a reference point. The international standard is 1013.25 mb (equivalently, 1013.25 hPa) at sea level, which also corresponds to 29.92 inches of mercury on older barometer scales.1National Oceanic and Atmospheric Administration. Air Pressure That number represents the average weight of the entire column of air above a point on the Earth’s surface at sea level under typical conditions. It is the yardstick against which every weather map, forecast discussion, and barometer reading is compared.
When a weather map marks an “H” somewhere, it means the pressure at the center of that system is higher than the pressure surrounding it. An “L” marks the opposite. These labels are relative: a high-pressure center reading 1018 mb could sit next to a low-pressure center reading 1005 mb, but both figures live within a fairly narrow band compared to what barometric pressure can theoretically do. In most mid-latitude locations on a given day, the surface pressure fluctuates between roughly 980 mb and 1040 mb. Readings outside that window are unusual and usually mean serious weather is nearby.
What Counts as High Pressure
In everyday weather forecasting, a surface pressure reading above about 1020 to 1025 mb is generally spoken of as high pressure. Strong high-pressure systems in winter, especially over cold continental land masses, can push well above 1040 mb. The highest reliably recorded sea-level pressure on Earth was 1083.3 hPa, measured on December 31, 1968, at Agata in Russia’s Evenhiyskiy region. A separate record for stations above 750 meters elevation stands at 1089.4 hPa, recorded on December 30, 2004, in Tosontsengel, Mongolia.2Wiley Online Library. The Tosontsengel Mongolia world record sea-level pressure extreme: spatial analysis of elevation bias in adjustment-to-sea-level pressures Both of those readings came during brutally cold Siberian-type winter conditions, which compress and densify air near the surface. You will never see numbers like that in temperate or tropical climates.
For most people checking a home weather station, a reading in the 1025 to 1035 mb range (about 30.2 to 30.6 inches of mercury) is solidly high. Skies under high pressure tend to be clear or partly cloudy, winds are lighter, and the atmosphere is relatively stable. That stability happens because air in a high-pressure system sinks, which warms it slightly and discourages cloud formation.
What Counts as Low Pressure
Low pressure is the mirror image. Readings below about 1009 to 1013 mb are generally treated as low, and strong storms can push pressure much further down. Mid-latitude cyclones commonly bring readings into the 990s or 980s. A powerful nor’easter or a deep extratropical cyclone can dip below 970 mb, and the lowest pressures ever measured at sea level on Earth occur inside intense tropical cyclones, sometimes dropping below 900 mb in the eyes of Category 5 hurricanes.
Low-pressure systems produce clouds and precipitation because air converges near the surface and is forced upward. As that air rises, it cools, and the moisture in it condenses into clouds, which often produce rain or snow.3UCAR Center for Science Education. The Highs and Lows of Air Pressure The deeper the low, the stronger the pressure gradient between its center and the surrounding air, and the stronger the winds. This is why dramatically low readings are associated with severe storms.
Why the Numbers on Your Barometer Might Not Match the Forecast
If you live at any meaningful elevation above sea level, the raw reading on your barometer will be lower than the “sea-level pressure” reported in forecasts. Barometric pressure drops as you gain altitude because there is simply less air stacked above you. Within the lower atmosphere, pressure decreases roughly in proportion to the height above sea level.4PubMed Central. High-altitude physiology and pathophysiology: implications and relevance for intensive care medicine A city like Denver, sitting around 1,600 meters (about 5,280 feet), has an average station pressure near 840 mb, far below the 1013 mb sea-level standard. Forecasters correct for this by mathematically adjusting every station’s reading to what it would be if the station were at sea level, which is why the weather map can compare a coastal city and a mountain town on the same scale.
If you buy a home barometer or set up a personal weather station, it will typically ask you to enter your elevation so it can make this same correction. Without that step, the raw reading will look alarmingly low even on a perfectly calm day. The adjustment is just arithmetic; it does not change the actual air pressing down on you. At altitude, there really is less air overhead, which has real consequences for cooking, breathing, and even how a baseball carries through the air.
How Fast Pressure Changes Matters More Than the Absolute Number
For both weather and health, the rate of change in barometric pressure often matters more than whether the reading is “high” or “low” in absolute terms. A barometer that drops three or four millibars in a few hours signals an approaching storm, even if the starting pressure was already on the low side. Conversely, a slow rise over a day or two suggests improving conditions.
Experienced mariners and pilots pay close attention to this rate of change because a rapidly deepening low, sometimes called a “bomb cyclone” when it drops at least 24 mb in 24 hours, can generate dangerously high winds with little warning. Knowing the current pressure is 1005 mb tells you less than knowing it was 1020 mb six hours ago and is still falling.
Barometric Pressure and Migraines
Many people who suffer from migraines report that weather changes trigger attacks, and research has been working to pin down whether that connection is real or coincidental. A systematic review of the literature found that pressure drops and rapid pressure changes were significantly associated with increased migraine frequency across several studies.5Cureus. Impact of Barometric Pressure Changes on the Severity, Frequency, and Duration of Migraine Attacks: A Systematic Review of the Literature – Section: Results The relationship was clearest for how often attacks occurred rather than how bad they were: few studies found a link between barometric pressure and migraine severity, and none found a connection to how long individual attacks lasted.
What this means in practical terms is that if you track your migraines, the day the barometer starts dropping is more likely to be a trigger day than a day of stable but low pressure. The shift itself appears to be the provocation, not the absolute level. Some migraine sufferers use barometric pressure apps specifically to anticipate these dips and plan medication or schedule changes accordingly. The evidence is strong enough to take seriously but not so uniform that every migraine patient will experience the same pattern. Your own tracking data matters more than any population-level study for deciding whether pressure changes are a meaningful trigger for you.
Barometric Pressure and Joint Pain
The folk wisdom that people “feel the weather in their joints” also has some scientific support, particularly for inflammatory conditions like rheumatoid arthritis. A study of 326 patients with rheumatoid arthritis found a significant inverse relationship between air pressure and joint swelling: as air pressure dropped, joint swelling and tenderness increased.6PubMed Central. Inverse Association between Air Pressure and Rheumatoid Arthritis Synovitis – Section: Results The effect was statistically clear at the group level, though individual variation was wide.
The proposed mechanism is mechanical. Joint capsules are somewhat sealed compartments. When the outside atmospheric pressure drops, the relative pressure inside the joint capsule is slightly higher, which could cause tissues to expand and press on pain-sensitive structures. The effect is small in absolute terms since even a major storm drops pressure by only a few percent, but for joints already inflamed and sensitive, that small physical change may be enough to cross the pain threshold.
People with osteoarthritis, old injuries, and other musculoskeletal conditions report similar patterns, though the research base is thinner for those groups than for rheumatoid arthritis. If you notice your knees ache before a rainstorm, the barometer probably is part of the explanation, but temperature drops and rising humidity often accompany falling pressure, making it hard to isolate which weather variable is doing the most work.
How Weather Maps Use Pressure to Predict What Is Coming
On a standard surface weather map, pressure is shown using isobars, which are lines connecting points of equal pressure. Where those lines are packed tightly together, the pressure gradient is steep, and winds are strong. Where the lines are spread far apart, the gradient is weak, and winds are light. High-pressure centers, marked with an “H,” push air outward and downward, producing the clear skies and calm conditions most people associate with good weather. Low-pressure centers, marked with an “L,” pull air inward and upward, generating the clouds and precipitation that most people associate with storms.3UCAR Center for Science Education. The Highs and Lows of Air Pressure
These systems move. A high-pressure ridge sliding east across North America can bring several days of dry, pleasant weather to each area it passes over, while a low-pressure trough behind it brings clouds and rain. The pressure readings at your location will rise as the high approaches, flatten out while it is overhead, and fall as the low takes its place. This cycling between higher and lower pressure is the basic rhythm of mid-latitude weather, and it repeats every few days to a week depending on the jet stream pattern.
In the tropics, pressure variations are much smaller day to day. The normal diurnal cycle, a slight rise and fall driven by solar heating, can be larger than the difference between one tropical weather system and the next. This is why tropical meteorologists watch even tiny pressure drops with great interest: a reading that would barely register as noteworthy in Chicago can signal a developing tropical disturbance near the equator.
Seasonal and Geographic Patterns
Barometric pressure is not randomly distributed around the globe. Certain semi-permanent features dominate: the Bermuda-Azores High in the North Atlantic, the Siberian High over central Asia in winter, the Icelandic Low and the Aleutian Low in the northern oceans during winter. These features set the baseline for what “normal” pressure looks like in a given region and season.
In continental interiors during winter, cold, dense air masses create very high pressure. The record-setting readings from Siberia and Mongolia mentioned earlier came from exactly this setup: frigid air pooling over high-latitude land surfaces for weeks.2Wiley Online Library. The Tosontsengel Mongolia world record sea-level pressure extreme: spatial analysis of elevation bias in adjustment-to-sea-level pressures In summer, the same regions warm up, pressure drops, and the semi-permanent highs shift and weaken. Coastal and maritime climates experience narrower swings because ocean temperatures moderate the air above them.
For practical purposes, this means “high” and “low” are somewhat local. A reading of 1020 mb in Miami in July is above average and suggests fair weather. That same reading in Fairbanks in January would be unremarkable or even slightly low by local standards. The sea-level standard of 1013.25 mb is a useful global reference, but the range of normal for your specific location depends on latitude, elevation, proximity to oceans, and season.
Living at Altitude and Chronically Low Pressure
For the roughly 140 million people worldwide who live above 2,500 meters, “low” barometric pressure is not an occasional weather event but a permanent fact of life. At that elevation, the ambient pressure is roughly 75% of the sea-level standard, and the oxygen available in each breath drops proportionally.4PubMed Central. High-altitude physiology and pathophysiology: implications and relevance for intensive care medicine The body compensates over days to weeks by producing more red blood cells, breathing faster at rest, and shifting how hemoglobin binds oxygen. These adaptations are well-studied in populations in the Andes, the Tibetan Plateau, and the Ethiopian Highlands, each of which has evolved slightly different physiological strategies for the same pressure challenge.
Visitors to high altitude without these adaptations encounter the pressure deficit as acute mountain sickness: headaches, nausea, fatigue, and sometimes more dangerous fluid accumulation in the lungs or brain. The trigger is not the pressure per se but the reduced partial pressure of oxygen that accompanies it. You can reproduce the same physiological stress by lowering the oxygen percentage in a sealed room at sea level, which is exactly what altitude-simulation chambers do for athletic training and research.
This distinction matters because the barometric pressure number on a weather report at altitude does not directly tell you how your body will respond. Two locations at the same pressure can have very different oxygen levels if one is cold (denser air, slightly more oxygen per breath) and the other is hot. Temperature, humidity, and individual fitness all modulate how any given pressure reading translates into how you feel and perform.
Reading Your Home Barometer
If you own a barometer or a home weather station, the single most useful habit is to note the trend rather than fixate on the absolute number. A rising barometer generally forecasts improving weather; a falling one forecasts deteriorating conditions. A sharp drop of more than about 6 mb in a few hours is a meaningful signal that a strong storm is approaching and winds may pick up.
Most digital weather stations display pressure in either millibars, hectopascals (the same number as millibars, just a different name), or inches of mercury. For inches of mercury, the equivalent conversions are roughly: 30.5 inHg is clearly high pressure, 29.9 inHg is standard, and anything below about 29.5 inHg is getting into low territory. Below 29.0 inHg, a serious storm is likely in the area. These cutoffs are approximate because local climate shifts what counts as normal, but they give a reasonable first approximation for most mid-latitude locations.
Analog barometers often have labels printed on the dial: “Stormy,” “Rain,” “Change,” “Fair,” and “Very Dry.” These correspond loosely to pressure ranges from low to high. They are not precision instruments, but the trend arrow telling you which direction the needle has been moving over the past few hours is genuinely useful information, often more so than the labeled zones themselves.