Atmospheric pressure is the force exerted by the weight of the air above any given point.
Atmospheric pressure is not a constant value; it's always in flux, even on a calm, sunny day. One of the most predictable changes is a slight daily cycle, where pressure tends to be at its highest in the morning and evening, and at its lowest around mid-day and midnight. This pattern, known as the "atmospheric tide," is caused by the heating and cooling of the air throughout the day, which creates subtle wave-like movements in the atmosphere.
However, these daily fluctuations are often overshadowed by larger, more significant changes driven by the weather.
Atmospheric pressure is a fundamental driver of weather. Changes in pressure create wind and are often an indicator of what kind of weather is on the way.
High-pressure systems: When a column of air is particularly heavy, it forms a high-pressure system. Air in these systems is sinking, which warms and dries as it descends. This process prevents clouds from forming, which is why high-pressure systems are typically associated with calm, clear skies and fair weather.
Low-pressure systems: Conversely, a low-pressure system is a region where the air is lighter. In these systems, air is rising. As the air rises, it cools, and the moisture within it condenses to form clouds and precipitation. For this reason, low-pressure systems are almost always associated with cloudy, unsettled, and stormy weather. The greater the pressure difference between a high and low-pressure system, the stronger the winds will be as the air moves from the high-pressure area to the low-pressure area to balance the weight difference.
This question might seem like a bit of a trick, since we just said pressure impacts the weather. However, it's a dynamic relationship where both factors influence each other. Local weather conditions can, in turn, affect the pressure.
For example, a strong thunderstorm can cause a temporary drop in pressure as the rapidly rising air creates a localized low-pressure area. The passing of a cold front, where colder, denser air moves in, can cause a sudden rise in pressure. Ultimately, the large-scale weather patterns—the formation of cyclones and anti-cyclones, the movement of fronts, and temperature changes—are the very things that create the high and low-pressure systems we use to forecast the weather.
Atmospheric pressure decreases with altitude. The higher you go, the less air there is above you, and therefore the less weight pushing down. This is why mountaintops, airplanes, and high plateaus all have lower atmospheric pressure than sea level.
This drop in pressure with height has practical consequences. People may experience shortness of breath or altitude sickness at high elevations because there are fewer oxygen molecules per breath. It also affects how pressure is measured and interpreted, especially in weather reporting and aviation.
Because pressure naturally decreases with elevation, meteorologists use a correction factor to make pressure readings comparable across different altitudes. This corrected value is called sea level pressure—an estimate of what the pressure would be if the station were located at sea level.
The uncorrected reading at the actual elevation of the station is known as station pressure. For example, a weather station in a mountainous area might measure 900 millibars of station pressure, but after correction for altitude, the sea level pressure might still be 1013 millibars—indicating average atmospheric conditions.
Without this standardization, weather maps would be misleading, as high-altitude areas would always appear to be under low pressure.
Atmospheric pressure plays a vital role in both aviation and meteorology.
Pilots use atmospheric pressure to set their altimeters—crucial for knowing altitude above sea level, especially during landings. A misreading due to incorrect pressure settings can result in serious navigational errors.
In weather forecasting, pressure is used to identify large-scale systems like cyclones and anti-cyclones. Meteorologists draw isobars—lines of equal pressure—on maps to visualize how air will move. Closely packed isobars indicate strong winds and steep pressure gradients, which are essential for storm forecasting and wind modeling.
While most people experience pressure changes of just a few millibars from day to day, some weather events bring dramatic extremes.
Hurricanes, for instance, are characterized by deep low-pressure centers. The lower the pressure, the more intense the storm. Hurricane Wilma in 2005 had one of the lowest pressures ever recorded in the Atlantic basin: 882 millibars.
On the other end of the spectrum, extremely high pressures can occur during strong wintertime anticyclones in Siberia and Mongolia, with recorded values exceeding 1080 millibars. These systems often bring cold, dry, and stagnant conditions.
Atmospheric pressure is measured with a device called a barometer. The earliest barometers used a column of mercury, and the height of the mercury column would rise or fall in response to changes in air pressure. Today, most barometers use electronic sensors or an aneroid capsule, which is a sealed metal box that expands and contracts with pressure changes.
The standard unit for measuring atmospheric pressure is the millibar (mb), though hectopascals (hPa) are also used and are numerically identical. In the United States, pressure is often reported in inches of mercury (inHg). A standard atmospheric pressure at sea level is approximately 1013.25 mb (or hPa), or 29.92 inHg.
Published:
August 1, 2025
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