Most people think of air pressure as something that’s important for bike and car tires, but it also impacts weather and even animal behavior. But what is atmospheric pressure?
The air around us is made up of tiny molecules that are constantly moving, and air pressure is the force pushing against the molecules. In this guide, you will learn how atmospheric pressure is measured, how it affects weather, and what normal atmospheric pressure is.
- What Is Air Pressure?
- How Is Air Pressure Measured?
- How Does Atmospheric Pressure Affect Weather?
- What Is Normal Atmospheric Pressure?
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What Is Air Pressure?
Air pressure, also called atmospheric or barometric pressure, is the force of the air in the atmosphere pushing on everything it touches, so wherever you are standing, the air in the atmosphere is pushing down on you.
When you are at the lowest level of our atmosphere, also known as sea level, the air is pushing down on you with an average force of 1,013.2 millibars. As you go higher into the atmosphere, there is less atmospheric pressure because there are fewer molecules of air above your head, and the force of the air pushing down on you is less.
How Is Air Pressure Measured?
Meteorologists use a barometer to measure the amount of atmospheric pressure. That is why you may see or hear meteorologists refer to atmospheric pressure as barometric pressure.
Barometers report in a few common units:
- mb or mbar: Millibars
- hPa: if you’ve wondered, “What is hPa?” it’s shorthand for the hectopascal unit, which is numerically the same as millibars (e.g., 1,013.2 mb = 1,013.2 hPa).
- inHg: inches of mercury, common in the United States
- psi: pounds per square inch.
Readings change with altitude and weather systems, so stations are adjusted (or “reduced”) to air pressure at sea level to make comparisons meaningful. Many apps and stations also smooth short-term wiggles so you can see the air pressure trend at a glance.
Want to get hands-on with instruments? Explore our guide to types of weather instruments to see how pros and hobbyists measure weather phenomena.
How Does Atmospheric Pressure Affect Weather?
Atmospheric pressure affects weather almost everywhere, almost all the time. Air is constantly moving from higher pressure toward lower pressure, and that movement drives winds and helps shape clouds and storms.
High pressure usually means air is sinking and drying out. Skies tend to be sunnier, winds lighter, and temperatures can be more extreme at night. So what is high air pressure? As a rule of thumb, readings above 1013.2 mb are often considered “high” at sea level.
Low pressure is typically associated with rising air, cloud formation, and a higher chance of precipitation. What is considered low air pressure? Anything below 1,013.2 mb is a common benchmark for “low,” with deeper lows often tied to stronger storm systems.
Pressure trends add extra context: a rapidly falling barometer can signal approaching unsettled weather; rising pressure often follows a front, bringing clearing conditions.
For a deeper dive into weather pattern basics, see our cold vs. warm fronts guide. Or learn how barometric pressure affects fish behavior.
What Is Normal Atmospheric Pressure?
At sea level, “normal” or standard pressure is defined as 1,013.2 mb, a value used as a reference point for weather maps and instrument calibration. That said, what feels “normal” in your location varies with altitude, season, and regional climate. For instance, mountainous sites have lower station pressure simply because there’s less atmosphere above them. To compare places, meteorologists convert values to atmospheric pressure at sea level.
You might also see the phrase normal barometric pressure used interchangeably—same concept, different wording.
|
Range (hPa / mb) |
Range (inHg) |
Typical Label |
What It Often Means for Weather |
|
≤ 1000 |
≤ 29.53 |
Low |
Greater chance of clouds, rain/snow, or windy conditions; stronger lows can indicate storms. |
|
~1001–1019 |
29.56–30.09 |
Near Normal |
Mixed or changing conditions; watch the trend (falling or rising) for clues. |
|
≥ 1020 |
≥ 30.12 |
High |
Usually calmer, drier, and sunnier; can mean heat in summer or cold, clear nights in winter. |
What Is The Atmospheric Pressure At Sea Level?
The internationally recognized value is 1,013.2 millibars, which equals 1,013.2 hPa, 29.92 inHg, or about 14.7 psi. This standard gives forecasters a baseline for comparing observations, maps, and model output across regions.
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