SolriseBarometric pressure: what rising and falling pressure mean
Barometric pressure is the weight of the atmosphere pressing down at your location, and its direction matters more than its exact value. Falling pressure usually signals an approaching storm system, while rising pressure usually means clearer, calmer weather is moving in.
What barometric pressure is
The air above you has weight, and barometric pressure measures it. In the US it is usually given in inches of mercury (inHg); much of the world uses millibars or hectopascals (1 mb equals 1 hPa). Weather reports adjust readings to sea level so places at different elevations can be compared fairly. The standard atmosphere at sea level is 29.92 inHg, or 1013 hPa.
The pressure scale
| Sea-level pressure | Level | What it usually means |
|---|---|---|
| Under 29.60 inHg (under about 1002 hPa) | Very low | Deep low pressure: often a strong storm system. |
| 29.60 to 29.84 inHg (about 1002 to 1010 hPa) | Low | Lower than average: unsettled weather is more likely. |
| 29.85 to 30.19 inHg (about 1011 to 1022 hPa) | Typical | Close to average sea-level pressure. |
| 30.20 to 30.49 inHg (about 1023 to 1032 hPa) | High | Higher than average: usually calmer, settled weather. |
| 30.50 inHg and up (about 1033 hPa and up) | Very high | Strong high pressure: settled, often clear and still. |
Source: bands set around the standard atmosphere of 29.92 inHg (1013 hPa). There is no single official pressure scale, so treat these as a guide.
Why pressure rises and falls
Falling pressure usually signals an approaching storm system; rising pressure usually means clearer, calmer weather is moving in. The bigger the difference in pressure between two nearby areas, the stronger the wind, because the atmosphere is trying to even that difference out.
The trend tells you more than the number. A reading of 29.80 inHg that is climbing is better news than 30.00 inHg that is dropping fast.
What pressure means for you
Everyday
A fast drop over a few hours is a reliable early sign of incoming rain or storms, often before the sky looks like much. It is also why some people feel more joint stiffness or headaches on low-pressure days, which is worth knowing if a walk or run feels harder than usual for no obvious reason.
Outdoors
If your watch or GPS uses a barometric altimeter, falling pressure makes it think you are climbing, and rising pressure makes it think you are descending. Recalibrate at a known elevation, like a trailhead sign, before trusting it. If you are boating or fishing, a steady pressure drop over a few hours is a reason to head back to shore early, before wind and waves build.
Flying
Flying from high to low pressure without updating the altimeter makes it read higher than you actually are: "high to low, look out below." Each inch of mercury off is roughly 1,000 feet of error, so a setting 0.10 inHg off puts you about 100 feet off. Above 18,000 feet in the US, all pilots set their altimeters to the same standard value, 29.92 inHg.
Common questions
What is normal barometric pressure?
The standard atmosphere at sea level is 29.92 inHg (1013 hPa). Readings from about 29.85 to 30.19 inHg are close to average.
What does falling barometric pressure mean?
Falling pressure usually signals an approaching storm system. A fast drop over a few hours is a reliable early sign of incoming rain or storms.
What does rising barometric pressure mean?
Rising pressure usually means clearer, calmer weather is moving in.
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