Barometric pressure is the weight of the atmosphere above you. Standard sea-level pressure is 29.92 inches of mercury: the reference every other reading is measured against.
High pressure generally means sinking air, which suppresses cloud growth and favors fair weather. Low pressure means rising air, which builds clouds and storms. That's the physics in one line.
What your pressure reading means
People look at the barometer, see a number, and want a verdict. Here is the honest lookup, with the caveat this whole guide is built on: the trend outranks the number.
Working in millibars? Multiply inches by 33.86: standard 29.92 inHg is 1013 mb.
So what does a 30.05 pressure reading mean?
It is the most-asked number, and the answer is the guide in miniature. A reading of 30.05 inches (about 1018 mb) sits just above standard: on its own, mildly fair-weather. If it got there by rising, expect quiet skies. If it is falling through 30.05, a change is on the way, and the reading itself never had a vote. Check the arrow next to the number, not the number.
The direction is the forecast
A single pressure reading is a snapshot, and snapshots don't tell you what's coming. The three-hour trend does. The same 30.05 means opposite things depending on where it came from:
- Rising: conditions improving or holding. Storms have to fight uphill.
- Steady: whatever you have now, expect more of it.
- Falling slowly: change is on the way, hours out.
- Falling fast (0.06 inches or more in three hours), something is organizing, and it often shows here before it shows on radar.
Falling pressure alongside a dew point of 70° or higher is the classic setup: plenty of moisture to work with, and a trigger arriving to lift it. Two measured numbers, one honest storm signal, no model required.
Why airports report it so precisely
Aircraft altimeters are barometers. A pilot setting the wrong pressure flies at the wrong altitude, so airport stations report pressure to hundredths of an inch and update it constantly. That precision is the reason a pressure trend built from airport observations is trustworthy enough to forecast with.