Barometric Pressure Trend Calculator
Barometric pressure trend is one of the most reliable short-term weather forecasting tools available to anyone with a barometer. The rate of change of atmospheric pressure, called pressure tendency, tells you whether weather conditions are likely to improve or deteriorate. This calculator takes two pressure readings (in hPa or inHg) and the time between them, then computes the pressure change rate in hPa per hour and classifies the trend per WMO pressure tendency guidance. It also returns a plain-language weather outlook based on the trend.
Pressure tendency formula
Change (hPa) = P2 - P1
Rate (hPa/hr) = Change / Time (hr)
The WMO standard observation interval is 3 hours. To convert inHg to hPa: multiply by 33.8639. Standard sea-level pressure: 1,013.25 hPa = 29.92 inHg.
WMO pressure tendency classification
- Change below -6 hPa in 3 hrs: Very rapid fall. Intense storm imminent; high confidence of severe weather.
- Change -3 to -6 hPa in 3 hrs: Rapid fall. Strong storm approaching; deteriorating conditions expected.
- Change -1 to -3 hPa in 3 hrs: Falling. Low pressure or frontal system approaching; weather likely to worsen.
- Change -1 to +1 hPa in 3 hrs: Steady. Current conditions likely to persist for the next few hours.
- Change +1 to +3 hPa in 3 hrs: Rising. Weather improving; high pressure building.
- Change above +3 hPa in 3 hrs: Rapid rise. Post-storm clearing; fair weather expected.
Barometric pressure trend: frequently asked questions
What does a falling barometer indicate?
A falling barometric pressure generally indicates an approaching low-pressure system, which typically brings clouds, wind and precipitation. A rapid fall (more than 2 hPa per hour, or 6 hPa in 3 hours) is associated with the approach of a strong storm system. The WMO classifies pressure tendency (change over 3 hours) as Rising (positive), Falling (negative) or Steady (less than 0.1 hPa change).
What does a rising barometer indicate?
A rising barometric pressure generally indicates an approaching high-pressure system, which typically brings clearing skies and improving weather. A rapid rise (more than 2 hPa per hour) after a storm suggests that the low-pressure system is moving away and fair weather is returning. Sustained high pressure is associated with calm, dry conditions.
What is standard atmospheric pressure?
Standard atmospheric pressure at sea level is defined as 1,013.25 hPa (hectopascals) or millibars (mb), equivalent to 29.92 inches of mercury (inHg) or 101,325 Pa. This is the International Standard Atmosphere (ISA) reference pressure defined by ICAO and ISO 2533. Actual surface pressure varies with weather systems and altitude.
How fast does barometric pressure change before a storm?
The WMO classifies 3-hour pressure tendencies: a change of 1 to 3 hPa is significant; 3 to 6 hPa is rapid; over 6 hPa in 3 hours is very rapid and indicates an approaching intense storm. For weather forecasting, a fall of more than 3 hPa in 3 hours (characteristic of an extratropical cyclone) is enough to prompt NWS watches or warnings.
What are the WMO pressure tendency codes (a-values)?
The WMO Manual on Codes (No. 306) defines eight pressure tendency characteristics: 0 = rising then falling; 1 = rising then steady; 2 = rising steadily or unsteadily; 3 = falling or steady then rising; 4 = steady; 5 = falling then rising; 6 = falling then steady; 7 = falling steadily or unsteadily; 8 = steady then falling. This calculator uses the simpler 3-hour net change to classify the tendency direction and rate.
Official sources
- WMO Manual on Codes, Vol. I.1, Part A (No. 306): library.wmo.int.
- NOAA NWS Glossary of Meteorology: weather.gov/glossary.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.