Pressure Tendency Calculator
Atmospheric pressure tendency is one of the oldest and most reliable tools in weather forecasting. A barometer that is falling rapidly warns of approaching storms, while a rising barometer signals improving conditions. Meteorologists report pressure tendency as the 3-hour pressure change in millibars or inches of mercury. Enter the current pressure and the pressure 3 hours ago to see the tendency, the rate of change per hour, and a plain-language weather outlook based on the WMO tendency characteristics.
Pressure tendency formula
3-Hour Change = P(now) - P(3 hours ago) [mb]
Rate = 3-Hour Change / 3 [mb/hr]
A negative value means pressure is falling; a positive value means pressure is rising. The WMO classifies tendency characteristics: 0-3 rising, 4 steady, 5-8 falling (with various patterns of steady/rising/falling within those groups). For practical purposes, a change more negative than -3 mb in 3 hours indicates significant pressure falls.
Interpreting pressure tendency for weather
- Falling rapidly (below -3 mb/3 hr): storm likely approaching within 12-24 hours.
- Falling slowly (-1 to -3 mb/3 hr): deteriorating conditions expected.
- Steady (less than 1 mb change): no major change forecast in the near term.
- Rising slowly (1 to 3 mb/3 hr): improving conditions, storm moving away.
- Rising rapidly (above 3 mb/3 hr): post-storm clearing, possible strong winds behind cold front.
Pressure tendency: frequently asked questions
What is pressure tendency?
Pressure tendency is the change in atmospheric pressure over a standard 3-hour period, expressed in millibars (hPa) or inches of mercury. Meteorologists measure it continuously as one of the most reliable indicators of approaching weather systems. A rapidly falling barometer often signals an approaching storm.
How fast must pressure fall to indicate a storm?
A pressure fall of 1 mb or more per hour (3 mb or more over 3 hours) is considered rapid and indicates significant weather development. Falls of 6 mb or more in 3 hours are associated with explosive cyclogenesis (bomb cyclones) and severe weather. A fall of 3-6 mb in 3 hours warrants attention to developing conditions.
Does rising pressure always mean improving weather?
Generally yes, but the rate matters. A slow, steady pressure rise after a storm typically means clearing conditions. A very rapid rise behind a cold front can produce strong gusty winds and sudden temperature changes. The NWS uses both the amount and rate of pressure change in its forecasting.
What units are used for pressure tendency?
Standard meteorological practice uses millibars (mb) or the equivalent hectopascals (hPa). Surface weather observations in the US use inches of mercury (inHg) for station pressure, where 1 inHg equals approximately 33.86 mb. METAR reports include pressure tendency in tenths of millibars.
Where is pressure tendency coded in a METAR?
In international METAR format, pressure tendency appears in the remarks section as a 5-digit code (5appp), where 'a' is the tendency characteristic (0-8) and 'ppp' is the change in tenths of hPa over the past 3 hours. Automated US stations often omit this group, but synoptic observations include it.
Official sources
- NOAA/NWS: JetStream: Atmospheric Pressure.
- WMO: WMO CIMO Guide: Pressure Measurements.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.