Definition
The amount and direction of change in atmospheric pressure at a station over the past three hours, reported on a surface analysis chart. It shows whether pressure has been rising, falling, or steady, and by how much (typically expressed in tenths of a millibar/hectopascal).
Plain English
How the air pressure at a weather station has been moving over the last three hours — going up, going down, or holding steady — and by how much.
Context Anchor
Seen on surface analysis charts as part of the weather information plotted around a reporting station.
Derivation
‘Tendency’ comes from the Latin ‘tendere’, meaning ‘to stretch toward’ or ‘lean in a direction’. In weather use it keeps that sense — the pressure isn’t just a number now, it’s leaning a certain way over time.
Why Pilots Care
Indicates whether weather is likely improving or worsening, helping pilots anticipate wind shifts, fronts, or changes in ceiling and visibility.
Grounding Statement
It’s the recent trend of the barometer at one station, captured in a single number.
Intuition Check
Do not read “tendency” as a vague guess or forecast. On a surface analysis chart, pressure change/tendency is a reported recent pressure trend at that station.
Example Sentence 1
The surface analysis chart showed a falling pressure tendency along our route, so we expected the ceilings to lower by the time we arrived.
Example Sentence 2
Before takeoff the pilot noted the rising pressure tendency and decided conditions were improving.