Pressure tendency
Air pressure barometric tendency three-hour pressure tendency
Pressure tendency is the amount air pressure rises or falls over a specified period. It describes the change, not the pressure itself.
Two readings make a tendency
A barometer reads 1015 hPa at noon and 1011 hPa at 15:00. Pressure has fallen by 4 hPa in three hours. The tendency is −4 hPa over three hours, not −4 hPa per hour; for that average rate, divide by three.
What rising and falling pressure can suggest
- Falling pressure: May indicate an approaching low or front.
- Rising pressure: May occur behind a front or as high pressure builds.
- Little change: Only tells us that pressure has changed little. The weather itself need not stay the same.
A barometer cannot promise sunshine
The old “fair” and “rain” labels on some barometers are rather optimistic forecasts. Persistent fog can sit under high pressure, while pressure may already be rising behind a cold front with showers still passing through.
Wind, clouds and the weather chart provide the context. Compare the same type of pressure at the same place. During a hill walk, surface pressure also falls as you climb, without any approaching low.