Module 13 - Meteorology
Barometer Trends and Short-Range Prediction
What you will learn
Calculate a three-hour pressure tendency, use the official rate bands and combine the result with forecast and observed change.
Pressure tendency is more informative than one reading. It shows how quickly pressure is changing, but not by itself what exact weather will occur.
Use a calibrated barometer and record pressure at consistent times. The Met Office station-report bands describe a 0.1–1.5 hPa change in three hours as slow, 1.6–3.5 hPa as rising or falling, 3.6–6.0 hPa as rising or falling quickly, and more than 6.0 hPa as rising or falling very rapidly.
A fall can support evidence of approaching or deepening low pressure; a rise can follow a front or support improving pressure. Neither direction guarantees the wind, sea or visibility. Strong winds and showers can continue after pressure begins to rise.
Use the trend to test the forecast sequence. A faster-than-expected fall, delayed rise or changed wind may justify an earlier reef, shorter route, larger sea-room margin or alternative harbour. Check instrument error and updated official information before assigning a cause.
Worked example
A barometer falls from 1006 hPa to 998 hPa between 1200 and 1500.
- 1998 − 1006 = −8 hPa.
- 2The interval is three hours.
- 3More than 6.0 hPa in three hours is ‘falling very rapidly’.
- 4Confirm the instrument, current warnings, forecast and observations, then take early passage precautions.
Sense check: The rate justifies urgent attention; it does not identify the exact local wind or sea without other evidence.
| Three-hour change | Official tendency wording | Use |
|---|---|---|
| 0.1–1.5 hPa | Rising or falling slowly | Note a gradual trend |
| 1.6–3.5 hPa | Rising or falling | Compare with forecast sequence |
| 3.6–6.0 hPa | Rising or falling quickly | Reassess timing and margins |
| More than 6.0 hPa | Rising or falling very rapidly | Confirm and take prompt precautionary action |
Convert readings into an action cue
Pressure is 1013 hPa at 0600 and 1008 hPa at 0900 while the forecast expects a front later in the day.
1. Subtract
1008 − 1013 = −5 hPa.
2. State the interval
The 5 hPa fall occurred in three hours.
3. Classify
Five hPa lies in the official 3.6–6.0 hPa ‘falling quickly’ band.
4. Apply
Check the latest forecast and observations, then preserve an earlier, safer option if change is ahead of plan.
Sense check: ‘Falling quickly’ is a measured tendency band, not a guarantee that one named storm or front is at a fixed distance.
Common mistake or limitation
- Calling every fall ‘bad weather’ or every rise ‘good weather’.
- Quoting a pressure change without the time interval.
- Using an unverified instrument trend instead of checking calibration, forecast, warnings and observations.
Recap
- Always state pressure change with its time interval.
- Five hPa in three hours is ‘falling quickly’; eight hPa is ‘falling very rapidly’.
- Use the trend with current official information and observations, not alone.
Optional quick check
Section 8 of 13
Pressure falls from 1012 to 1007 hPa in three hours. How should it be described?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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