Module 13 - Meteorology
Pressure Systems
What you will learn
Use isobar pattern, system movement and a three-hour pressure tendency to support a weather decision without treating one pressure value as a complete forecast.
Highs, lows and isobars describe a pressure pattern. They help explain wind and likely weather, but the current marine forecast, fronts, local effects and observations still control the passage decision.
Atmospheric pressure is measured in hectopascals (hPa), numerically equivalent to millibars. A high or low is defined relative to the surrounding pattern, not by one universal safe or unsafe number. In the northern hemisphere, surface flow is generally clockwise around a high and anticlockwise around a low, while friction and local geography keep the actual wind from following a perfect circle.
Isobars join places with equal sea-level pressure. Close spacing shows a steep pressure gradient and usually stronger wind; wider spacing suggests a weaker gradient and lighter wind. The chart also shows whether a system, front, ridge or trough is approaching, weakening or moving away. Compare successive charts rather than reading one frozen pattern as a timetable.
High pressure is often settled, but can still bring fog, poor visibility, strong edge gradients or persistent easterly conditions. Low pressure is often unsettled and may carry fronts, but conditions vary across the system and with its track. A barometer trend can confirm that the situation is changing; it does not identify the exact wind, weather or timing on its own.
Worked example
Pressure changes from 1013 hPa to 1008 hPa in the preceding three hours.
- 1Subtract the later reading from the earlier: 1013 − 1008 = 5 hPa.
- 2The direction is falling because the later value is lower.
- 3Five hPa in three hours lies inside the official 3.6–6.0 hPa ‘falling quickly’ band.
Sense check: Do not average a six-hour change and call it the official three-hour tendency unless the actual preceding three-hour readings support it.
| Evidence | What it supports | What it does not prove |
|---|---|---|
| Close isobars | A steeper pressure gradient and usually stronger wind | Exact local wind or sea state |
| High / low circulation | Broad clockwise / anticlockwise pattern in the northern hemisphere | A perfect circular surface wind at the vessel |
| Three-hour tendency | Whether pressure is rising or falling, and at what classified rate | The precise weather sequence without forecast context |
| Successive charts | System and front track, development and timing trend | Guaranteed arrival time at one harbour |
| Observed cloud, wind and sea | Whether actual conditions match the forecast story | A replacement for current warnings and updates |
Use the barometer to test the forecast story
The latest chart shows a low and fronts approaching. The yacht’s corrected barometer has fallen from 1013 hPa to 1008 hPa during the preceding three hours.
1. Calculate
1013 − 1008 = a 5 hPa fall in three hours.
2. Classify
The Met Office glossary calls a 3.6–6.0 hPa change in the preceding three hours ‘falling quickly’.
3. Corroborate
The rapid fall supports the approaching-low forecast; compare cloud, wind, visibility and sea with the expected sequence.
4. Act
Obtain the latest forecast and reassess timing, exposure, crew, shelter and alternatives before conditions remove options.
Sense check: The useful evidence is the measured change over a stated period. ‘1008 hPa’ alone does not tell you whether pressure is rising or falling.
Common mistake or limitation
- Treating every high as fair and every low as uniformly severe; location within the system, fronts, season and local effects matter.
- Using one barometer reading without time, trend, instrument check or forecast context.
- Assuming surface wind follows an isobar exactly or that isobar spacing removes the need for the local marine forecast.
Recap
- Pressure is a pattern and trend, not a standalone go/no-go number.
- Close isobars normally mean stronger wind; high and low circulation is clockwise and anticlockwise in the northern hemisphere.
- Use charts and a timed barometer tendency to corroborate, then act on current forecasts, warnings and observations.
Optional quick check
Section 4 of 6
A corrected barometer falls from 1013 to 1008 hPa in the preceding three hours. What is the sound interpretation?
Sources and factual review
Reviewed 19 August 2026 - Day Skipper Revision.
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