Module 13 - Meteorology
Pressure Systems
What you will learn
Read pressure centres, isobar spacing and chart validity as a weather model, then compare that model with the route forecast and observed pressure tendency.
Highs, lows and isobars explain broad circulation and pressure gradient, but none guarantees a single local wind, visibility or sea state.
On a northern-hemisphere surface chart, air circulates broadly clockwise around high pressure and anticlockwise around low pressure. Closely spaced isobars indicate a stronger pressure gradient and usually stronger gradient wind than widely spaced isobars.
A high is not a guarantee of clear visibility or calm conditions. It can contain low cloud or fog, and its edge can carry a meaningful pressure gradient. A low is not equally severe everywhere: the track, depth, fronts, gradient, movement and your position within the system matter.
A surface-pressure chart is an analysis or forecast for a stated valid time, not a live picture at the yacht. Use the chart to form an expectation, then compare it with the relevant marine forecast, warnings, barometer, wind, cloud and visibility.
Worked example
The calibrated barometer reads 1013 hPa at 0600 and 1008 hPa at 0900.
- 1Calculate the change: 1008 − 1013 = −5 hPa.
- 2The interval is three hours, so the tendency is a 5 hPa fall in three hours.
- 3The Met Office station-report band of 3.6–6.0 hPa in three hours is ‘falling quickly’.
- 4Check the current forecast, warnings and observed wind, cloud and visibility before changing the passage plan.
Sense check: A pressure tendency is strong evidence of change, but it cannot by itself identify the system track or local sea state.
| Chart cue | What it supports | What it does not prove |
|---|---|---|
| High / low centre | Broad circulation and system position | Exact local wind or visibility |
| Isobar spacing | Relative pressure gradient | One exact gust or wave height |
| Front or trough | Likely zone of weather change | Fixed timing or severity |
| Valid time | When the model applies | Conditions before or after without another source |
Read the chart as a testable expectation
A forecast chart valid at 1200 shows a low northwest of the route, close isobars over the western approaches and a front crossing the coast later.
1. Check time
Confirm whether the chart is an analysis or forecast and note its valid time.
2. Locate the route
Place the passage relative to the low, fronts and strongest gradient.
3. Predict cautiously
Expect a broad wind and weather sequence without inventing exact local timing.
4. Verify
Compare the current forecast and observations with the expected sequence.
Sense check: A 1200 forecast chart does not prove the surface conditions at 0900 or at one harbour entrance.
Common mistake or limitation
- Equating high pressure with guaranteed clear, calm weather or low pressure with identical conditions everywhere.
- Ignoring whether a chart is an analysis or forecast and when it is valid.
- Using isobar spacing to invent an exact local wind without the forecast and observations.
Recap
- Highs and lows give broad northern-hemisphere circulation, not guaranteed local weather.
- Close isobars mean a stronger pressure gradient.
- Always use the chart valid time and verify the model against current information and observations.
Optional quick check
Section 4 of 15
What is the strongest conclusion from closely spaced isobars?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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