Module 13 - Meteorology
Pressure Systems
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.
| 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 |
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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