Pressure
What Is a Millibar Pressure Change? A Sailor's Guide
Published · Last checked October 2026
A millibar (mb) is a unit of air pressure, identical to a hectopascal (hPa). Average sea-level pressure is about 1013 mb. For sailors the trend matters more than the number: a change of 1 to 3 mb in three hours is normal, but a fall of 6 mb or more in three hours usually means strong wind is on its way, and a fall of 10 mb or more in three hours often means gale force or worse.
What a millibar actually measures
Air pressure is the weight of the atmosphere pressing down on the surface. The millibar is one thousandth of a bar, and one millibar equals one hectopascal, which is why the Met Office and most barometers now show hPa. At sea level, pressure usually sits between about 960 mb in a deep depression and 1040 mb in a strong anticyclone, with 1013 mb as the long-term average.
The absolute figure tells you something: below about 1000 mb you are probably under or near a depression; above 1025 mb you are probably in a high with lighter winds. But two boats with the same reading can face very different weather. What separates them is how quickly the pressure is changing.
Why the rate of change matters
Wind is air moving from high to low pressure, deflected by the Earth's rotation. When pressure changes fast at your position, it usually means the pressure gradient nearby is steep and the isobars are packed close together. Close isobars mean strong wind. So a rapidly falling barometer is one of the oldest and most reliable warnings of a coming blow.
| Term | Change in 3 hours | What it suggests |
|---|---|---|
| Steady | Less than 0.1 mb | Settled for now |
| Rising or falling slowly | 0.1 to 1.5 mb | Gradual change |
| Rising or falling | 1.6 to 3.5 mb | Weather changing; watch it |
| Rising or falling quickly | 3.6 to 6.0 mb | Strong wind likely |
| Rising or falling very rapidly | More than 6.0 mb | Gale or severe gale possible |
These are the same words you hear in the coastal station reports of the shipping forecast. Learning them lets you match what you see on your own barometer with what the forecasters are describing.
Rules of thumb at sea
- A steady fall of 1 mb an hour for several hours: expect the wind to build, often to force 6 or more.
- A sharp rise after a fall: the front or low has passed. Wind often veers and can blow hard for a while in the cold air behind.
- 'Long foretold, long last; short notice, soon past': a slow, prolonged fall brings longer-lasting bad weather than a sudden dip.
- Rapid change in either direction means wind. A fast rise can bring strong gusty northwesterlies just as much as a fall brings southwesterlies.
Putting it into practice
Before a passage, check the forecast and note the pressure. Once under way, compare your readings with the forecast. If the barometer is falling faster than the forecast implied, the system may be deeper or quicker than expected, and it is time to reef early, review your bolt-holes and consider turning back. The weather forecasting lesson covers how depressions and fronts produce these pressure patterns.