How to Read a Barometer for Sailing

To read a barometer for sailing, tap it gently (for an aneroid dial), note the reading in millibars and log it every hour. Ignore the 'Rain' and 'Fair' words on the dial: what matters is the trend. Steady pressure means settled conditions; a slow fall means change on the way; a fall of more than about 6 mb in three hours warns of gale-force wind. Compare your trend with the forecast to spot weather arriving early.

Close-up of an aneroid barometer dial marked in hectopascals — An aneroid barometer marked in hPa, the same as millibars. Photo: Ka23 13, CC BY 4.0.

Types of barometer aboard

A graph of hourly barometer readings, steady at first, then falling 7 mb in three hours — Hourly logging turns the readings into a trend you can act on.

Aneroid dial: the traditional brass barometer with a moving needle and a setting hand. Reliable, no power needed.
Barograph: a recording barometer that draws or stores a pressure trace. The trace makes trends obvious.
Electronic: built into chartplotters, instruments, watches and phones. Many show a three-hour or 24-hour graph, which is ideal.

Any of these will do the job. What counts is that you read it regularly and write the figure down.

Setting and reading an aneroid barometer

Set it to sea-level pressure once by comparing with a nearby coastal station report or a reliable app, using the adjusting screw on the back.
Before each reading, tap the glass lightly. The needle can stick, and tapping lets it settle.
Read the pointer against the millibar scale, not the weather words.
Turn the brass setting hand so it lies over the needle. Next time you read it, the gap between the two shows how far pressure has moved.
Record the reading and time in the logbook.

Interpreting the trend

A simple guide to barometer trends at sea.
Trend (3 hours)	Likely meaning	What to do
Steady	Settled weather continues	Carry on, keep logging
Slow fall, 1 to 3 mb	Low or front approaching	Check the next forecast
Fall of 3 to 6 mb	Wind freshening, front near	Reef early, plan a refuge
Fall over 6 mb	Gale-force wind likely	Seek shelter, prepare the boat
Sharp rise after a fall	Front or low passed	Expect a veer and gusty showers

A falling barometer combined with a backing wind, thickening high cloud and a growing swell is the classic sign of an approaching depression in the UK. A rising barometer with the wind veering northwest and clearing skies usually marks the cold air behind it.

Using the barometer with the forecast

A barometer does not replace the forecast; it checks it. Note the forecast pressure for your area and time. If your readings fall faster or earlier than forecast, the system is probably deeper or quicker than expected. That is your cue to act on the worse end of the forecast. The weather awareness lesson and the guide to the millibar pressure change explain the numbers in more detail.

Remember that pressure also has a small daily rhythm, rising and falling by around 1 mb twice a day in temperate latitudes. Do not read too much into a single small dip.

Related guides

Frequently asked questions

What barometer reading means a storm is coming?

There is no single number. A rapid fall, more than about 6 mb in three hours, is the warning sign, wherever it starts from.

How often should you read a barometer on a boat?

Every hour on passage, or at least at every watch change, and write each reading in the log so you can see the trend.

Is a phone barometer accurate enough for sailing?

Usually yes for trends. Many phones have a pressure sensor; the absolute figure may be slightly off, but the rate of change is what you need.

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