Using the barometer at sea
1The lesson in plain words
A barometer only forecasts when you read it at fixed times, work out the 3-hour change and act on it. Make the readings a habit, not an afterthought.
Trust the trend
Pressure systems taught you the tendency words, from falling slowly to falling very rapidly. Here you put them to work. A single reading tells you little; the change since your last readings tells you what the weather is doing now. Read at fixed times, at least every 3 hours on passage and every hour when the pressure falls. Always use the same instrument, so its own small error cancels out.
Check your instrument
Before passage, compare your reading with a nearby official sea-level pressure for the same time. Its index error is your reading minus the official value; subtract that error from each reading. Height matters little on a yacht: pressure falls about 1 hPa for every 8 m you rise, so a cabin barometer 2.5 m above the sea reads about 0.3 hPa low. Re-check after big temperature changes or a knock.
Work out the tendency
Take the latest reading minus the reading 3 hours earlier. Example: 1011.4 hPa at 1200 and 1007.6 hPa at 1500 is a fall of 3.8 hPa: falling quickly. At 1800 it reads 1002.9 hPa: another 4.7 hPa. The fall is accelerating: compare it with the wind, sky and latest forecast. Write the sign (+ or −) and the tendency word in your log every time.
Look at the shape
The characteristic of the tendency is its shape over the 3 hours: falling steadily, falling then steady, or falling then rising. A fall that slows and stops often marks a warm front passing. A fall that turns quickly into a sharp rise usually marks a cold front or trough. A quick rise after a low often brings strong, gusty wind behind it.
Electronic barometers
Many plotters and instrument displays log pressure and draw a graph. Set an alarm for a fall bigger than you are willing to ignore, for example more than 3.5 hPa in 3 hours. Check that the logged history survives a power cut. Keep a simple aneroid or handheld backup and write readings down, because a screen graph is lost when the electrics fail.
2Key data
- Tendency
- Latest reading − reading 3 hours earlier, with its sign
- Reading routine
- At least 3-hourly on passage; hourly when falling
- Height effect near sea level
- About 1 hPa per 8 m of height
- Worked example
- 1011.4 → 1007.6 hPa in 3 h = −3.8 hPa, falling quickly
- Spanish term
- Tendencia barométrica; barómetro aneroide
3Key points
- 1
Read the barometer at fixed times and log every reading with its time.
- 2
Tendency = latest reading minus the reading 3 hours earlier.
- 3
An accelerating fall is a warning to check other weather signs and the latest forecast.
- 4
A fall turning into a sharp rise usually marks a cold front or trough.
- 5
Know your instrument's index error and keep a written backup log.
4Frequent mistakes
Wrong: Read the barometer only when the weather starts to look bad.
Right: Read it at fixed times all passage. Without earlier readings you cannot work out a tendency when you need it.
Wrong: Correct a yacht's barometer for height before every reading.
Right: On a yacht the height effect is a few tenths of a hectopascal. Apply the known index error; the tendency matters far more.
Wrong: The first rise after a low means the bad weather is over.
Right: A quick rise behind a low or cold front often comes with strong, gusty wind. Keep the reefs in until the wind eases.
5Practice case & exam keys
On passage your log shows 1016.0 hPa at 0600, 1014.2 hPa at 0900 and 1010.1 hPa at 1200. The morning forecast gave south-west force 4 to 5, increasing 6 later. What do you conclude?
Show the answer
The two 3-hour tendencies are −1.8 hPa (falling) and −4.1 hPa (falling quickly). The fall is accelerating, but these readings alone do not show that force 6 will arrive early. Get the latest forecast and warnings, prepare for the forecast increase, and change to hourly readings.
Exam keys
- Tendency = change over the last 3 hours, written with its sign.
- Read at fixed times; hourly when the pressure is falling.
- Apply the instrument's index error; a yacht's height effect is negligible.
- A sharp rise after a fall can mark a cold front or trough; gusty wind may follow.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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