Air pressure, the barometer and wind
1The lesson in plain words
Differences in air pressure make the wind. The barometer's trend, not its number, is your best early warning.
What pressure is
Air pressure is the weight of air pressing on each unit area. Forecasts give it in hectopascals (hPa); one hPa equals one millibar (mb). The standard sea-level value is 1013.25 hPa. Real sea-level pressure usually lies between about 950 and 1050 hPa. On a chart, an isobar is a line joining places of equal pressure.
The barometer
On board you use an aneroid barometer, a sealed metal capsule that flexes as pressure changes, or an electronic one. Set it against a nearby official sea-level reading. Then log it regularly, at least every three hours at sea, and more often when it is falling.
Tendency matters
The words on an old dial, "rain" or "fair", are unreliable. What matters is the tendency, the change over the last three hours. Met Office terms: slowly 0.1–1.5 hPa, plain rising or falling 1.6–3.5 hPa, quickly 3.6–6 hPa, very rapidly over 6 hPa. A quick fall warns of strengthening wind; check the forecast at once.
Why the wind blows
A pressure gradient is a change in pressure over distance. A steeper gradient generally drives stronger wind. Air is pushed from high towards low pressure. Earth's rotation, the Coriolis effect, turns moving air right in the Northern Hemisphere and left in the Southern. Northern Hemisphere winds circle lows anticlockwise and highs clockwise; both reverse in the south.
Buys Ballot's law
In the Northern Hemisphere, stand with your back to the wind: low pressure is on your left. At the surface, friction turns the wind a little towards the low, so the low is slightly ahead of your left side. In the Southern Hemisphere, low pressure is on your right. The rule fails near the equator and in local winds.
2Key data
- Standard sea-level pressure
- 1013.25 hPa; 1 hPa = 1 mb
- Falling slowly
- 0.1–1.5 hPa in 3 hours
- Falling quickly
- 3.6–6 hPa in 3 hours
- Falling very rapidly
- More than 6 hPa in 3 hours
- Northern Hemisphere circulation
- Anticlockwise round a low, clockwise round a high
- Buys Ballot (north)
- Back to the wind: low pressure on your left
3Key points
- 1
Pressure is given in hPa (= mb); standard is 1013.25 hPa.
- 2
Log the barometer; the 3-hour tendency is what counts.
- 3
A quick fall, over 3.5 hPa in 3 hours, warns of stronger wind.
- 4
North: wind goes anticlockwise round lows, clockwise round highs.
- 5
Back to the wind in the north: the low is on your left.
4Frequent mistakes
Wrong: A reading of 1000 hPa always means bad weather.
Right: The absolute value means little on its own. Watch the trend: a steady 1000 hPa can be quiet, a quick fall from 1020 hPa can bring a gale.
Wrong: Wind blows straight from high to low pressure.
Right: The Earth's rotation turns it, so it blows round the systems, crossing the isobars only slightly at the surface.
Wrong: Buys Ballot's law is the same in both hemispheres.
Right: In the south it is reversed: back to the wind, the low is on your right.
5Practice case & exam keys
At 0900 your barometer reads 1012 hPa, at 1200 it reads 1007 hPa. You are in the Mediterranean with your back to a south-easterly wind. What do you learn?
Show the answer
A fall of 5 hPa in 3 hours is "falling quickly": the wind is likely to strengthen soon. With your back to a south-easterly, you face north-west. The low is roughly south-west of you, slightly ahead of your left side. Check the latest forecast and warnings now, and head for shelter if they confirm stronger wind.
Exam keys
- Standard pressure 1013.25 hPa; isobars join equal pressure.
- Tendency over 3 hours is the useful barometer reading.
- Buys Ballot: Northern Hemisphere, back to the wind, low on the left; reversed in the south.
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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