METEOROLOGY · Lesson 1 of 6
Air Pressure and Weather Systems
25 min · Beginner friendly
You’ll learn toRelate isobar spacing to wind strength · Describe weather typical of high and low pressure · Apply Buys Ballot's law to locate a depression
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Every sailing decision — when to leave, which way to go, whether to go at all — rests on one question: what is the weather about to do? Weather over the sea is not random. It is driven by differences in air pressure, and those differences are measured, mapped and forecast with remarkable skill. Learn to read pressure and you learn to read the weather.
Take a guess
Take a guess before the lesson: tomorrow's weather map shows the lines of equal pressure packed very close together over your sea area. Windy or calm?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
Pressure, isobars and wind
Air has weight. The push of the atmosphere on the surface is , averaging about 1013 hectopascals (hPa) at sea level — the older term millibars is the same unit. The sun heats the Earth unevenly, so pressure varies from place to place: where air warms and rises, pressure falls; where air cools and sinks, it rises. Air then flows from high pressure towards low, like water running downhill — and that flow is wind. The bigger the pressure difference over a given distance, the stronger the wind. This single idea unlocks almost everything else in this module.
- Atmospheric pressure
- The weight of the air pressing down on the surface, measured in hectopascals (hPa). One hectopascal equals one millibar.
- Isobar
- A line on a weather map joining places with equal pressure, usually drawn every 4 hPa. Think of them as contour lines for the atmosphere.
- Pressure gradient
- How quickly pressure changes with distance. A steep gradient (isobars close together) means strong wind.
- Barometer
- An instrument that measures atmospheric pressure. On a boat, its trend over time matters far more than any single reading.
Knowledge check
On a synoptic chart, the isobars over your sea area are packed very close together. What should you expect?
Highs and lows
A high (or anticyclone) is a region of slowly sinking air: sinking air warms and dries, cloud is suppressed, and the weather is settled. A low (or depression) is a region of rising air: rising air cools, its moisture condenses, and the result is cloud, rain and wind. Most of the changeable weather of north-west Europe arrives as a conveyor belt of depressions rolling in from the Atlantic.
| Feature | High (anticyclone) | Low (depression) |
|---|---|---|
| Air movement | Sinking, spreading out | Rising, drawing air in |
| Typical weather | Settled: sunshine or persistent gloom, light winds | Unsettled: cloud, rain, strong winds |
| Wind circulation | Clockwise, spiralling gently outwards | Anticlockwise, spiralling inwards |
| Speed of movement | Slow-moving, can sit for days or weeks | Fast-moving, often crosses in 24 to 48 hours |
| Barometer | High and steady | Low and often falling fast |
Why does the wind circle the systems instead of blowing straight into the low? Because the Earth rotates: moving air is deflected to the right in the northern hemisphere (the Coriolis effect), so it ends up flowing almost to the isobars. Surface friction slows it a little and lets it lean roughly 15 degrees in across the isobars towards the low.
Highs have quirks. In winter they can trap cold, grey, damp air for days (anticyclonic gloom), and their clear, calm nights are the classic recipe for in harbours — more in the lesson. Because highs move slowly, whatever they bring tends to last; depressions cross in a day or two, so their weather changes by the hour.
Knowledge check
You have three days off, and a large anticyclone has parked itself over your cruising area. What sailing weather do you plan around?
Buys Ballot's law
In the northern hemisphere, with your back to the wind and the centre of low pressure is on your left hand, slightly forward — a rule published by the Dutch meteorologist Buys Ballot in 1857. It works because wind circulates anticlockwise around a low: wherever you stand on that spiral, the centre sits to the left of the airflow.
Knowledge check
Standing on deck in the northern hemisphere with your back to the wind, where is the centre of low pressure?
Why care where the centre is? Depressions generally travel west to east across UK waters, so fixing the centre tells you whether the system — its fronts, wind shifts and worst weather — is coming at you or sliding past. It also lets you sanity-check the forecast against what you feel on deck.
Worked example
Locating a depression from the cockpit
You are day-sailing off the fictional of Westhaven. The forecast mentioned a low approaching, and you want to know roughly where its centre is right now. The wind on deck is a steady south-westerly.
- A south-westerly wind blows from the south-west, so turn until it is on your back: you are now facing north-east.
- Stretch out your left arm. Facing north-east, your left hand points north-west.
- Apply Buys Ballot: the low centre lies roughly where your left hand points, a little forward of abeam — so to the north-west of you.
- Depressions usually track eastwards, so a centre to your north-west is likely to pass to the north of you — expect the wind to veer (shift clockwise) as it goes by.
The depression centre bears roughly north-west of your position.
Work it out
You are at anchor and the wind is a steady northerly (blowing from the north). Using Buys Ballot's law, in roughly which direction does the centre of low pressure lie?
As a three-figure bearing — due east is 090. Roughly is fine.
Key points
- Pressure is measured in hectopascals (millibars); sea-level average is about 1013 hPa.
- Wind is air flowing from high pressure towards low, bent by the Earth's rotation to run almost along the isobars.
- Closely spaced isobars = steep = strong wind; wide spacing = light wind.
- Highs bring settled weather and circulate clockwise; lows bring wind and rain and circulate anticlockwise (northern hemisphere).
- Buys Ballot's law: back to the wind, the low is on your left (northern hemisphere).
- Watch the barometer trend: a fall of more than 3 hPa in 3 hours warns of strong winds.
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