The big patterns on a weather chart, the highs and lows with their families of ridges and troughs, set the scene for everything else. Before the fronts and the showers, it is the pressure systems that decide whether a day is settled or stormy and which way the wind will blow. Reading them is the first step in reading the weather.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Pressure, isobars and the map
A surface pressure chart draws lines of equal pressure called isobars, the contour lines of the atmosphere, and the pattern they make is the pressure systems. Where the isobars close into a centre of low pressure you have a low or depression; where they close into a centre of high pressure you have a high or anticyclone. The spacing of the isobars is as important as the shape: closely packed isobars mean a steep pressure gradient and strong winds, while widely spaced isobars mean a slack gradient and light winds. Our guide to reading a surface analysis chart covers the chart itself; this guide is about what the systems on it do.
Low pressure: the unsettled half
A low is an area of low pressure into which surface air converges and is forced to rise. Rising air cools, its moisture condenses, and the result is cloud, precipitation and unsettled, often windy weather. Depressions are where the active weather lives, and they usually carry fronts, the warm and cold boundaries covered in our guide to air masses and fronts, spiralling out from the centre. Because the pressure gradient around a vigorous low is steep, the winds are strong, and the closer the isobars, the stronger they blow.
The air does not flow straight into the low; the Earth's rotation deflects it, so it circulates around the centre. In the northern hemisphere the wind blows anticlockwise around a low (clockwise in the southern), spiralling slightly inward. That rotation is why a depression appears as a great swirl of cloud on a satellite picture.
High pressure: settled, but not always clear
A high is the opposite: an area of high pressure where air descends, or subsides. Sinking air warms and dries, which suppresses cloud, so a high usually brings settled weather, clear skies and light winds, the fine, calm days of an anticyclone. The winds circulate the other way, clockwise around a high in the northern hemisphere (anticlockwise in the southern), spiralling slightly outward.
But "settled" is not the same as "clear", and this is the trap with highs. The descending air can form an inversion near the surface, trapping moisture and pollution beneath it, so especially in the cooler months a high can give persistent low cloud, mist, fog and poor visibility, the dreary spell sometimes called anticyclonic gloom. Our guide to temperature inversions and stable air explains the mechanism. A high promises calm winds; it does not always promise good visibility.
Ridges and troughs
Between the closed centres, the isobars bulge into elongated extensions, and these have their own names and weather:
- a ridge is an elongated extension of high pressure, a finger of high reaching out from an anticyclone, and it brings a spell of fair, settled weather as it passes, a brief improvement;
- a trough is an elongated extension of low pressure, and it brings a band of unsettled weather, cloud, showers, or a front, as it goes through, a brief deterioration.
Reading a ridge or trough on the chart tells you about the timing of a change: a ridge building in promises a fine interlude, a trough approaching warns of a passing band of weather even without a fully formed depression.
Cols and the slack between systems
Between two highs and two lows there is sometimes a region of slack pressure called a col, where the isobars are widely spaced and the gradient is weak. Winds are light and variable in a col, and the weather is whatever the local conditions make of it: fog and low cloud can form readily in the calm, or it can simply be a quiet, featureless patch between the systems. A col is the pause between the active weather, and its lightness of wind is its defining feature.
Wind, the gradient and Buys Ballot's law
The link between the pressure pattern and the wind is direct: the wind blows roughly along the isobars, not across them, deflected by the Earth's rotation, and its strength follows the gradient, the isobar spacing. A handy rule ties it together. Buys Ballot's law says that in the northern hemisphere, if you stand with your back to the wind, the low pressure is on your left (and the high on your right); in the southern hemisphere it is reversed. So the wind itself tells you where the systems lie, and the chart's isobars tell you how hard it will blow.
Systems move and develop
A pressure chart is a snapshot, and the systems on it are alive: they move, and they grow and decay. A low can deepen (its central pressure falling, its winds and weather intensifying) or fill (weakening and dying), while a high can build (strengthening and expanding) or decline. They are steered by the flow in the upper atmosphere, the broad pattern aloft and the jet stream, which acts as a kind of conveyor for the surface systems beneath it.
For a pilot, this means the timing matters as much as the pattern. A low that is deepening and racing toward you is a very different proposition from one that is filling and slipping away, even if the snapshot looks similar, and a ridge building in promises a longer fine spell than one already breaking down. Reading a sequence of charts, not just one, tells you which way the systems are trending, so you can judge whether the weather on your route will be improving or deteriorating by the time you get there. The forecast charts add the missing dimension to the snapshot: where the systems will be, and how strong, at the hour you fly.
A worked example
You compare two charts. On the first, a deep low sits to your northwest with tightly packed isobars sweeping across your route and a cold front trailing from it. You read the picture at once: strong winds circulating anticlockwise around the low, unsettled weather with the front's cloud and rain, and, by Buys Ballot's law, with your back to the strong southwesterly the low is on your left where the chart shows it. It is a day for strong-wind and frontal-weather planning.
On the second chart a broad high sits over your area with widely spaced isobars. You read light winds, circulating clockwise around the high, and settled conditions, but because it is a cool, damp season you flag the risk of an inversion trapping fog and low cloud beneath the subsiding air, the anticyclonic-gloom trap. A ridge extending from the high promises a fair interlude, while a trough marked further out warns of a band of weather to come. From two pressure patterns you have read the wind, the weather and the timing of the changes, before looking at a single METAR.
Common pitfalls
- Equating high pressure with good flying weather. A high is settled but can trap fog and low cloud under an inversion.
- Ignoring the isobar spacing. The gradient, not just the system, sets the wind strength; tight isobars mean strong winds.
- Getting the circulation backwards. In the northern hemisphere it is anticlockwise around a low and clockwise around a high, reversed in the south.
- Overlooking ridges and troughs. They mark the brief improvements and deteriorations between the main systems.
- Forgetting the col. Slack pressure means light, variable winds and a tendency to fog and low cloud in the calm.
In Pilot EFB
Pilot EFB is a study and planning companion that helps you make sense of the pressure pattern behind a forecast, alongside the decoded METAR, the surface analysis chart and the rest of your briefing in one offline-first place. A briefing you have already pulled stays readable away from a signal, and pulling fresh weather needs a connection. Pilot EFB does not forecast the weather and is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.