A synoptic chart is a map of atmospheric pressure at a moment in time. Read it in three passes: find the pressure centres, look at how tightly the isobars are packed, then locate the fronts. Those three readings tell you whether a place is about to get rain, wind, sunshine or fog.
What the lines and symbols mean
Isobars are lines joining points of equal atmospheric pressure, measured in millibars (mb), also written as hectopascals (hPa). They work exactly like contour lines on an Ordnance Survey map — except they join equal pressure rather than equal height, and they move.
Two rules follow immediately, and they generate most of the marks available:
- Closely spaced isobars mean strong winds. A steep pressure gradient over a short distance drives air quickly, just as a steep slope makes a fast descent. Widely spaced isobars mean light winds or calm.
- Wind blows roughly along the isobars, not straight across them. The Earth's rotation deflects moving air, so in the Northern Hemisphere air circulates anticlockwise around a low and clockwise around a high, spiralling slightly inwards to the low and outwards from the high.
Pressure centres are marked L for low and H for high, with the central pressure in millibars. Around 1013 mb is roughly average at sea level, so anything well below is a low and well above is a high.
Fronts are boundaries between air masses of different temperature, drawn as lines with symbols:
| Front | Symbol | What it brings |
|---|---|---|
| Warm front | Semicircles pointing in the direction of travel | Gradual cloud build-up, then steady, prolonged rain; temperature rises as it passes |
| Cold front | Triangles pointing in the direction of travel | Heavy, short-lived downpours, sometimes thunder; temperature drops sharply behind it |
| Occluded front | Alternating semicircles and triangles on the same line | A cold front has caught the warm front, lifting the warm air clear of the ground; cloud and rain, often the end of the system |
Low pressure: the depression
A depression is an area of low pressure. Air at the centre rises. As it rises it cools, water vapour condenses, and cloud and precipitation form. That single chain — rising, cooling, condensing — explains everything a depression does.
Recognise one by tightly packed, roughly circular isobars with a low central value and fronts attached, usually trailing to the south and east of the centre in a distinctive comma shape.
Depressions reaching the UK typically travel from west to east across the Atlantic, so the sequence of weather at any one place follows a predictable order:
- Ahead of the warm front — pressure falling, high wispy cloud thickening and lowering, then steady rain over several hours as the front approaches.
- In the warm sector, between the two fronts — milder, cloudy, often drizzly, with lighter and more variable winds.
- At the cold front — a burst of heavy rain, possibly thunder, then a sharp change.
- Behind the cold front — pressure rising, temperature falling, brighter with scattered showers and gusty winds.
Writing that sequence out, in order, is one of the most reliably rewarded answers in the whole topic.
High pressure: the anticyclone
An anticyclone is an area of high pressure. Air at the centre sinks. Sinking air warms as it descends, cloud evaporates rather than forming, and the weather settles.
Recognise one by widely spaced isobars over a large area, a high central value, and no fronts.
The weather it brings depends on the season, and the seasonal contrast is a favourite exam point:
- Summer anticyclone — clear skies, hot days, light winds, long sunny spells; if it persists, drought and heatwave conditions, and poor air quality in cities because the sinking air traps pollutants.
- Winter anticyclone — clear skies allow heat to escape overnight, so cold, frosty nights, fog forming in valleys and often lingering all day, and calm conditions.
The same pressure system, opposite temperatures, one explanation: clear skies mean strong daytime heating in summer and strong nocturnal cooling in winter.
Reading a chart, step by step
Step 1 — Locate the pressure centres
Find every L and H and read their central values. Note where they sit relative to the UK, and note their direction of travel if arrows are given — systems in this part of the world usually move west to east.
Step 2 — Read the isobar spacing at the place you are asked about
Do not read the spacing at the centre of the system; read it where the question points. Tightly packed means strong winds; widely spaced means light. Then read the actual isobar value nearest that place to give the pressure in millibars.
Step 3 — Work out the wind direction
Trace the isobar through your location and follow the circulation rule: anticlockwise around a low, clockwise around a high, with a slight inward or outward angle. Remember that wind direction is named for where the wind comes from — a westerly blows from the west.
Step 4 — Find the fronts and work out what has passed
Identify each front by its symbol and see which side of it your location sits on. If a warm front has just passed and a cold front is approaching, the place is in the warm sector, and you can describe both the current weather and what is coming next.
Step 5 — Write the prediction as cause and effect
Never simply state the weather. Link it back to the pressure.
Cardiff lies close to the centre of a depression of 984 mb with tightly packed isobars, so winds will be strong. A warm front lies just to the east, meaning the city is in the warm sector, with mild temperatures and cloudy skies. As the cold front approaches from the west, heavy rain is likely, followed by a drop in temperature and brighter, showery conditions.
That answer names the value, uses the isobar spacing, locates the fronts and predicts a sequence — everything the mark scheme is looking for.
Common mistakes
| Mistake | Fix |
|---|---|
| Confusing high and low pressure effects | Anchor to the process: rising air makes cloud, sinking air clears it |
| Reading isobar spacing at the wrong place | Read the spacing at the location the question names |
| Getting the wind circulation backwards | Learn one rule — anticlockwise around a low in the Northern Hemisphere — and derive the other |
| Muddling the front symbols | Triangles are sharp, like the sharp temperature drop at a cold front |
| Naming wind direction by where it is going | Wind is named for where it comes from |
| Describing weather with no explanation | Every statement needs "because the air is rising/sinking" |
Frequently asked questions
What does it mean when isobars are close together?
It means the pressure changes sharply over a short horizontal distance — a steep pressure gradient — and air moves faster in response, so winds are strong. The comparison with contour lines on an OS map is exact: closely spaced contours mean a steep slope, closely spaced isobars mean a steep pressure gradient. This is why depressions bring gales while anticyclones bring calm, since a deep low has tightly packed isobars around it and a high typically has them spread widely across a large area. Always read the spacing at the place the question asks about, not at the centre of the system.
How do I tell a warm front from a cold front?
By the shape of the symbols and by what happens to the temperature. A warm front is drawn with semicircles and brings a gradual change — cloud thickening over hours, then steady, prolonged rain, with temperature rising after it passes. A cold front is drawn with triangles and brings an abrupt change: heavy, sometimes thundery rain over a short period, then a sharp fall in temperature and brighter, showery weather behind. The physical reason is the angle at which the air masses meet — warm air rides gently up over cold air at a warm front, while advancing cold air undercuts warm air steeply at a cold front, forcing it up fast and producing towering cloud.
Why does an anticyclone bring hot weather in summer but freezing fog in winter?
Because in both cases it brings clear skies and light winds, and clear skies have opposite effects in the two seasons. In summer, long days and unbroken sunshine heat the ground strongly, so temperatures climb and can persist for days. In winter, days are short and the same clear skies allow heat radiated from the ground to escape into space overnight without a cloud layer to trap it, so temperatures fall sharply, frost forms, and moist air near the surface cools to its dew point and produces fog. The light winds of an anticyclone then allow that fog to sit undisturbed, sometimes for the whole day.
Do I need to know the actual pressure numbers?
You need to be able to read them off the chart and to know roughly where the dividing line sits. Sea-level pressure averages around 1013 mb, so values well below that indicate a low and values well above indicate a high; deep Atlantic depressions reaching the UK are often below 980 mb, while a strong anticyclone may exceed 1030 mb. You will not be asked to recall specific figures from memory, but you will be expected to quote the value from the chart in front of you, because using the data given is what distinguishes an answer about this chart from a general answer about weather.
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