Weather Fronts and Weather Maps

When two air masses collide

Large bodies of air, called air masses, can cover thousands of kilometres. One might be cold and dry after sitting over the Arctic; another warm and moist after drifting up from the tropics. These air masses do not mix easily. Where they meet, they form a boundary called a weather front — and fronts are where a great deal of our most active weather is born.

The word "front" was borrowed from the battlefronts of the First World War, because forecasters pictured masses of warm and cold air pushing against each other like opposing armies. Understanding fronts, and the maps that show them, lets you forecast the weather yourself.

Why fronts make weather

When two air masses meet, the warmer, lighter air is forced to rise over the cooler, denser air. As that warm air rises, it cools, its water vapour condenses, and clouds and precipitation form. So almost every front comes with a band of cloud and rain. The type of front decides exactly how that weather unfolds. This builds directly on the idea that rising air cools and condenses — the same process behind Types of Clouds.

Warm fronts: a gentle, drawn-out change

A warm front is where advancing warm air rides up over retreating cold air. Because the slope is shallow, the warm air rises gradually over a wide distance.

As a warm front approaches, you typically see:

1. High, wispy cirrus clouds first, hours ahead.
2. Then thickening, lowering layers of cloud.
3. Finally a long spell of steady, light-to-moderate rain.
4. After it passes, the air turns warmer and more humid, with skies often staying dull.

On a weather map, a warm front is drawn as a line with semicircles (traditionally red) pointing the way it is moving.

Cold fronts: a sharp, dramatic change

A cold front is where advancing cold air undercuts warm air and shoves it steeply upward. Because the lift is sudden and strong, the weather change is sharper and briefer.

As a cold front passes you usually get:

1. A short burst of heavy showers or even thunderstorms along the front itself.
2. A noticeable drop in temperature.
3. Often a swing in wind direction and a rise in pressure.
4. Behind it, clearer, cooler, fresher air with bright skies and puffy cumulus clouds.

A cold front is drawn as a line with triangles (traditionally blue) pointing the way it moves. Cold fronts travel faster than warm fronts, which is why they sometimes catch up to form an occluded front.

Reading the rest of the map: isobars and pressure

Fronts are only part of a weather map. The thin curved lines you see are isobars — lines joining places with the same air pressure. They work just like the contour lines on a hiking map, but for pressure instead of height.

Isobars tell you two key things:

• Wind strength. Where isobars are packed close together, pressure changes steeply over a short distance, so winds are strong. Widely spaced isobars mean light winds. (This is the pressure-gradient idea explained in Air Pressure and Wind.)
• Wind direction. Air spirals inward and anticlockwise around a low-pressure centre in the Northern Hemisphere (clockwise in the Southern), and outward and clockwise around a high.

A low (often marked L) is the stormy centre where fronts coil around a rising core of warm air — expect clouds, wind and rain. A high (marked H) is a calm dome of sinking air — expect dry, settled weather.

Putting it together: a typical depression

Many mid-latitude weather systems are depressions (areas of low pressure) with a warm front and a cold front trailing from them. As one passes over you in a single day, you might experience: high cirrus, then steady rain (warm front), then a milder humid spell (warm sector), then a band of heavy showers (cold front), then cool, bright, breezy air behind. Once you recognise this sequence, a glance at the map tells you the story of the day ahead.

Activity: forecast from a real weather map

Put your new skills to the test with this weather-watch challenge.

1. Find today's weather map (a "surface pressure chart" or "synoptic chart") from a national weather service online.
2. Locate your area. Identify the nearest L and H centres and any fronts crossing toward you. Note whether the isobars near you are tightly packed (windy) or spread out (calm).
3. Based on the symbols, write a short forecast for the next 24 hours: Will a warm or cold front arrive? Will it be wet then clear, or calm and dry? Will winds be strong?
4. The next day, compare your forecast with what actually happened, and with the official forecast.

Keep this up for a week or two. You will quickly learn to read the sky and the map together — exactly how a meteorologist works. To understand the bigger high-altitude winds that steer these systems, read The Jet Stream.