Air masses and fronts

Air takes on the character of wherever it sat still for long enough. Weather happens where two of them meet — and the steepness of that boundary decides what you will fly through.

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Air masses

An air mass is a large body of air with roughly uniform temperature and humidity. It gets those properties by sitting over a source region long enough to take on its character — and it is classified by exactly that.

Temperature gives you arctic (extremely cold and deep, mostly a winter phenomenon over ice and snow), polar (cooler, shallower, high latitudes) and tropical (warm to hot, low latitudes). Moisture gives you continental (dry, formed over land) and maritime (moist, formed over water).

Maritime arctic seldom if ever forms. And an air mass changes as it travels — continental polar air crossing the Great Lakes in winter picks up moisture and becomes something quite different by the time it reaches the far shore.

Finding a front without a chart

A front is the boundary between two air masses, and it is named for whichever air mass is doing the replacing. Three surface signatures give it away:

• A significant temperature gradient — the air mass either side is different, which is the whole definition.
• Converging winds — the wind shifts across the boundary.
• Pressure falling as it approaches and rising after it passes. This is why a falling altimeter setting is worth more than it looks, and why the pressure trend rankings find fronts hours before a ceiling ranking does.

How each one is drawn

The symbols are the chart's grammar, and they are readable once you know that the pips point the way the front is moving. Blue triangles for cold, red semicircles for warm; a stationary front has both, on opposite sides, because neither air mass is winning; an occluded front has both on the same side.

Slope is the whole story

Fronts have vertical structure, and the steepness of that structure decides what the weather looks like.

A cold front has a steep slope. Warm air is forced up abruptly, so if it is unstable you get a narrow band of showers and thunderstorms along or just ahead of the front. Violent, and over quickly.

A warm front has a gentle slope. Warm air glides up the frontal surface gradually, so if it is stable you get widespread layered cloud and precipitation spread out ahead of the front. Benign-looking, and it goes on for hours — which is its own hazard, because it produces long exposure to low ceilings and, in the cold season, to icing.

A stationary front can have any slope; cloud and precipitation still form in the warm air rising along it, and the whole thing stays put.

An occluded front forms because cold fronts typically move faster than warm ones and eventually catch up. The cold air undercuts the cooler air ahead of it, lifting the already-rising warm air further. The result is cloud and precipitation along, ahead of and behind the surface position — the one front where being past it does not mean being clear of it.

What is on the chart right now

The Weather Prediction Center redraws the surface analysis every three hours. Here is what it currently carries over the United States — the counts are worth a moment on their own, because the mix of frontal types is a fair summary of what kind of day it is across the country.

You will usually see troughs outnumber everything else, and they are counted separately above on purpose: a trough is an elongated area of low pressure with no air-mass boundary across it. It is not a front, it gets no pips or scallops, and calling it one is a common slip. It still marks convergence and therefore lift, so weather often forms along it — which is exactly why it is easy to mistake for a front.

The chart itself draws each front with the pips and scallops you learn to read; this is a summary of the same data, and the EFB plots the real thing.

Where you meet it next

• Stability — why the same front gives showers in one air mass and drizzle in another
• Thunderstorms — the cold-front case at its worst
• Icing — the warm-front case at its worst
• Live pressure trends — fronts, found by falling altimeters

Sources

Air mass classification and frontal structure are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28). The live analysis is the Weather Prediction Center surface chart.