Understanding weather fronts: warm front and cold front

An air mass forms when air sits over a region for a long time and takes on its characteristics: over warm seas it becomes warm and moist, over snow-covered land cold and dry. When such an air mass moves on, it takes its characteristics with it. The boundary zone with another air mass is called a front.

At every front essentially the same thing happens: the warmer, lighter air is lifted. As it rises it expands, because air pressure is lower higher up, and cools as it does so. Cold air can hold less water vapour than warm air. The surplus vapour condenses into tiny droplets or ice crystals – clouds form, and eventually precipitation.

The difference between warm and cold fronts lies in the slope of the boundary. At a warm front it is gentle, the air rises slowly over a long distance, and extensive layered clouds form. At a cold front it is steep, the air is lifted quickly and forcefully, and tall heaped clouds grow, up to thunderclouds.

A warm front often announces itself many hours ahead. First, high, wispy feathery clouds (cirrus) appear. Then they thicken into a milky veil through which the sun or moon sometimes shines with a ring, known as a halo. After that the cloud base lowers, turns grey and dense, and steady, long-lasting rain begins. Air pressure usually falls steadily during this time.

A cold front is shorter and more violent. Tall, sharply outlined heaped clouds approach, the sky darkens, the wind freshens with gusts and changes direction. Heavy showers follow, sometimes thunderstorms or hail. After it passes it becomes noticeably cooler, air pressure rises again, and it often clears with good visibility.

A barometer – or the altimeter on a watch, as long as you stay in the same place – can show falling air pressure. A clear, sustained drop in pressure is a sign that the weather is getting worse. It does not, however, tell you exactly when or how badly.

Knowledge of fronts helps above all with understanding forecasts and weather maps. If the forecast says “cold front passing in the afternoon”, you know to expect gusts, showers and possibly thunderstorms – and that it will be cooler afterwards. On the move, you can tell whether the forecast's timing is right or whether the weather is arriving earlier.

The frontal model is a simplification, though. Mountains, coasts and lakes create their own local weather that can mask the typical signs. In the mountains, thunderstorms and fog can develop very quickly even without a recognisable front. Your own observation therefore never replaces the official forecast and the weather services' warnings.

Before every trip, check the current forecast and the severe-weather warnings from the national weather service, ideally on the same day and not just the evening before. In the mountains, set a fixed turnaround time and stick to it, even if the goal seems close.

Approaching high cloud is no reason to panic, but it is a reason to pay attention and review your plan. When dark heaped clouds approach, the wind freshens and you hear the first thunder, it is time to seek shelter – details are in the entry on lightning safety outdoors.

Cold fronts often move faster than warm fronts and eventually catch up with them. The warm air between them is then lifted completely off the ground. This is called an occlusion; the weather there shows a mix of features of both types of front.

Many old weather sayings describe exactly these sequences, for example that a ring around the moon can announce rain. Some sayings are right more often, others hardly at all. They are nice to know, but not reliable enough to base a safety decision on.