In meteorology, a front refers to an air mass boundary. In the case of a cold front, a colder air mass displaces a warmer one, and vice versa in the case of a warm front. The third form is a mixture of these two, known as a mixed front or occlusion. In this article, however, we will focus on the rarest variant here – the warm front.
Sliding warm air
In a warm front, warmer air pushes towards a colder air mass. However, as the warmer air is lighter, it slides onto the colder air mass and is lifted in the process. The slip angle is relatively flat with an inclination of 1:100 to 1:400, forming a horizontally extensive slip surface that can extend over several hundred to a thousand kilometers. Clouds form in this area. High veil clouds (cirrus, especially cirrostratus) form far ahead of the front.
Fig. 1: Cirrostratus, flat veil clouds of ice crystals; Source: Wikipedia
The air pressure slowly begins to drop as more and more of the relatively heavier cold air is replaced by lighter warm air in the column of air above the observation point. As the front approaches, the cloud cover thickens and the cloud base drops. At first, the sun still shines through, but the sky becomes increasingly milky and the light more diffuse. Soon altostratus sets in, a flat, medium-high cloud cover with only blurred contours. As a rule, the sun barely gets through here. As the weather progresses, the mountains become increasingly cloudy from above and the first unproductive precipitation appears. In the run-up to the actual ground front, nimbostratus finally follows, a vertically powerful cloud form with a low lower boundary. The precipitation spreads and also increases in intensity. The air pressure drops further and the wind picks up. While cold fronts often show sharp and defined rain bands on the precipitation radar, well-developed warm fronts produce a large precipitation carpet.
Fig. 2: Schematic representation of a warm front.; Source: MeteoNews
Land rain or snow?
There are some special forms of precipitation depending on the season. In the summer months, a warm front brings prolonged and sometimes heavy land rain with a very high snow line. If the warm air is unstable, embedded thunderstorms are also possible. Paradoxically, in winter, warm fronts can sometimes bring considerably more snow than some cold fronts. This is partly due to the fact that cold fronts move faster – the whole thing tends to happen according to the motto "short and heavy". Warm fronts move more slowly and the precipitation lasts longer. After a calm winter high-pressure phase with little wind, there is usually still cold air in the valleys and basins. If warmer air now rises at altitude, it can snow into this old cold air for a long time. In some cases, the vertical temperature distribution is almost isothermal (e.g. 0 degrees at ground level, 0 degrees at 1000 meters and 0 degrees at 2000 meters). If the temperatures continue to rise at altitude and the freshening wind gradually begins to mix the lower levels and clear out the cold air, the snow line sometimes rises sharply. Warm fronts in winter can also be the trigger for freezing rain or sleet (if it was cold enough beforehand).
Warm fronts on the weather map
On an isobar or weather map, the warm front is represented by red lines and semicircles. As mentioned above, warm fronts move much more slowly than cold fronts (blue). The latter therefore eventually catches up with the warm front, forming a mixed front or occlusion (purple). The area between the warm and cold fronts is called the warm air sector. As a rule, the low-pressure areas and their fronts do not form near us, but over the North Atlantic, for example. When the frontal systems finally reach the Alpine region, they have already had some time to develop. They are usually cold fronts or occlusions; well-developed warm fronts are the rarest type of front in comparison.
Fig. 3: Isobar and front map for January 24, 2024, 1 p.m.; Source: MeteoNews
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