When air masses meet

When a moving cold air mass meets a warm air mass, that is lighter, it tends to wedge below the latter, thus giving origin to a cold front. The warm air is forced upwards and its ascent causes the formation of clouds.  Since the surface of contact between the two masses is quite steep and ascent is rapid, the clouds will be prevalently of the cumuliform type. The passage of a cold front is accompanied by widespread cloud systems and precipitations, with a sharp drop in the temperature, an increase in the pressure and precipitations  often characterized by thunderstorms.
On the weather charts, cold fronts are indicated using a black line with triangles. Cold fronts form typically at our latitudes, when the cold dry air from the Polar regions meets the humid air coming from the tropical regions.
Warm fronts
If, on the contrary the  warm air mass is moving towards the cold air mass, the warm air mass  slowly slips over the cold one, and rises along a vast gently sloping surface: in this case  we can speak of a warm front which also brings clouds and perturbation systems. The margins of a warm front are less marked that those of a cold front, changes are more gradual and the perturbations arrive less rapidly. The passage of a warm front is marked by a rise in the temperatures, a decrease in pressure and persistent rains, which are however of a moderate intensity. Since the clouds rise slowly,  the cloud system that forms generally consists of stratified clouds.  On the weather charts, warm fronts are indicated with a black line with semicircles.
Occluded fronts
An occluded front forms when a cold front reaches a warm front, forcing all the warm air to rise to higher altitudes and the cold air is stratified near the ground. At our latitudes,  over the Atlantic, cyclonic areas form continuously, these are fed by the Azores anticyclone and the Polar anticyclone. Here, the tropical warm and humid air and the cold dry polar air meet, and generally a depression vortex is formed in which a warm front and a cold front are active. The cold front is generally more active and advances more rapidly than the warm front. When the cold front reaches the warm front an occluded front is originated. After having discharged the remaining humidity on to the occluded front, the weather generally returns to good weather,  the air on the ground starts to warm again: and the perturbation is exhausted.
On the weather charts occluded fronts are indicated with a black line with alternating triangles and semicircles.
The presence of mountain ranges on the course of a front can provoke variations and deformations of various types, that can lead to evolutions in a perturbation that are difficult to forecast.

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