Occluded front

When occlusion is called an operation in a dynamic area of ​​low pressure in which the warm sector is lifted off the ground by union of two fronts in meteorology. For dynamic low-pressure areas in middle latitudes, called the Ferrel cell or west wind zone, it comes in the wake of cyclogenesis to form two fronts, the warm front and cold front. Here, both fronts rotate with the low pressure area, ie counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

Okklusionstypen

There are two basic types of occlusion, the Kaltfrontokklusion and Warmfrontokklusion. In both cases, a warm front (red line) is overtaken by a faster cold front ( blue line). The point at which they unite in an occlusion is called the point of occlusion; from this point there is only a front which occlusion ( purple line ). Near the Okklusionskern prevail weak circulating winds. In the edge region of the core of the wind can take storm strength. Only at a greater distance, the wind speed drops again.

When Kaltfrontokklusion the air behind the cold front is colder than the air in front of the warm front. Thus both of the air masses are lifted prior to the cold front, since they have the higher temperature, the lower density. The warm front is " rolled over " on the surface weather map and forms to the line formed by occlusion and cold front, a kink.

Conversely, in the Warmfrontokklusion: Here, the warm front very cold air behind the cold front only cool air. These slides on to the cold air at point of occlusion as the warm front. The front line of the warm front is not changed on the map, therefore, occlusion and warm front form a continuous line. By the cold air in front of the front passage also of the bottom is cooled by the convection behind the occlusion is small, and there are only a few cumulus clouds new.

The cold temperatures of both sectors ( here: dark blue = cold, light blue = cool ) differ only slightly, compared to the warm sector are both colder. Nevertheless, the difference in density is usually sufficient even with a small temperature difference to be dominated by the occlusion of one of the fronts. If the temperature difference is too small or not there can occasionally be found such dominance. It then also takes no conceptual distinction more. This phenomenon can also be regarded as a third Okklusionstyp.

In mid-latitudes Kaltfrontokklusionen occur preferentially in the summer months and Warmfrontokklusionen preferably in the winter months.

Image description

In the right graph, the previously mentioned concepts are presented along with typical types of clouds and precipitation events.

The first field shows the same surface weather map with a total area of ​​low pressure. A warm front is represented here by half circles on the front line, a cold front by triangles and occlusion by triangles and half circles side by side. The orange horizontal dashes mark certain areas of the low pressure system, which are shown in more detail in the following sub-images. It must be noted that a low pressure area has the shape shown only in the mid-latitudes and even in the northern hemisphere.

The second subplot shows a section through the low pressure area in line (a). It represents the occlusion, ie the union of the two fronts.

The third subplot shows a section through the area of ​​low pressure in line (b). Here, the fronts are still separated. So this is the state before occurrence of occlusion.

The corresponding temperature profile in the fourth field indicates the type of occlusion.

• Pressure areas and fronts