What is in front of the purple in the picture? I haven't seen it before. Meteorological knowledge.

On the weather map, the purple front is blocked.

Blocking front is a kind of front, which is formed when a cold front catches up with a warm front, or when two cold fronts meet head-on (or when two sections of the same cold front meet head-on). The formation of blocking front often occurs in the mature stage of temperate cyclones, usually in the middle and high latitudes. The occlusion front is displayed as follows:

There are three main conditions for the formation of occlusion front:

(1) The terrain in front is blocked by mountains;

(2) Two cold fronts meet head-on;

(3) The cold front in the extratropical cyclone catches up with the warm front.

In these cases, the warm air in front of the cold front is lifted off the ground and trapped in the air. Therefore, the interface between the cold air mass behind the cold front and the cold air mass in front of the warm front is called the blocking front. Generally speaking, bad weather prevails near the blocking front, and the precipitation area is located between the front and the 700 HPA trough line.

Occlusion fronts can be divided into three types:

(1) The cold air mass before the warm front is colder than that after the cold front.

(2) The cold air mass behind the cold front is colder than that before the warm front.

(3) There is little difference in air mass properties between neutral blocking front and blocking front.

Cloud system characteristics of blocking front;

(1) The sheltered frontal cloud system is a cloud belt about 300 kilometers wide. From the top of the warm zone to the vortex center, the spiral cloud system must coincide with the atmospheric circulation center.

(2) At the back of the sheltered frontal cloud system, due to the invasion of cold air, a tongue-shaped black cloud-free area with neat and smooth back boundary is formed, which is called dry tongue. The shape and amplitude of the tongue stem often indicate the activity of cold air.

(3) Occlusion front cloud belt is often characterized by dark hue along spiral cloud belt, and the cloud height at spiral center is the lowest.