What is quasi-stationary frontal weather?

Cold and warm air masses on both sides of a quasi-stationary front often form a "confrontation" state. The warm air mass advances and is blocked by the cold air mass. The warm air mass is forced to slide along the front. The situation is similar to that of a warm front. The cloud system that appears is similar to that of a warm front. Warm front cloud systems are roughly the same. Since the slope of a quasi-stationary front is smaller than that of a warm front, the warm air sliding along the front can extend far away from the front, so the cloud area and precipitation area are wider than those of a warm front. However, the precipitation intensity is small and lasts for a long time, which may cause continuous rainy weather with "very heavy rain and continuous rain".

Quasi-stationary front weather is generally divided into two categories: one is that cloud systems develop on the front and there is obvious precipitation. For example, quasi-stationary fronts in South China mostly evolve due to the weakening of cold fronts. The weather is similar to the first type of cold front, except that the front slope is smaller and the cloud area and precipitation area are wider. The precipitation area is not limited to the frontal area. Extending to a large range behind the front, the precipitation intensity is relatively small and it is continuous precipitation. Because the quasi-stationary front moves slowly and often swings back and forth, the rainy weather lasts from ten days to half a month, or even more than a month. "Rains abound during the Qingming Festival" is a reflection of this kind of weather in the Jiangnan region. This kind of rainy weather will not change until the quasi-stationary front turns into a cold front or the warm front moves out of the area or the front disappears. In early summer, if the humidity of the warm air mass increases and the lower levels heat up, the air layer may become unstable, and cumulonimbus clouds and thundershowers may also form on the front.

The other type is a quasi-stationary front where the main cloud system develops under the front and there is no obvious precipitation. For example, the Kunming quasi-stationary front is a cold air moving southward that is blocked by mountains and becomes stationary. The warm air above the front is dry and slides slowly, unable to produce large-scale cloud systems and precipitation, while the cold air below the front slides along the slopes. Mixed with turbulent flow, less thick nimbostratus clouds can be formed under the front, often accompanied by continuous precipitation.

China's quasi-stationary fronts mainly appear in South China, southwest China and the north side of the Tianshan Mountains, mostly in the winter half of the year, and have a great impact on the weather in these areas and nearby areas.