Why does it rain when a cold front crosses the border?

Reason: Precipitation needs air to rise and meet cold, and then precipitation is formed. When a warm front meets cold air in motion, it will rise actively, forming conditions for precipitation, so there will be precipitation on the moving front. On the contrary, when a cold front encounters relatively hot air, it often moves downward, and the air behind the front is forced to rise, forming precipitation.

The intensity of cold and warm air masses in cold and warm fronts is different, so the front type and direction are also different. A cold front is the front where the cold air mass actively moves to the warm air mass (as shown in Figure A). In the picture, the part of the cold air mass generally depicts the direction arrow of the cold air mass moving forward, which is opposite to the direction of the airflow arrow of the warm air mass forced to rise on the front.

A warm front is the front where the warm air mass actively moves to the cold air mass (as shown in Figure B). In the figure, the airflow direction arrow of the warm air mass climbing along the front is consistent with the movement direction of the cold air mass on the other side of the front.

Extended data:

The difference between cold and warm fronts:

1, the front slope (that is, the angle between the front line and the ground) is different. Because the cold air mass moves quickly, the cold air mass in the cold front pushes the front to move quickly, and the warm air mass rises quickly, so the slope of the cold front is large. In the warm front, the warm air mass slowly climbs above the cold air mass, pushing the front to move slowly, so the slope of the warm front is small.

2. The location and range of precipitation in cold and warm fronts are different. Cold front precipitation mainly appears behind and near the front, and the rain area is narrow; The warm front precipitation is in front of the front, and the rain area is wide.

3. The precipitation time and intensity in cold and warm fronts are different. The cold front has short precipitation time and high intensity. Warm front has long precipitation time and low intensity.

4. The weather changes during and after cold and warm fronts transit are different. When a cold front crosses the border, it usually appears cloudy, windy, rainy (snowy) and cool. After the cold front crossed the border, the cold air mass occupied the position of the original warm air mass, and the temperature dropped, the humidity dropped, the air pressure increased, and the weather turned fine.

When the warm front crosses the border, the clouds become thicker and cover the sky, often forming continuous precipitation (that is, long-term continuous precipitation). After the warm front crossed the border, the weather was controlled by the warm air mass, and the temperature increased, the humidity increased, the air pressure decreased, and the weather turned fine.

Frontal system is the main weather system that affects China. The precipitation and some disastrous weather in China are mostly related to frontal activity, in which the cold front is the main influencing factor. The summer rainstorm in northern China is mostly frontal rain formed by cold front; The cold wave that broke out in winter was also formed when the cold front went south.