Why does the air pressure drop first and then rise before and after the cold air passes through the border?

It is that the air pressure first drops and then rises when the warm front passes, and it is that the air pressure first rises and then falls when the cold front passes.

Cold front is a front system formed by strong cold air mass entering warm air mass. When the cold front crosses the border, the affected areas are controlled by a single cold air mass. Because of the hot low pressure and cold high pressure, the air pressure will increase when the cold front passes. Moreover, the rainfall of the cold front is behind the front.

When the influence of cold air mass gradually weakens, the temperature rises, the humidity rises, the air pressure drops and the weather turns fine in this area. Therefore, after the cold air passes through the border, the air pressure should rise first and then fall.

During the front movement, the cold air mass plays a leading role, pushing the front to move to one side of the warm air mass. This front is called a cold front. A cold front is what people often call a cold air front. It is Depth Charge with cold air heading south, and it is the most common weather system that affects China, especially in winter.

Every few days in winter, a cold air invades the inland from northwest and north China. When the cold front crosses the border, it will be accompanied by the increase of northerly wind, the increase of air pressure and the decrease of temperature, which will sometimes cause rain and snow and even cause heavy rain in summer. Generally, after the cold front crosses the border, the local area will be controlled by cold high pressure, and the weather will become sunny.