Why is the trough with low air pressure rainy?

The air density in the low-pressure trough area is lower than the surrounding air density, so it is easy to rise (this is like a wooden block in water, because its density is lower than that of water, it will float), and the air temperature will decrease during the rising process.

A basic concept in meteorology, namely specific humidity, will be quoted here, which refers to the ratio of the mass of water vapor in a wet air to the total mass of air (water vapor mass plus dry air mass), that is, how many grams of water vapor are contained in each gram of wet air. When the air pressure is constant, the specific humidity is a function of temperature, and decreases with the decrease of temperature. That is, when the temperature of an air mass is low, it can contain less water vapor, just as the northwest wind in winter is a dry and cold air mass from Siberia and Mongolia.

As we know, normal air contains a certain amount of water vapor more or less (there is humidity observation in the real-time observation of China Weather Network), so as soon as the temperature drops, the water vapor in the air becomes less, so what about the excess water vapor? After condensation, it comes to the ground as precipitation.

Here is just a brief introduction to your basic knowledge. If you want to know more, you can refer to Baidu Encyclopedia or meteorological textbooks.