Why is the air pressure low when it rains?

Because there is more water vapor in the air in rainy days or before it rains, the average molecular weight of air decreases, that is, the air density decreases, which leads to the decrease of air pressure. The higher the temperature, the more the pressure drops in rainy weather, so people will feel chest tightness before it rains in summer (people's inhalation is forced in by external atmospheric pressure, and of course the change of oxygen content in the air is also related). If you want to know more, you need to look at high school physical chemistry.