The reason why the air pressure is lower in rainy days than in sunny days

The atmospheric expansion in cloudy areas is more intense, which leads to the lateral outward diffusion of the atmosphere in cloudy areas and reduces the density of the air. At the same time, the atmospheric humidity in cloudy areas is high, which also reduces the density of the atmosphere. Due to these two factors, the air pressure on cloudy days is lower than that on sunny days.

Causes of atmospheric pressure: Atmospheric pressure is the result of the earth's gravity. Due to the gravity of the earth, the atmosphere is "sucked" to the earth, thus generating pressure, and the atmospheric pressure near the ground is the highest. Atmospheric pressure in meteorological science refers to the weight of atmospheric column per unit area (atmospheric pressure), that is, the pressure exerted by atmospheric column per unit area.

Unit of air pressure: There are two units of air pressure: millimeter and millibar. The unit of air pressure is expressed by the height of mercury column, and the unit is millimeter (mm). For example, the air pressure is 760 mm, which means that the atmospheric pressure at that time is equal to the pressure generated by a mercury column with a height of 760 mm, and the other is millibar (mb) which is often heard in weather forecast broadcasts. It is a unit of air pressure expressed by the pressure of atmospheric pressure column per unit area. 1 millibar = 1000 dynes/cm2 (1 bar = 1000 millibar). Therefore, 1 millibar means that there is a force of 1 000 dynes in an area of1cm2. When the air pressure is 760mm, it is equivalent to 10 13.25mbar, which is called a standard atmospheric pressure.