Generally speaking, under the control of high pressure, the weather is fine; It often rains in places controlled by the bottom air pressure. Can you explain why?

Generally speaking, the ground constantly radiates long waves to the atmosphere effectively, and the atmosphere is also constantly radiating back to the ground. On sunny days, the heat on the ground can be transported to the outside through the effective radiation and upward divergence of the convective gas layer. On cloudy days, clouds reduce the outward divergence of troposphere atmosphere. The effect of clouds on preserving the surface and heating the liquid layer is called "greenhouse effect". In this way, 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 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.

In the same place, atmospheric pressure is not static, it changes with time. In a day, atmospheric pressure is usually the highest at 9 am and the lowest at 3 pm. The air pressure values are also different throughout the year. On the mainland, the air pressure is the highest in winter and lower in summer. On the contrary, the sea and mainland have higher air pressure in summer and lower air pressure in winter.

The main factors affecting atmospheric pressure are humidity and temperature. On cloudy days, the humidity of the atmosphere is relatively high, which means there is more water in the atmosphere. As we all know, the molecular weight of water is lower than the average molecular weight of air, but in some cases, the number of molecules per unit volume is very small. In this way, the atmospheric pressure of air with high humidity is low, and vice versa.

Temperature also affects atmospheric pressure. When the temperature is high, the air expands, the air density per unit volume decreases, and the atmosphere is depressed, and vice versa. But when the difference is not too big, the temperature changes little and the influence of temperature is not obvious.