Why is the air pressure high in sunny days and low in cloudy days, high in winter and low in summer?

Because there is more water in the air on rainy days, the molecular weight of water is smaller than the average molecular weight of air 29 18, so the air density on cloudy days is relatively small. Atmospheric pressure is related to air density. The air pressure density is high and the air pressure density is low, so the rainy weather is low.

Under normal circumstances, the ground continuously 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.