Why is the air pressure greater in sunny days than in rainy days, and greater in winter than in summer?

The reason why the atmospheric pressure is higher in sunny days than in rainy days: At the same temperature, the greater the atmospheric pressure, the greater the air density, the higher the floating height of water vapor and the sunny weather. If the air pressure is low, water vapor will float at low altitude or on the ground, so the air pressure in sunny days is higher than that in rainy days.

The reason why the atmospheric pressure in winter is higher than that in summer: Because of the low temperature in winter and the high air density in summer, the atmospheric pressure in winter is higher than that in summer.