Why is the air pressure sometimes high and sometimes low in the same place? Otherwise, why is there a saying that low pressure will be rainy and high pressure will be sunny?

1. Because the average molecular weight of air is 29 and the molecular weight of rain (water vapor) is 18, there is less water vapor in the air on a sunny day. There is a lot of water vapor in rainy days, so the atmospheric pressure is high in sunny days and low in rainy days.

2. The air in the high pressure area often sinks and the weather is fine. The air in the low-pressure area is easy to rise, gather in the air, and condense into clouds when it is cold, causing rainfall, so it is easy to become rainy weather. -Fuzzy analysis.

3. Under normal circumstances, the ground continuously radiates long waves to the atmosphere effectively, and at the same time, the atmosphere is 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 area, the value of atmospheric pressure is different at different times of the year. This is called the annual variation of atmospheric pressure. The annual variation of atmospheric pressure can be divided into three types, namely continental type, ocean type and mountain type. Among them, the annual variation of atmospheric pressure in ocean type is just the opposite to that in continental type. Generally speaking, "the atmospheric pressure in winter is higher than that in summer" refers to the annual variation law of atmospheric pressure in mainland China. This is briefly analyzed below (the other two situations will not be discussed). Because the atmosphere is in the open space around the earth and has no specific boundary, it is different from the gas in a closed container. In summer, the temperature of the mainland is higher than that of the ocean, and the atmospheric humidity is relatively high (compared with winter), which makes the mainland air diffuse to the ocean continuously, leading to its pressure reduction. In winter, the temperature in the mainland is lower than that in the ocean, and the air humidity in the mainland is also lower than that in summer, which makes the air in the ocean spread to the mainland and increases the pressure in the mainland. This is why the atmospheric pressure in mainland China is higher in winter than in summer (atmospheric temperature is also a factor that affects atmospheric pressure, but the factor that determines the change of atmospheric pressure here is not temperature, but the flow and density of the atmosphere). -That's the right answer.