Why is the atmospheric pressure higher on a sunny day than on a cloudy day?

1? The reason for the generation of atmospheric pressure: The atmosphere is as fluid as liquid, and both are affected by gravity due to the attraction of the earth. Therefore, the atmosphere around the earth also exerts pressure on the objects immersed in it. , this pressure is called atmospheric pressure. The famous Magdeburg Hemisphere Experiment in history proved the existence of atmospheric pressure.

2. The value of atmospheric pressure can be determined using Torricelli's experiment. The value of 1 standard atmosphere is equal to the pressure generated by a 760 mm high mercury column, which is 1.01 × 10?5 Pascals.

3? Changes in atmospheric pressure

(1) The higher it is from the ground, the thinner the air and the smaller the density of the air. Within an altitude of 2,000 meters, it can be approximately considered that for every 12 meters of altitude increase, the atmospheric pressure decreases by 1 mm of mercury.

(2) Generally, the atmospheric pressure on a clear day is higher than that on a cloudy day, and the atmospheric pressure in winter is higher than that in summer. Atmospheric pressure is related to air density. The density of water vapor is less than that of air (mainly due to the two main components of nitrogen and oxygen). There is more water vapor in the air on a cloudy day, so the air pressure becomes lower!

Although air is a mixture containing a variety of gases, it can be simply divided into two parts: water vapor and other dry air without water vapor. When atmospheric changes are regarded as a quasi-static process and there is no heat exchange, that is, the formation of clouds, fog, rain, snow, etc. is not considered, the atmospheric pressure is the sum of the pressures generated by these two parts, which is the weight of water vapor and the weight of dry air in the air column. sum.

Assume that the entire air column is divided into countless small air columns with different heights. Each air column contains 1 mol of air, and its temperature and pressure can be regarded as uniform. According to Avogadro's law, the number of air molecules in a small air column is Avogadro's constant NA. The relationship between it and the number of water vapor molecules N water and the number of dry air molecules N dry is:

N water + N dry = NA. (3)

The pressure is: Δp = Δp water + Δp dry = M water g + M dry g

= n water μ water g + n dry μ dry g,

p>

And because the number of moles of water vapor and dry air in the small air column is And:

Among them: μ dry = 28.97 × 10?3 kg/mol, μ water = 18 × 10?3 kg/mol, that is (μ dry? μ water) > 0. It can be seen from equation (4) that when the content of water vapor molecules in the air changes. It will cause changes in atmospheric pressure:

(l) When N water → 0, the air column will contain no water vapor and will be all dry air, then p → ∑ μ dry g. That is, the size of the atmospheric pressure tends to the sum of the weight of all dry air;

(2) When N water → NA, the air column will not contain dry air and will be all water vapor, then p → ∑μ water g is the atmospheric pressure The size of tends to the sum of the weight of all water vapor;

(3) When the N water content increases and the air becomes humid, then p will become smaller. That is, when the weather changes from sunny to overcast, the atmospheric pressure will decrease;

(4) When the N water content decreases and the air becomes dry, that is, p will become larger. That is, when the weather changes from cloudy to sunny, the atmospheric pressure will increase.

In the textbook, "Generally speaking, the atmospheric pressure on a sunny day is higher than that on a cloudy day." This refers to the change in atmospheric pressure caused by changes in water vapor content when the temperature remains unchanged. It is often said that "areas controlled by high-pressure ridges are mostly sunny" and "areas controlled by low-pressure troughs are mostly cloudy", which also means the same.