How are sunny, cloudy and rainy days formed?

1? Causes of atmospheric pressure: The atmosphere and liquid are both fluids. Because of the gravity of the earth, the atmosphere around the earth also exerts pressure on the objects immersed in it. This pressure is called atmospheric pressure. The famous experiment in the hemisphere of Fort Madre in history proved the existence of atmospheric pressure.

2? The value of atmospheric pressure can be measured by Torricelli experiment. The value of 1 standard atmospheric pressure is equal to the pressure generated by 760mm high mercury column, that is, 1.0 1× 10? 5 Pascal.

3? Variation of atmospheric pressure

(1) The higher from the ground, the thinner the air and the smaller the air density. Within 2000 meters above sea level, it can be approximately considered that the atmospheric pressure decreases by 1.2 meters with an increase in altitude.

(2) Generally, the atmospheric pressure in sunny days is higher than that in cloudy days, and that in winter is higher than that in summer. Atmospheric pressure is related to the density of air, and the density of water vapor is less than that of air (mainly nitrogen and oxygen). There is more water vapor in the air on cloudy days, and the air pressure becomes lower!

Although air is a mixture containing many gases, it can be simply divided into two parts: water vapor and other dry air without water vapor. When the atmospheric change is regarded as a quasi-static process without heat exchange, that is, the formation of clouds, rain and snow is not considered, the pressure generated by these two parts adds up to form atmospheric pressure, which is the sum of the weight of water vapor in the above-mentioned gas column and the weight of dry air.

Assuming that the whole gas column is divided into countless small gas columns with different heights, each gas column contains 1 mol of air, and its temperature and pressure can be regarded as uniform. According to avogadro's law, the number of molecules of air in a small gas column is Avogadro constant n a, and its relationship with the number of molecules of water vapor, water and dry air is as follows:

N water +N dry =NA. (3)

The pressure is Δ p = Δ p water+Δ p dry =M water g+M dry g.

=n water μ water g+n dry μ dry g,

Because the number of moles of water vapor and dry air in the small gas column is

The pressure of the whole gas column on the bottom surface, that is, atmospheric pressure P, is equal to the sum of the pressures provided by all small gas columns on the bottom surface:

Where: 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 Formula (4) that when the content of water vapor molecules in the air changes. Will cause changes in atmospheric pressure:

(l) When N water →0, the air column will not contain water vapor, and it is all dry air, then P →Σ μ is dry g, that is, the atmospheric pressure tends to the sum of the weights of all dry air;

(2) When N water →NA, the air column will not contain dry air, but all water vapor, then P →∑μwater g, that is, the atmospheric pressure tends to the sum of all water vapor weights;

(3) When the water content of N increases and the air becomes humid, P becomes smaller. That is, the weather will turn from sunny to cloudy, and the atmospheric pressure will be reduced;

(4) When the water content of N decreases, the air becomes dry, that is, P becomes larger. That is, the weather will turn from cloudy to sunny and the air pressure will rise.

Generally speaking, the air pressure on sunny days is higher than that on cloudy days. Refers to the change of atmospheric pressure caused by the change of water vapor content when the temperature is constant. It is often said that "the control area of high pressure ridge is sunny" and "the control area of low pressure trough is cloudy", which also means this.