When is the atmospheric pressure low? Why?

(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.