Three-cell theory

Air moves along a closed trajectory, or tends to move circularly along a closed trajectory.

Refers to the basic situation of large-scale atmospheric movement on a global scale, with a horizontal scale of more than several thousand kilometers and a vertical scale of more than ten kilometers. It is the background condition for the development and movement of weather systems at different scales.

Solar radiation, earth rotation, earth surface inhomogeneity, ground friction.

Below 200 (12), regardless of spring, summer, autumn and winter, the average temperature gradient points from the equator to the poles;

The temperature difference between north and south in winter is greater than that in summer;

In summer, the stratospheric temperature gradient points to the equator with its poles, and the cold center is located at the equator.

High-level: the polar potential height is low and the equatorial potential height is high, which causes the equatorial potential gradient pointing to the polar region and moving to the polar region at high altitude.

Low-level: The polar cold air sinks, and the mass accumulates, and the low-level air produces a pressure gradient pointing to the equator, and the low-level air moves to the equator.

This constitutes a single-circle circulation: equatorial rising, polar sinking, high-level south wind and low-level north wind.

Direct thermal cycle: the cycle caused by uneven atmospheric heating, in which the heating zone rises and the cooling zone sinks.

Under the influence of the earth's rotation, the air is influenced by Coriolis force, with the northern hemisphere leaning to the right and the southern hemisphere leaning to the left, forming a meridional three-circle circulation.

Under the action of Coriolis force, the air flowing northward over the equator deflects to the right and turns to the west wind from left to right, where it converges, where the mass accumulates, the ground pressure rises and the airflow diverges. Under the action of Coriolis force, one of the southerly winds turns to the northeast wind, which is relatively stable and is called the northeast trade wind. In the southern hemisphere, it is the southeast trade wind, and the two trade winds meet and rise at the equator, thus forming a direct circulation circle called Hadley circulation.

Polar energy is insufficient, the temperature is low and the density is high, so the air pressure decreases with height. At high altitude, there is a pressure gradient pointing to the low-latitude polar regions, while at low altitude, there is a pressure gradient pointing to the low-latitude polar regions. Then the low-level air has a movement pointing to the lower latitude, and the right deviation is the northeast wind under the action of Coriolis force; Under the action of Coriolis force, the south wind in the upper air turns to the southeast wind, forming a polar circulation circle.

Hadley circulation diverges in the sinking divergent airflow, and the northbound airflow merges with the southbound airflow in the lower level of the polar circulation circle, and diverges in the upper level, and one of them moves southward. In this way, there is a circulation between Hadley circulation and polar circulation, which is indirect circulation, also called Ferrer circulation, as opposed to direct circulation.