Atmospheric movement

The atmosphere is in motion all the time, and the form and scale of its movement are extremely complex. There are both horizontal and vertical movements; there are both global large-scale movements and local small-scale movements.

(1) The driving force of atmospheric movement

The generation and form of atmospheric movement depend on the action of air pressure. The so-called air pressure refers to the weight of the air column supported by the unit area, and the unit is Pascal (recorded as Pa). The air pressure in a place decreases with increasing height. The main factors affecting the change of air pressure with height are the height and density of the atmospheric column above the place; in the horizontal direction, the difference in temperature will also cause changes in the density of the atmosphere in that direction, and Causes changes in the level of air pressure. Because there is a pressure difference in the atmosphere in the vertical or horizontal direction, a pressure gradient force is generated. Its direction is from the high pressure area to the low pressure area along the direction perpendicular to the isobaric surface. Its size is the change in air pressure per unit distance in this direction. quantity. The pressure gradient force can be divided into horizontal pressure gradient force and vertical pressure gradient force. Usually the vertical pressure gradient force is larger, which is 1 million times the horizontal pressure gradient force. Although the vertical air pressure gradient force is large, it is always in balance with gravity due to the influence of the earth's gravity; while the horizontal air pressure gradient force is small, but there is no other substantial force to balance it, and under certain conditions it can cause larger Large horizontal movement of air. Therefore, the force that actually causes the horizontal movement of the atmosphere is the horizontal pressure gradient force.

In addition to the horizontal gradient force, the moving atmosphere is also affected by geostrophic deflection force, inertial centrifugal force and friction force. The geostrophic deflection force is caused by the rotation of the earth and the spherical effect of the earth. Normally, the atmosphere on the Earth's surface rotates from west to east along with the solid Earth. The linear velocity is large near the equator and gradually becomes zero toward the poles. If the atmosphere with a large rotational linear velocity at a low latitude moves to a high latitude with a smaller linear velocity, the linear velocity difference between the two places will cause the moving atmosphere to generate an eastward additional velocity, and the actual wind direction must be eastward; on the contrary , the atmosphere flowing from high latitudes to low latitudes is equivalent to having an additional westward speed, causing the wind to shift westward. In fact, the movement of all objects on the earth (including solids, liquids and gases) will be deflected by the earth's rotation, and its deflection direction is the same as that of the atmosphere. This phenomenon is as if a force is exerted on a moving object to change the direction of its motion. This imaginary force is generally called the Coriolis force (after the French physicist Coriolis in 1835 It is named after this physical phenomenon was first studied in 2001), and for the moving atmosphere, it is called geostrophic deflection force (Figure 2-4). The geostrophic deflection force only changes the direction of atmospheric motion but not its speed; in the northern hemisphere it deflects the airflow to the right of the original direction of motion, and in the southern hemisphere it deflects the airflow to the left.

Figure 2-4 Schematic diagram of geostrophic deflection force

When the atmosphere moves in a curve, it will also be affected by the inertial centrifugal force of the atmosphere. In addition, the horizontal movement of the atmosphere near the ground will also be affected by ground friction; there is also friction between the moving atmosphere and the atmosphere.

Among the above-mentioned forces, the horizontal pressure gradient force is the driving force of atmospheric movement, and other forces only act after the atmospheric movement begins.

(2) Atmospheric circulation

Global and regular atmospheric movements are usually called atmospheric circulation. It reflects the basic pattern of atmospheric movement, and nurtures and restricts smaller-scale airflow movements. It is the background condition for the occurrence, development and movement of weather systems at various scales.

1. Low-latitude circulation

In the equatorial region, the surface temperature is hot all year round. The air expands due to heat, becomes lighter and rises in density, forming an "equatorial low pressure zone". Because the air in this zone mainly moves vertically, and the updrafts often carry a lot of water vapor, it is easy to condense and rain when it reaches high altitudes, resulting in the "equatorial windless zone" and a humid, hot and rainy climate. After the air on the equatorial surface rises to high altitudes, it forms high pressure at high altitudes, causing the air high at the equator to flow south and north. Due to the geostrophic deflection force, the direction of the airflow gradually increases and deflects eastward, and is basically parallel to the latitude line at an altitude of about 30° south and north latitude. In this way, the air flow can no longer flow south or north, causing the accumulation and density of high-altitude air, and the air flow is forced to move toward the ground, forming a "subtropical high pressure zone" (quiet wind zone), and leading to an arid desert climate. Near the surface, the air flow moves from the subtropical high pressure belt to the equatorial low pressure belt, and due to the geostrophic deflection force, the air flow direction gradually deflects westward, forming a "northeast or southeast trade wind (trade wind) belt" (Figure 2-5). As a result, a low-latitude atmospheric circulation system is formed between the equator and 30° south and north latitudes.

2. Mid-latitude circulation and high-latitude circulation

Because the polar regions are cold all year round, the atmosphere cools and shrinks, forming "polar high pressure belts" in the south and north near the surface. Between the polar high pressure belt and the subtropical high pressure belt, a relatively low pressure belt is formed in the area about 60° south and north latitude, called the "subpolar low pressure belt". Therefore, under the simultaneous action of the pressure gradient force, geostrophic deflection force and friction force, an easterly wind is formed between the polar high pressure belt and the subpolar low pressure belt, which is called the "polar easterly belt"; from the subtropical high pressure belt to the subpolar low pressure belt The low-pressure belt forms a westerly wind, which is called the "prevailing westerly belt". Updrafts are formed when polar easterly winds and prevailing westerly winds meet in the subpolar low-pressure belt. Updrafts flow to the subtropics and polar regions at high altitudes.

As a result, mid-latitude circulation circles and high-latitude circulation circles were formed.

Figure 2-5 Atmospheric circulation model and pressure zones and wind zones on the earth

Due to the results of atmospheric circulation, seven relatively stable pressures are formed in the global near-surface atmosphere. belt and six wind belts (see Figure 2-5). Surface topography and land-sea distribution characteristics can cause changes in atmospheric circulation patterns in local areas. For example, due to the blocking effect of the towering Himalayas and the Tibetan Plateau, the atmospheric circulation structure in western my country has been significantly changed. Some people believe that this may be an important reason for the gradual desertification of western my country.