Atmospheric movement and changing weather

Material throughout the atmosphere is in constant motion. But for a long time, people only had some understanding of the movement of the lower atmosphere, mainly the troposphere. It was not until rocket technology was developed for military purposes and rockets were launched to detect the atmosphere after World War II that people realized that there were still gases existing in the form of atoms or ions in the upper space near the earth. The movement of the atmosphere here cannot be detected by human senses, and can only be clearly shown by measuring it with instruments.

However, the atmospheric movement that people are generally concerned about now is still the lower atmosphere, mainly the movement of air molecules in the troposphere. In the troposphere, hot air rises due to volume expansion and density reduction; cold air sinks due to volume contraction and density increase. The curling smoke allows us to see this vertical movement, but what we can feel more is the wind. "On the day of the beginning of spring, the east wind thaws" (Book of Rites), "The autumn wind blows, and the waves in the Dongting are under the leaves" (Qu Yuan's "Xiangjun"). The Chinese ancestors who made a living by farming have long noticed that the wind is a A natural phenomenon that has an important impact on human life. However, due to the long-term failure to recognize the existence of air and being misled by the idea of ??the unity of heaven and man, the result is that the generation of wind is regarded as the result of gods, emperors or other human actions. "The master of man is master of heaven" and "if he unites with the people's hearts, then auspicious winds will come". Praying to heaven for wind and rain has become the universal consciousness from emperors to common people.

After Europeans learned that air has mass and had the concept of air pressure, starting from the 16th to 17th centuries, they were able to get inspiration from the flow of water from high pressure to low pressure, and thought that air also has Flow from high pressure areas to low pressure areas. The movement of air is wind. If the distribution of the atmosphere is uniform, the air pressure should be the same at the same altitude. But it actually differs from place to place. Connecting points with the same air pressure appears as a curve on the plane, similar to terrain contours, and is called a pressure contour. The change in air pressure in an area is called the pressure gradient by meteorologists. Due to the existence of air pressure gradient, air will flow from high pressure areas to low pressure areas. The greater the air pressure gradient value, the faster the air flows, that is, the stronger the wind blows. The difference between hot and cold is the fundamental cause of high and low air pressure. Why is there uneven heating and cooling in the atmosphere? First of all, it’s because there is more or less heat from solar radiation. Secondly, it depends on how much heat can be left here. The amount of solar radiation received on the earth varies from time to time and from place to place, and is affected by different factors such as season, latitude and ground conditions. The equator and its vicinity are often exposed to direct sunlight, and the sunshine time is long, so the amount of solar radiation received is large. The air is heated and expands, forming a low pressure area near the ground; A high pressure area is formed. Therefore, the air close to the ground may flow from the poles to the equator; and the air at the equator, after rising to the upper troposphere, should flow to the high altitudes of the poles, where it will shrink and sink. In this way, when only considering the impact of solar radiation on the atmosphere, it seems that an "ideal" circulation should be formed between the equator and the poles. It is said to be ideal because there is no such operating mode. Another important factor is that when the earth rotates, the linear speed of rotation is different (the same angular speed) at different latitudes, causing the airflow from the pole to the equator to deflect to the right in the northern hemisphere and to the left in the southern hemisphere.

The French engineer and mathematician G.G. Coriolis (1792-1843) perfected the scientific understanding of this phenomenon in 1835, and later generations named this force Coriolis. force, or rotation deflection force. This effect becomes stronger the closer you get to the poles, and is equal to zero at the equator. Therefore, under the influence of the Coriolis force, the atmospheric circulation cannot be formed directly from the poles to the equator, because when the air flow reaches 60° and 30° north and south latitudes, the direction of the wind has changed significantly. A subpolar low-pressure belt is formed near 60° north and south latitude on the surface, and the air flow is dominated by upward-blowing easterly winds. A high pressure area is formed near 30° north and south latitude on the surface, called the subtropical high pressure zone. The air flow is dominated by downward blowing westerly winds. As a result, three atmospheric circulation circles are formed in the southern and northern hemispheres respectively (Figure 4-4): Taking the northern hemisphere as an example, from the pole to 60° north latitude, polar northeasterly winds form near the surface, while southwesterly winds return at high altitudes. In the polar regions; near the ground between 30° and 60° north latitude, westerly winds (mainly southwesterly winds) prevail; between 30° north latitude and the equator, northeasterly trade winds develop near the ground (people often rely on this wind when sailing by sailboat) Navigation can bring information, so this wind is called the trade wind, also known as the trade wind). The air temperature is the highest near the equator, and the airflow rises vertically. At the height of this belt, it returns to the subtropical high pressure belt in the form of southwesterly winds. What is described above are the basic characteristics of atmospheric circulation.

The rotation deflection force will also cause the atmosphere to move in a vortex around the center of the low pressure area or high pressure area, forming a cyclone (clockwise rotation) with the low pressure area in the Northern Hemisphere as the center, and the high pressure area in the Northern Hemisphere as the center. An anticyclone (counterclockwise rotation) forms in the center. Tropical storms such as typhoons and tornadoes are all manifestations of cyclones, which often carry large amounts of rain to nearby continents.

Figure 4-4 The three-circle pattern of atmospheric circulation and the wind direction zone and pressure zone near the surface (quoted from J.P. Davidson et al., 1997)

The amount of solar radiation remains stable Under such circumstances, how much heat can be absorbed by the surface layers of the hydrosphere, biosphere and lithosphere under the atmosphere plays a decisive role in the temperature there. On the earth, on average, more than 30% of the heat radiated from the sun is reflected into space and lost, about 18% is absorbed by the atmosphere (mainly clouds and carbon dioxide), and 50% is absorbed by the surface of oceans and land. absorbed. Since the heat in the atmosphere is easily lost to space, the heat absorbed by seawater, rocks, soil and organisms can be slowly released, becoming the main factor in maintaining the temperature. The amount of heat they absorb varies depending on their nature and the conditions in which they exist. For example, white snow can reflect most of the heat radiated from the sun back. Forests, grasslands and soil can absorb most of this heat. The ocean's heat absorption capacity is also very strong, but it depends on the angle of the sun. When the sun is overhead, most of it is absorbed; when it shines obliquely in the morning and evening, most of it is reflected. Overall, the reflectivity of the ocean surface is smaller than that of land, and the heat capacity of seawater is on average about twice greater than that of the materials that make up land. Therefore, the ocean has become a more important regulator of temperature heating and warmth than land. There are also differences in the cold and heat in the ocean itself and in various places on the land, which will directly or indirectly affect the movement of the atmosphere. Therefore, the movement of the atmosphere takes a variety of forms, and changeable weather appears on the earth.

At first glance, the atmosphere appears colorless and transparent, but in fact the distribution of gaseous substances is uneven. In the troposphere, where a variety of weather phenomena occur, many air masses (air masses) with different types of cold, warm, dry and wet can be separated. An air mass can extend for millions of meters in the horizontal direction, at least hundreds of thousands of meters, and its thickness ranges from several hundred meters to several thousand meters until it reaches the top of the troposphere. In the same air mass, it is relatively uniform, and the physical properties such as temperature, humidity, and transparency are almost the same in the horizontal direction; in the vertical direction, they vary with height. Air masses are formed in certain geographical environments. For example, cold air masses that have a great impact on my country in winter are mostly formed in Siberia; warm and humid air masses that have a strong impact on my country in summer are mostly formed in the tropical areas of the Western Pacific.

When cold and warm air masses meet, a narrow transition zone is formed at their junction, called a front. The front looks like an inclined surface in space, with cold air masses at the bottom and warm air masses at the top. The temperatures on both sides of the front rise and fall sharply, with a difference of about 10°C. If the cold air mass pushes toward the warm air mass, it is called a cold front; otherwise, it is called a warm front (Figure 4-5). If the cold and warm air masses remain at a stalemate and the front rarely moves, it is called a quasi-stationary front. Weather changes are closely related to the activity of air masses, the slope and moving speed of the front, and the vertical movement of the air. When our place of residence is shrouded in dry and cold air masses, we feel the sky is high and the air is refreshing; while the warm and humid air masses bring sultry heat and humidity. The cold wave or heat wave we are familiar with is the arrival of cold or warm air masses. When cold and warm air masses meet, the water vapor in the warm air mass is cooled and condensed, so the weather near the front changes drastically, often cloudy and rainy.

Figure 4-5 Weather changes caused by warm and cold air masses