As we all know, the earth we live on is surrounded by a layer of air thousands of meters thick, and the horizontal movement of the air relative to the ground is called wind. Under normal circumstances, according to different scales of atmospheric circulation, wind can be divided into three types, namely atmospheric circulation, monsoon circulation and local circulation.
1. Atmospheric circulation
Atmospheric circulation is the global circulation of air. Studies have shown that the macroscopic movement of the atmosphere around the earth is the combined result of the following two forces:
(1) Solar radiation heats the atmosphere surrounding the earth. However, the atmospheres of different latitudes receive different solar radiation intensities and different degrees of temperature changes, so there is an atmospheric pressure difference between the atmospheres of different latitudes. Under the action of the atmospheric pressure difference, the air starts to flow. In the equator or low latitude areas, the sun is relatively small, the sunlight is almost direct, the radiation intensity is large, and the sunlight is exposed for a long time, the surface of the earth and the atmosphere above it receives more solar radiation, and the temperature rises greatly; on the contrary, in high latitude areas, the sun is high, the sunlight is oblique, the radiation intensity is small, and the sunlight exposure time is short, and the ground surface and the atmosphere above it receive less solar radiation, and the temperature rise is smaller. In this way, there will be a temperature difference between the air at high latitudes and the air at low latitudes, which in turn creates a pressure gradient in the atmosphere from above the equator to the north and south poles. Driven by the pressure gradient, the air moves horizontally. Theoretically, its movement direction flows from high pressure area to low pressure area along the direction perpendicular to the isobar.
(2) The earth never stops rotating and exerts a horizontal force in the same direction as its rotation on the atmosphere above the earth. People call this force the geostrophic deflection force or Coriolis force. Under the effect of the deflection force of the geostrophic force, the airflow in the northern hemisphere of the earth deflects to the right, while the airflow in the southern hemisphere of the earth deflects to the left.
Figure 1 shows a schematic diagram of the formation and direction of atmospheric circulation on the earth’s surface. It can be seen from this figure that under ideal conditions, the earth’s surface is surrounded by three atmospheric circulation circles, namely the equatorial trade wind circle (equator to latitude 30°), the mid-latitude prevailing westerly circle (latitude 30-60°) and polar east wind circle (latitude 60~90°). This is the famous “three-circle circulation” theory. Of course, the “three-circle circulation” is only an ideal circulation model. In fact, affected by factors such as topography and oceans, such as uneven distribution of land and sea, the difference in temperature changes between oceans and continents, and the diversity of continental topography, the actual circulation is much more complicated than the ideal model.
2. Monsoon circulation
The monsoon circulation is a large-scale regional circulation, and its prevailing wind direction or pressure system has obvious seasonal changes. This kind of wind that changes the direction of the wind regularly with the seasons within a year is called the monsoon. The main reason for the formation of the monsoon is the difference in thermal power between the sea and the land. Because the land has a small heat capacity and does not flow, the land cools and heats up faster than the ocean. In winter, the land is colder than the ocean, the continental pressure is higher than the ocean, the pressure gradient force is from the continent to the ocean, and the wind blows from the continent to the ocean; on the contrary in summer, the land quickly warms, the ocean is relatively cold, the land pressure is lower than the ocean, the pressure gradient force is from the ocean to the continent, and the wind blows from the ocean to the continent. In the upper troposphere, compensating air flows in the opposite direction will inevitably appear, forming a circulation.
3. Local circulation
The local wind is a small-scale local circulation. Due to the different nature of the underlying surface in a small area, local circulations are often formed. The most common ones are sea-land breeze and valley breeze. The details are as follows:
(1) Sea and land breeze. Different types of materials have different heat capacities, and oceans have higher heat capacities than continents. When the sun shines (during the day), the temperature of the land is higher than the temperature of the sea surface, so the temperature of the air above the land (hot air) is higher than the temperature of the air above the sea (cold air). As a result, near the coastline, the cold air on the sea will flow to the land, forming wind, which is called sea breeze. Conversely, when there is no sun (dark night), the temperature of the land is lower than the temperature of the ocean, so the temperature of the air above the land (cold air) is lower than the temperature of the air above the sea (hot air). As a result, near the coastline, the cold air on the land will flow to the sea, forming a wind, which is called land breeze. From the above discussion, it can be known that due to the alternate day and night, the direction of the sea and land breeze in coastal areas also changes alternately.
(2) Valley wind. Sunlight travels in a straight line. During the day when the sun’s intensity is sufficient, the sun-facing side of the hillside receives far more solar radiation than the concave valley. The temperature of the air (hot air) above the hillside facing the sun is higher than that of the air above the valley (cold air), so the cold air in the valley will flow toward the hillside to form wind, which is called valley wind. On the contrary, when there is no sunlight at night, the air temperature on the hillsides and valleys will drop to the same level. Because the temperature of the hot air above the hillside decreases by a large extent, the density also increases, so that the high-density air will flow down the hillside to form wind, which is called mountain breeze. Obviously, the valley wind is closely related to the local topography.