Is it sunny or rainy when the temperature is reversed?
The reasons for the reversal mainly include the following aspects:
1. Ground radiation cooling: On a clear night without wind or breeze, the ground quickly cools down, and the atmosphere near the ground also cools down. The closer the air is to the ground, the greater the influence of the ground, so the closer it is to the ground, the lower the temperature; The farther away from the ground, the less the temperature drops, thus forming an inverse temperature from the ground (see figure 1). With the intensification of ground radiation cooling, the inversion temperature gradually expands upward and reaches its strongest at dawn. Generally, after sunrise, the solar radiation gradually increases, the ground heats up rapidly, and the inversion gradually disappears from bottom to top. The summer nights are short, the inversion layer is thin and disappears quickly, while the winter nights are long and the inversion layer gradually disappears.
2. Air advection: When warm air moves horizontally above the cooled ground, water surface or air layer, the air at the bottom layer cools rapidly due to the influence of the underlying surface, while the air at the top layer cools less due to the long distance, so there is an inversion. The intensity of inversion mainly depends on the temperature difference between warm air and cold ground. The greater the temperature difference, the stronger the inversion. In winter, in mid-latitude coastal areas, due to the significant temperature difference between land and sea, when warm air is at sea,
3. Sinking in the air: mostly in mountainous areas. The cold air on the hillside sinks to the bottom of the valley, and the warm air at the bottom of the valley is pushed up by the cold air, resulting in inversion. This inversion is mainly formed under certain terrain conditions, so it is also called terrain inversion. For example, Los Angeles in the United States is surrounded by mountains on three sides, and there are more than 200 days of inversion every year.
4. Frontal factor: In the troposphere, cold and warm air meet, and the density of warm air is low, so that an inclined transition front is formed between them. On the front, if the temperature difference between cold and warm air is significant, inversion will also occur. From the vertical distribution of temperature above point A, it can be seen that the isotherm from the cold air below to the warm air above will be tortuous when passing through the front area, and the degree of tortuous depends on the temperature comparison between the two air masses. There is an inversion layer in the zigzag section (as shown in section 2 BC). Because the front is inclined above the cold air from the ground, the inversion layer is also inclined with the inclination of the front. Therefore, inversion can only be observed in the area controlled by cold air, and the height of inversion is related to the position of observation point relative to the ground front. The closer the observation point is to the ground front, the lower the inversion height.
No matter what causes the inversion, it will have a great impact on air quality. It hinders the vertical convection of air and the diffusion of smoke, pollutants and water vapor condensate. Tens of meters or even hundreds of meters thick inversion layer covers the city like a thick quilt, and the pollutants near the ground have no way to go, so they have to "stay where they are" and accumulate more and more, and the smoke will cover the sky and the air pollution will inevitably increase.