Why is the valley wind dry? Why does the land breeze blow in winter?

Due to the thermal difference between the valley and the nearby air, the daytime wind blows from the valley to the hillside, which is called "valley wind"; At night, the wind blows from the hillside to the valley, which is called "mountain wind". Mountain breeze and valley breeze are always called mountain breeze.

The formation principle of mountain-valley wind is similar to that of land-sea wind.

During the day, the hillside receives more sunlight and the heat of the sun, becoming a small "heating stove", and the air warms more; Above the valley, the air at the same height is far from the ground and the temperature is less. So the warm air on the hillside keeps rising, flowing from the hillside to the upper valley, and the air at the bottom of the valley supplements the mountain along the hillside, thus forming a thermal cycle between the hillside and the valley. The lower wind blows from the valley bottom to the hillside, which is called the valley wind (see Figure 5-4 1). At night, the air on the hillside is affected by the radiation cooling of the hillside, and the "heating stove" becomes a "cooler", and the air is cooled more; Above the valley, the air at the same height is far from the ground, and the temperature drops less. So the cold air on the hillside flows into the valley along the hillside because of its high density, and the air at the bottom of the valley rises due to convergence, flowing from the top of the mountain to the top of the mountain, forming a thermal circulation opposite to that during the day. The low-level wind blows from the hillside to the valley, which is called mountain wind (see Figure 5-42).

The average speed of the valley wind is about 2~4 meters per second, and sometimes it can reach 7~ 10 meters per second. When Gu Feng passed the mountain pass, the wind speed increased. The wind speed of mountain wind is smaller than that of valley wind, but in the canyon, the wind is stronger, which sometimes damages the crops in the valley. Generally, the thickness of the valley wind is about 500~ 1000 meters above the valley bottom, and this thickness increases with the increase of gas layer instability. Therefore, the tensile thickness in the afternoon is the largest in a day. The thickness of mountain wind is relatively thin, usually only about 300 meters.

On sunny days, the wind from the valley sends warm air to the mountain, which raises the temperature on the mountain and promotes the germination, flowering, fruiting and early maturity of plants, crops and fruit trees on the hillside. Winter can reduce the chill. The valley wind brings water vapor to the top of the valley, which increases the air humidity in the mountains and reduces the air humidity in the valley. This phenomenon is especially obvious at noon. If there is enough water vapor in the air, the summer breeze will often condense clouds and cause rainfall, which is very beneficial to the growth of trees and crops in mountainous areas; At night, the mountain wind brings the water vapor from the mountain to the valley, so the air humidity in the mountain decreases and the air humidity in the valley increases. In the growing season, mountain breeze can reduce the temperature, which is very beneficial to the accumulation of nutrients in plants and the growth and expansion of root and tuber plants.

Mountain breeze can also transport fresh air to urban areas and factories, and take away the smoke and chemicals floating in the air, which is conducive to improving and protecting the environment. The construction and layout of the factory should consider the regular change of wind direction. The wind direction changes regularly and the wind force is relatively stable, so it can be used as a power resource to study and utilize, give play to its advantages, control its disadvantages and serve the socialist construction.

It is worth noting that, in addition to mountainous areas, the wind similar to the valley wind can also appear on the plateau and basin edge of China: the wind direction and wind speed have obvious daily changes. The mountain-valley wind on the edge of the Qinghai-Tibet Plateau, especially in the area adjacent to the Sichuan Basin, has a great influence on the weather on the edge of the Qinghai-Tibet Plateau. In the case of sufficient water vapor, the white clouds condense over the hillside and cause rainfall at the edge of the basin at night.