How many levels are there in the troposphere?

The troposphere is the lowest layer in the Earth's atmosphere. The temperature in the troposphere decreases with increasing height, the convective movement of air is extremely obvious, and the horizontal distribution of air temperature and humidity is also very uneven. The thickness of the troposphere varies with latitude and season. Generally, the average thickness in low latitudes is 16 to 18 kilometers; in mid latitudes, the average thickness is 10 to 12 kilometers; and in high latitudes, the average thickness is 8 to 9 kilometers. In terms of seasons, in most areas of China, the troposphere is generally thick in summer and thin in winter. The troposphere concentrates about three-quarters of the mass of the entire atmosphere and almost all the water vapor. It is the layer with the most complex weather changes and the most important layer that affects flight. Almost all important weather phenomena encountered during flight appear in this layer, such as thunderstorms, dense fog, low cloud curtains, rain, snow, atmospheric turbulence, wind shear, etc. In the troposphere, according to the distribution characteristics of air flow and weather phenomena, it can be divided into three levels: lower layer, middle layer and upper layer.

The lower troposphere: also called the friction layer. Its range is from the ground to an altitude of 1 to 2 kilometers. However, the actual height in various places is related to factors such as surface properties and seasons. Generally speaking, its height is higher on rough surfaces than on smooth surfaces, higher in summer than in winter (Northern Hemisphere), and higher during the day than at night. In the lower levels, airflow is greatly affected by ground friction, and wind speed generally increases with height. In complex terrain and severe weather conditions, severe airflow disturbances often exist, threatening flight safety. Sudden downdrafts and strong low-altitude wind shear often cause aircraft crashes. In addition, abundant water vapor and dust often lead to dense fog and other phenomena that worsen visibility, posing serious obstacles to aircraft takeoff and landing. In order to ensure flight safety, each airport stipulates the meteorological conditions for the takeoff and landing of various types of aircraft. In addition, the diurnal changes in air temperature in the lower troposphere are extremely obvious, and the temperature difference between day and night can reach 10 to 40°C.

Middle troposphere: Its bottom boundary is the top of the friction layer, and its upper boundary is about 6 kilometers high. This layer is far less affected by the surface than the friction layer. Most of the clouds and precipitation phenomena in the atmosphere occur in this layer. In the upper part of this layer, the air pressure is usually only half that of the ground, and oxygen is required for flight there. Generally, light transport aircraft, helicopters, etc. often fly in this layer.

The upper troposphere: It extends from an altitude of 6 kilometers to the top of the troposphere. The temperature in this layer is below 0°C all year round, and the water vapor content is very small. All types of clouds are composed of ice crystals or supercooled water droplets. In mid-latitude and subtropical areas, there are often strong wind belts with wind speeds equal to or greater than 30 meters/second in this layer, which are the so-called high-altitude jet streams. When aircraft fly near rapids, they often encounter strong turbulence, which can cause discomfort to the passengers and even damage the aircraft structure and threaten flight safety.

In addition, between the troposphere and the stratosphere, there is a transition layer with a thickness of several hundred meters to 1 to 2 kilometers, called the tropopause. The tropopause has a great blocking effect on vertical air flow. Rising water vapor, dust particles, etc. mostly gather below it, and the visibility there is often poor.