Shulou(Shulou.com)11/24 Report--

We know that the cruising altitude of civil airliners is generally about 10,000 meters, which corresponds to the stratosphere of the atmosphere. One of the important reasons for choosing the stratosphere is that the stratospheric air moves mainly horizontally and vertically. This means that the air in the stratosphere is relatively stable, it is not easy to form convection and turbulence, and there are no complex weather phenomena, such as clouds, rain, thunderstorms and so on. In this way, when the aircraft is flying in the stratosphere, it can reduce bumps and wind resistance, and improve comfort and safety.

But have you ever wondered why the vertical movement of air in the stratosphere is so weak? In order to answer this question, we should start with the stratification of the atmosphere. So how to divide the different levels of the atmosphere? In fact, there is no uniform and fixed standard, and different disciplines and fields may adopt different methods according to different purposes and priorities. Here, we stratify the atmosphere according to the characteristics of temperature changing with height.

Generally speaking, the atmosphere can be divided into five layers according to temperature, namely, troposphere, stratosphere, mesosphere, thermosphere and escape layer. The boundary height between these levels is not fixed, but varies with latitude, season, day and night and other factors. If we use a curve to represent the change of atmospheric temperature with height, we will find that the curve is not smooth, but shows some tipping points, which are the transition regions between different levels.

So why are these discounts? This is mainly due to different heat sources and heat conduction modes in different levels of the atmosphere. For example, the troposphere is characterized by a temperature decreasing with height, with an average temperature decrease of 0.65 ℃ for every 100m increase. This is because the troposphere is mainly heated by the ground, which absorbs solar radiation and transfers heat to the air through convection and radiation. On the other hand, the temperature in the stratosphere increases with height, because there is an ozone layer inside the stratosphere, which absorbs ultraviolet rays from the sun and becomes the heat source of the stratosphere.

Now, let's assume that there is a small mass of air at the bottom of the troposphere, which is hotter than the surrounding air. In this way, the small mass of air will begin to rise. In the process of rising, two things will happen to this small mass of air. First, because the upper air temperature is lower, it will transfer heat to its surroundings and lower its own temperature; second, because the upper air pressure is low, it will expand outward and cause its own temperature to drop.

But in fact, the temperature drop caused by the former is minimal, so we can roughly think of the whole process as adiabatic expansion. When the temperature of this small mass of air drops to the same temperature as the local ambient temperature, it will stay there. Similarly, if there is a small mass of air in the upper troposphere that is cooler than the surrounding air, then the opposite process will take place. In other words, the vertical movement of air is common in the troposphere.

Next, we set the environment in the stratosphere. Similarly, if there is a small mass of air that is slightly warmer than its surroundings, it will rise and expand adiabatically, causing its own temperature to drop. Unlike the troposphere, stratospheric temperature increases with height. So when this small mass of air reaches a higher altitude, it will find that its temperature is lower than its surroundings, so it goes back down to its original place. Let it move up and down just to facilitate our analysis, in fact, it almost stays in place. Therefore, the vertical movement of stratospheric air is very weak.

In fact, this inversion gradient can also occur in the troposphere. For example, if the ground radiates heat for a long time at night without being replenished, the temperature of the air near the ground layer is lower than that of the upper layer in the early morning. In addition, the topographic influence and frontal influence will also produce this kind of inversion layer.

The inversion layer is like a lid, which makes it difficult for soot, impurities and harmful gases suspended in the atmosphere to spread upward through it, resulting in a decline in air quality and deterioration of visibility. Some serious air pollution events in the world, such as the photochemical smog in Los Angeles, are mostly related to the existence of the inversion layer. More than a decade ago, haze appeared frequently, and it was the thickest in the early morning, which was also related to the inversion layer.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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