The Atmosphere — Page 10, Lesson 11

Let’s start with the big picture. The atmosphere is the envelope of air surrounding the Earth, and for professional pilots we need to know its structure in detail. I’ll walk you through the three main layers introduced here: the troposphere, the tropopause, and the stratosphere. We begin with the troposphere. This is the lowest layer of the Earth’s atmosphere. Its key characteristic is that temperature decreases as height increases — that’s the normal lapse rate we’ll talk about more later. The troposphere contains three-quarters of the total weight of the atmosphere, and it contains almost all the weather — clouds, precipitation, turbulence, everything we fly through day to day. So when you’re climbing out after takeoff, you’re in the troposphere until you reach its top. Above the troposphere is the stratosphere. In the stratosphere, the temperature behaviour changes. Initially, temperature remains constant up to an average height of about 20 km. Then, from 20 km upward, temperature increases until it reaches -2.5°C at a height of 47 km. Why does it warm up? The reason is ultraviolet radiation acting in the formation of ozone — ozone absorbs UV energy and heats the surrounding air. Above 51 km, temperature starts to decrease again. The boundary between the stratosphere and the next layer above it, the mesosphere, is called the stratopause. Its average height in temperate latitudes is 50 km. Now, the critical boundary between the troposphere and the stratosphere is the tropopause. The tropopause marks the level where temperature ceases to fall with an increase in height. In practice, we take it as the height where the temperature fall is less than 0.65°C per 100 metres, which is equivalent to 2°C per 1000 feet. That’s the operational definition you’ll use. The height of the tropopause is not fixed — it’s controlled by the temperature of the air near the surface. The warmer the air, the higher the tropopause. The colder the air, the lower the tropopause. So variations due to latitude, season, and whether the surface is land or sea all cause the tropopause height to vary. There are two latitudes where the tropopause abruptly changes height or “folds” — at approximately 40° and 60° latitude. Let’s look at some average numbers. At the Equator, the tropopause is at 16 to 18 km with an average temperature of -75°C to -80°C. At the poles, it’s much lower — about 8 km with an average temperature of -40°C to -50°C. At 50° North, a typical mid-latitude value, the tropopause averages 11 km, which is 36,090 feet, with a temperature of -56.5°C. There’s an important relationship: the temperature of the tropopause is controlled by its height. The higher it is, the colder the temperature at the tropopause. The lower it is, the warmer the temperature at the tropopause. So over the poles, where the tropopause is low, the temperature can be as high as -40°C. Over the Equator, where it’s high, the temperature can be as low as -80°C. That figure shows the mean height of the tropopause along the Greenwich Meridian — it’s a good visual reference for how the tropopause dips at the poles and rises at the Equator, with those abrupt changes around 40° and 60° latitude. So to summarise: the troposphere is where weather happens and temperature drops with height; the tropopause is the boundary where that temperature drop slows to less than 0.65°C per 100 m; and the stratosphere above it initially has constant temperature, then warms due to ozone formation, up to the stratopause around 50 km. The tropopause height varies with surface temperature, and its own temperature varies inversely with its height.