Week 9 · Unit 2 The Atmosphere, opener; 2-1 Components and Layers of the Atmosphere (A): gases of the air and their importance, troposphere, tropopause, stratosphere, the ozone layer, mesosphere and the start of the thermosphere · 3 lessons
3
From the stratosphere and the ozone layer up to the thermosphere
Integrated Science Sec 1 Term 1, Unit 2: pp. 67–72
GoalDescribe the stratosphere, the ozone layer, the mesosphere and the start of the thermosphere, and how temperature changes through them.
New words
stratosphere · the layer above the troposphere, up to about 50 km, with calm, mostly horizontal airozone layer · a region in the stratosphere rich in ozone (O₃), which absorbs most harmful ultravioletmesosphere · the coldest layer, from about 50 to 80 km, where most meteors burn upthermosphere · the layer that begins at about 80 km (more next week)
Explanation
Above the troposphere is the stratosphere, up to about 50 km. Its lowest part has a steady temperature, then it becomes warmer higher up, in the ozone layer (about 20 to 40 km in the book), because ozone absorbs ultraviolet radiation. The ozone layer is inside the stratosphere; it is not a separate sphere. Ozone (O₃) absorbs most of the harmful ultraviolet that could damage the DNA of cells and cause skin cancer. The air in the stratosphere moves mostly sideways and is calm, so aircraft like to fly there. Above it, in the mesosphere (about 50 to 80 km), the temperature falls again, down to about −93 °C, the coldest of all; most meteors burn up here. The thermosphere starts at about 80 km. So the temperature falls in the troposphere, rises in the stratosphere, falls in the mesosphere and rises again in the thermosphere. The atmosphere has no sharp top. Heights depend on place and season, so book values are “about”.
Layer
Height, about
Temperature with height
troposphere
0–10 km
falls
stratosphere
10–50 km
steady, then rises (ozone)
mesosphere
50–80 km
falls, to about −93 °C
thermosphere
from 80 km
rises
Worked examples
Read the temperature sketch (standard values, extra): 15 °C at the ground, about −56.5 °C at 11 km and up to 20 km, about −2 °C at 50 km, about −90 °C at 85 km. The line falls, stays level, rises, then falls: this shows the layers. The heights are not evenly spaced.
Why does the temperature rise in the ozone layer? Ozone absorbs ultraviolet radiation from the Sun, and the energy absorbed warms the air there. That is why the layer is a shield and also why the stratosphere gets warmer with height.
Scale model: 1 cm stands for 5 km. The troposphere (10 km) is 10 ÷ 5 = 2 cm, the stratosphere top (50 km) is 10 cm and the mesosphere top (80 km) is 16 cm. A strip 16 cm long covers everything up to the thermosphere.
Class activity
Paper strip model, with a grown-up. Use a long strip of paper and the scale 1 cm = 5 km. Mark the top of the troposphere at 2 cm, the stratosphere at 10 cm and the mesosphere at 16 cm, colour the layers and write one feature for each. Mark where the ozone layer lies (20 to 40 km: from 4 cm to 8 cm). Then hold the strip upright and compare it with Earth’s radius drawn at the same scale: it would be about 1274 cm long.
Practice
1
Which layer lies directly above the troposphere, and about how high does it reach?
The stratosphere, up to about 50 km.
2
Which layer is the coldest, and what burns up there?
The mesosphere (down to about −93 °C); most meteors burn up there.
3
On a scale of 1 cm = 5 km, how long is the line for 40 km?
40 ÷ 5 = 8 cm.
4
Say whether the temperature rises or falls with height in each: troposphere, ozone layer, mesosphere.
