الأسبوع 9 · الوحدة الثانية «الغلاف الجوي» (افتتاحية) والموضوع ٢-١ «مكونات وطبقات الغلاف الجوي» (١): مكونات الهواء وأهمية الغلاف الجوي والتروبوسفير والستراتوسفير وطبقة الأوزون والميزوسفير · 3 دروس
2
Troposphere and tropopause: the layer of weather and life
Integrated Science Sec 1 Term 1, Unit 2: pp. 67–72
GoalDescribe the troposphere and tropopause, and calculate temperatures at different heights with the fall of 6.5 °C per km.
New words
troposphere · the lowest layer of the atmosphere, where almost all weather happenstropopause · the thin boundary at the top of the troposphere, at about 11 kmlapse rate · how fast temperature falls with height: about 6.5 °C for every kilometre in the tropospherebiodiversity · all the living things in a place
Explanation
The troposphere is the layer nearest the ground. It reaches about 10 km in the book, and the tropopause lies at about 11 km. Its height is really about 8 km near the poles and about 18 km near the equator (extra). It is the densest layer and holds nearly all the weather: clouds, rain and wind. The average temperature at the ground is about 15 °C, and it falls by about 6.5 °C for every kilometre up to the tropopause. The cause is that the air pressure falls with height, so the air expands, and expanding takes energy from the motion of its molecules, which cools it. The tropopause is a thin boundary that stops the cold air of the troposphere mixing with the warm air of the stratosphere. Almost all life, so most biodiversity, is in the troposphere, because it has oxygen, carbon dioxide and water vapour, a suitable temperature and a suitable pressure.
Worked examples
The ground temperature is 30 °C. What is it at 3 km? The fall is 3 × 6.5 = 19.5 °C, so the temperature is 30 − 19.5 = 10.5 °C.
Find the temperature at the tropopause (11 km) when the ground is 15 °C. The fall is 11 × 6.5 = 71.5 °C, so the temperature is 15 − 71.5 = −56.5 °C. At 8 km it is 15 − 8 × 6.5 = −37 °C.
At what height does the temperature reach 0 °C if the ground is 20 °C? The temperature must fall by 20 °C, and 20 ÷ 6.5 ≈ 3.1 km. So it reaches 0 °C at a height of about 3 km.
Class activity
Temperature ladder, on paper. Draw a tall ladder with steps for 0, 2, 4, 6, 8, 10 and 11 km. Choose a ground temperature (for example today’s temperature from a weather app) and fill in the temperature at each step with the fall of 6.5 °C per km. Mark where it reaches 0 °C. Then write two sentences on why the temperature falls with height, using the words pressure and expand.
Practice
1
Ground temperature is 25 °C. Find the temperature at 5 km.
25 − 5 × 6.5 = 25 − 32.5 = −7.5 °C.
2
A weather station on a hill is 2 km above a town where it is 24 °C. What temperature do you expect at the station?
24 − 2 × 6.5 = 24 − 13 = 11 °C.
3
What does the tropopause do?
It is a thin boundary that stops the cold troposphere mixing with the warm stratosphere.
4
Give two reasons why most life is found in the troposphere.
For example: it has oxygen, carbon dioxide and water vapour, and a suitable temperature and pressure.
