# Composition and structure  (Page 2/3)

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Earth's atmosphere

The Earth's atmosphere is a layer of gases that surround the planet, and which are held there by the Earth's gravity. The atmosphere contains roughly 78.1% nitrogen, 20.9% oxygen, 0.9% argon, 0.038% carbon dioxide, trace amounts of other gases, and a variable amount of water vapour. This mixture of gases is commonly known as air. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.

## The structure of the atmosphere

The earth's atmosphere is divided into different layers, each with its own particular characteristics ( [link] ).

## The troposphere

The troposphere is the lowest level in the atmosphere, and it is the part in which we live. The troposphere varies in thickness, and extends from the ground to a height of about 7km at the poles and about 18km at the equator. An important characteristic of the troposphere is that its temperature decreases with an increase in altitude. In other words, as you climb higher, it will get colder. You will have noticed this if you have climbed a mountain, or if you have moved from a city at a high altitude to one which is lower; the average temperature is often lower where the altitude is higher. This is because the troposphere is heated from the 'bottom up'. In other words, places that are closer to the Earth's surface will be warmer than those at higher altitudes. The heating of the atmosphere will be discussed in more detail later in this chapter.

The word troposphere comes from the Greek tropos , meaning turning or mixing . The troposphere is the most turbulent (or agitated) part of the atmosphere and is the part where our weather takes place. Weather is the state of the air at a particular place and time e.g. if it is warm or cold, wet or dry, and how cloudy or windy it is. Generally, jet aircraft fly just above the troposphere to avoid all this turbulence.

## The stratosphere

Above the troposphere is another layer called the stratosphere , where most long distance aircraft fly. The stratosphere extends from altitudes of 10 to 50km. If you have ever been in an aeroplane and have looked out the window once you are well into the flight, you will have noticed that you are actually flying above the level of the clouds. As we have already mentioned, clouds and weather occur in the troposphere, whereas the stratosphere has very stable atmospheric conditions and very little turbulence. It is easy to understand why aircraft choose to fly here!

The stratosphere is different from the troposphere because its temperature increases as altitude increases. This is because the stratosphere absorbs solar radiation directly, meaning that the upper layers closer to the sun will be warmer. The upper layers of the stratosphere are also warmer because of the presence of the ozone layer . Ozone (O ${}_{3}$ ) is formed when solar radiation splits an oxygen molecule (O ${}_{2}$ ) into two atoms of oxygen. Each individual atom is then able to combine with an oxygen molecule to form ozone. The two reactions are shown below:

#### Questions & Answers

Suggested how the difference in weight influences the size of the rocket that is needed to launch the astronaut into space from Earth
Sibongile Reply
newtons third law of motion
Nomvulah Reply
how to deal with newtons third law of motion
Nomvulah
help me with Newtons laws of universal gravitation
Ntsako Reply
Every particle in the universe attract every other particle with a force that is directly proportional to the product of their mass and inversely proportional to the square of distance between them
Delight
guys plz help me how to calculate net force?
Thabiso Reply
its FN +Fg+ Ff+ Ft+Fapplied = Net Force or any force acting on that object
Thimna
FN IS A NORMAL FORCE FG IS A GRAVITATIONAL FORCE FF IS A FRICTION FORCE FA IS A APPLIED FORCE THESE ARE FORCES THAT ACT IN EVERYTHING YOU TOUCH LIKE MUG, PEN, PHONE ,MOB EVERYTHING THAT MATTER
Simphiwe
net force is all the forces acting on an Object that is your FN +FG even T +fy or fx anyforce that will act on the object
Yanga
fnet=mass×acceleration
Yanga
What's Newton second law
Mahlatse
Newton's second law...is when resultant/net force acts on an object,the object will accelerate in the direction of the force at an acceleration directly proportional to the force and inversely proportional to the mass of the object
Cheyeza
How do yu calculate normal force on a inclined
Mahlatse
net force are FORCES that ACT on that object resultant is a direction or u can say it a magnitude
Simphiwe
Newton's second law : the resultant/net force act on an object is equal to the rate of change in momentum.
Alfred
guys what is the most effective way to know the law of gravitation
Romeo
any one who can help me with chemistry
george Reply
Yeah Sure. I will help
Ruthy
OK first can you pls teach me how to to calculate on chemistry chapter
george
What is a derivative?
Hulisani
Hydrocarbon derivatives are compounds that are made primarily of carbon and hydrogen atoms with specific groups of atoms attached. These specific groups of atoms are called functional groups. Hydrocarbon derivatives contain at least one element that is electronegative.
Ruthy
How to find the integral of a specific function?
Hulisani
how do u calculate the mass of an object if u have been given the gradient of a graph?
Hlelelwe Reply
what is endothermic reaction
Mphumuzi Reply
what is the difference between intramolecular and intermolecular forces
Xiluva Reply
please help me here how do the molecular masses of the compounds change?
Khutso Reply
how to calculate atom mass
Mbali Reply
how do water molecules form
Mukonazwothe Reply
water molecules consist of two atoms of hydrogen linked by covalent bonding to the same atom of oxygen ...hydrogen bonds are formed easily when two water molecules come close together , but are easily broken when the water molecule moves apart again
Nqobile
similarity between vander waals forces and hydrogen bond
Angel Reply
type of intermolecular forces between 2 hydrogen and 2 oxygen
Angel
when an atom X of an element in Group 1 reacts to become X+
Rhulani Reply
You have O.5 dm^3 of a KNO3 solution of concentration 0.2 M .You need to dilute this solution to get a solution of concentration 0.18M. calculate how many cubic cm of H2O you need to achieve this
Thimna Reply
im really struggling with this question please help me
Thimna
I don't know how to help i nee you help
Mphumuzi
what is a radium
Zieniengie Reply

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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