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Unit 2
Lesson 1
The atmospheric layers
The atmosphere : It is a gas envelope that is surrounded the Earth and rotates with it about
its axis.
The atmospheric pressure :It is the weight of air column of an atmosphere height on a unit
area.
The atmospheric pressure is measured by a unit called a bar or millibar.
1 bar (b) =1000 millibar (mb)
The atmospheric pressure changes by the changing of the height above sea level. As the
height of air column increases, the atmospheric pressure increase. So we go up above the
sea level ,the height of atmosphere column decreases , the atmospheric pressure decreases
and air density decreases.
Isobar : it is map which we use to determine the places which have high pressure and places
which have low pressure.
**Barometer is used to measure the atmospheric pressure and Aneriod is a type of
barometer which is used to determine the possible day weather
Altimeter is used to measure the elevation of the navigation (height) of the airplanes .
Layers of the atmospheric envelope: There are 4 layers which are classified according to the
change in atmospheric pressure and temperature into:( troposphere - stratosphere mesosphere - thermosphere )
*There is a region between each two successive layers. In these regions the temperature
remains constant.
1- Tropopause is the region between troposphere and stratosphere.
2- Stratopause is the region between stratosphere and mesosphere.
3- Mesopause is the region between mesosphere and thermosphere.
First layer:Troposphere: It means the disturbed layer as most of the weather changes occur
in it .
*the characteristics and the importance of the troposphere
1-It extends 13 km above the sea level to the tropopause.
2-The temperature decreases as we go up with the rate of 6.5oC each 1 km until it reaches
the lowest value (-60oC ) at the tropopause.
3-Atmospheric pressure decreases as we go up , where it becomes 100 mb.
4-It contains about 75% of the atmospheric mass , so that all the atmospheric phenomena
such as rain, wind, clouds…..etc happens in this layer.
5-It contains about 99%of the water vapour which organizes the Earth's temperature.
6- The air movement is vertical, where the warm air currents go up and the cold air
currents go down.
Second layer : Stratosphere: Which is called the ozone atmosphere because it contains
ozone layer.
*the characteristics and the importance of the stratosphere:
1-It extends from tropopause(13 km above the sea level )to the stratopause (50km)with
thickness of 37 km.
2-The temperature at the lower part is constant (-60oC ),then it increases gradually until it
reaches 0oC at the end of this layer, this is because the absorption of ultraviolet radiation by
the ozone layer.
3-Atmospheric pressure decreases as we go up , where it becomes 1 mb.
4-It contains most of the atmosphere ozone which is concentrated between 20-40km
above sea level.
5-The lower part is suitable for flying plans because:1- it doesn't contain clouds .
2-it doesn't suffer from any weather disturbances. 3the air moves in this part horizontally.
Third layer : Mesosphere: It means the middle layer
*the characteristics and the importance of the mesosphere:
1- It extends from stratopause (50 km above the sea level )to the mesopause (85km)with
thickness of 35 km.
2-The temperature decreases as we go up until it reaches the lowest value (-90oC ) at its
end, so this layer is the coldest layer.
3-This layer is much vacuumed because it contains a limited amount of helium and
hydrogen gases.
4-Meteors burn in this layer because of the friction with air molecules.
Fourth layer : Mesosphere: It means the hottest layer
*the characteristics and the importance of the thermosphere:
1-It has a thickness of about 590 km.
2- Temperature increases rapidly with going higher until it reaches 1200oC.
3- The upper part is known as ionosphere because it contains charged ions.
4-Ionosphere plays an important role in wireless communication and broadcasting because
it reflected radio waves that are transmitted by communication centers and radio stations.
5- Ionosphere is surrounded by two magnetic belts which are known as Van Allen Belts.
6-Van Allen Belts a--they are helped in dispersing the harmful charged cosmic radiation
away from the Earth.
b- They cause the Aurora phenomenon, which appears as brightly, coloured
light curtains at both North and South poles.
7-The atmosphere is inserted into the outer space via an area ,which known as
Exosphere.
8-The satellites float around the Earth and transmit weather condition
information and TV programs in Exosphere.