The ATMOSPHERE basically blankets the Earth,keeping the temperature stable and protecting living organisms from harmful solar radiation.This protective combination of gases extends hundreds of kilometers from the planets surface to space. 

THE ATMOSPHERE AND ITS LAYERS

THERMOSPHERE:

The thermosphere is the layer of the Earth's Atmosphere directly above the mesosphere and directly below the exosphere. Within this layer,(UV) ultraviolet radiation causes ionazation. The International Space station has a stable orbit within the middle of the thermosphere, between 320 and 380 kilometres (200 and 240 mi). Auroras also occur in the thermosphere.




MESOSPHERE:

The mesosphere  is the layer of the  Earth's Atmosphere  that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures below 130 K. The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary of the mesosphere is usually located at heights of about 50 km above the Earth's surface and the mesopause is usually at heights near 100 km, except at middle and high latitudes in summer where it descends to heights of about 85 km.


STRATOSPHERE:

The stratosphere is the second major layer of  Earth's Atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The border of the troposphere and stratosphere, the tropopause, is marked by where this inversion begins, which in terms of atmospheric thermodynamics is the equilibrium level. The stratosphere is situated between about 10 km (6 mi) and 50 km (30 mi) altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km (5 mi) altitude






TROPOSPHERE:

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols. The average depth of the troposphere is approximately 17 km (11 mi) in the middle latitudes. It is deeper in the tropics, up to 20 km (12 mi), and shallower near the polar regions, at 7 km (4.3 mi) in summer, and indistinct in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi) deep depending on the landform and time of day. The border between the troposphere and stratosphere, called the tropopause, is a temperature inversion.

VERTICAL STRUCTURE OF THE ATMOSPHERE PICTURE:

Q:Why are the four major layers of the atmosphere separated where they are?
A:They are separated because the different layers have different characteristics like their temperature , width ,  even chemical composition is a difference among them.


Shows height and the area span of the layers:
Exosphere: from 500 – 1000 km (300 – 600 mi) up to 10,000 km (6,000 mi), 
Ionosphere: It is located in the thermosphere and is responsible for auroras.
Thermosphere: from 80 – 85 km (265,000 – 285,000 ft) to 640+ km (400+ mi), temperature increasing with height.
Mesosphere: The mesosphere extends from about 50 km (160,000 ft) to the range of 80 to 85 km (265,000 – 285,000 ft), 
StratosphereThe stratosphere extends from the troposphere's 7 to 17 km (23,000 – 60,000 ft) range to about 50 km (160,000 ft). 
Troposphere: The troposphere is the lowest layer of the atmosphere; it begins at the surface and extends to between 7 km (23,000 ft) at the poles and 17 km (60,000 ft) at the equator,


Q:What increases the temperature in the stratosphere?
A:Ozone layer absorbs ULTRA VIOLET (UV) rays from the sun and converts this energy into heat,causing air temperature to increase


Q:Can planes fly in the mesosphere? Why or why not?
A:No they cant because the Air in the mesosphere is thin and also because solar radiation passes through freely in the mesosphere which is dangerous


Q:On what does the temperature in the thermosphere depend?
 A:The oxygen in the thermosphere absorbs ULTRA VIOLET rays  it gains from the sun.It is so hot there because nitrogen and oxygen in the atmosphere absorb a lot of radiation from space and convert it to heat.