Structure of the atmosphere: the troposphere and stratosphere

Detail

1. How the atmosphere is divided into layers

• Structure of the atmosphere means the way the air around the Earth is arranged in layers, one above the other.
• There are five layers. They are separated by how temperature changes with height, not by the gases in them.
• Order from the ground upward: troposphere → stratosphere → mesosphere → thermosphere → exosphere.
• Air density (how much air is packed into a given space) is highest at the surface and falls with height.
• Gravity pulls most of the air close to the ground.

• So the air gets thinner as you go up. Air pressure falls with it.

• Trap (Class 9 "Don't Miss Out"): temperature falls with height only in the troposphere and mesosphere.

• In the stratosphere and thermosphere, temperature rises with height.

• Temperature-height profile (Class 11 Fig. 7.1; Class 9 Fig. 3.3). Draw a zig-zag line:

• It slopes left (colder) through the troposphere.
• It runs straight up (constant temperature) at the tropopause.
• It slopes right (warmer) through the stratosphere up to about 50 km.
• It slopes left again in the mesosphere, then right in the thermosphere.

Layer	Approx. height	Temperature with height
Troposphere	0 to about 13 km (8 km at the poles, 18 km at the equator)	Falls
Stratosphere	Tropopause to about 50 km	Rises
Mesosphere	Above the stratosphere	Falls
Thermosphere	Above the mesosphere	Rises
Exosphere	Outermost layer	Very thin air, slowly merges into space

2. Troposphere: the weather layer

• Troposphere: the lowest layer of the atmosphere. It starts at the Earth's surface.
• Height:
• Average height is 13 km (Class 11).
• Class 9 gives about 12 km. Class 7 gives a range of 6–18 km.

• NOAA gives the top of the troposphere at about 10–15 km [4].

• Why it is thicker at the equator:

• Equator (about 18 km): strong heat → strong convection (warm air rising, carrying heat upward) → heat is carried high → the layer reaches higher.

• Poles (about 8 km): weak heating → weak convection → the layer stays shallow.

• What it holds:

• Nearly all the water vapour and dust particles in the atmosphere.
• So all weather (clouds, rain, storms, fog) and climate change take place here.

• It is the most important layer for life. All living things on land live in it.

• Lapse rate: temperature falls about 1°C for every 165 m of height.

• This is the normal lapse rate (the usual rate at which air cools as you go up). It is about 6.5°C per km.
• Why it gets colder upward: the ground absorbs sunlight and warms the air from below → air far from the ground gets less heat → higher air is colder.

• Full detail is in the insolation and heat budget note.

• Ozone here: only about 10% of Earth's ozone is in the troposphere [4].

3. Tropopause: the boundary zone

• Tropopause: the zone between the troposphere and the stratosphere. Here the temperature stays nearly constant with height.
• Its temperature:

Location	Height of tropopause	Temperature at tropopause
Over the equator	High (about 18 km)	About −80°C
Over the poles	Low (about 8 km)	About −45°C

• Why the equator's tropopause is colder (trap):
• The equatorial tropopause is much higher (about 18 km against 8 km).
• Air keeps cooling at the lapse rate all the way up.

• More height means more cooling → about −80°C, even though the surface below is the hottest on Earth.

• Jet streams blow near the tropopause.

• Jet stream: a fast, narrow band of strong wind high in the atmosphere, blowing mostly from west to east.
• Their role in the Indian monsoon is covered in the pressure-winds-circulation note.

4. Stratosphere: the ozone layer

• Stratosphere: the second layer. It starts at the tropopause and goes up to about 50 km [4].
• Temperature rises with height. This is the reverse of the troposphere.
• Ozone (O₃, a form of oxygen with three atoms) absorbs the Sun's ultraviolet (UV) rays.
• The absorbed UV energy turns into heat.

• The upper stratosphere gets more UV and is warmer. So temperature rises as you go up.

• Ozone layer: the part of the stratosphere with the most ozone.

• About 90% of Earth's ozone is in the stratosphere [4].
• The densest band, at about 15–35 km, is called the "ozone layer" [4].

• It protects life by blocking harmful UV rays.

• Calm and clear:

• It is dry (almost no water vapour) and nearly cloud-free.
• There is very little up-and-down air movement, because warm air sits on top of cold air.
• So Class 9 calls it ideal for flying aeroplanes.
• In practice, long-haul jets cruise at about 10–12 km, near the tropopause. This is the boundary between the two layers, not deep inside the stratosphere.

5. Volcanic eruptions and the stratosphere: Mount Pinatubo, 1991

• Where: Mount Pinatubo, Luzon island, Philippines. It erupted on 15 June 1991 [5].
• Step by step:
• Injection: about 20 million tonnes of sulphur dioxide and ash were blasted more than 20 km high [5]. About 15 million tonnes of SO₂ reached the stratosphere [6].
• Change: the SO₂ reacted with water and formed a haze of sulphate aerosols (tiny droplets, mostly sulphuric acid) [6].
• Spread: the material circled the globe in about three weeks. Stratospheric winds then spread it around the world over the next two years [5][6].

• Result: the aerosols scattered and absorbed incoming sunlight → less sunlight reached the ground → average global temperature fell by about 0.6°C (1°F) over the next 15 months [5][6].

• Why the cooling lasted so long: the stratosphere has no rain to wash particles out, and air there is stable. So aerosols stay up for a year or two. Dust in the troposphere is rained out within days or weeks.

6. Watching the upper air from India

• India Meteorological Department (IMD, set up in 1875) measures the troposphere and lower stratosphere using balloons.

• Radiosonde: a small instrument carried up by a hydrogen balloon. It radios back temperature, pressure and humidity at each height. This is how the temperature-height profile is measured.

• Upper-air network: 39 radiosonde and 62 pilot balloon observatories across India [2].

• Systematic upper-air observations began in 1905 at Shimla. An upper-air observatory followed in 1912 at Agra [2].

• IMD makes its own radiosondes at its Delhi workshop. Hydrogen for the balloons is produced at Agra [2].

• Ozone monitoring:

• IMD's National Ozone Centre is the WMO secondary regional ozone centre for Regional Association II (Asia) [3].
• Total column ozone (all the ozone in a column of air from ground to space) is measured with Dobson and Brewer spectrophotometers at five locations [3].
• There are ten surface ozone stations. Ozone at different heights is measured with ozonesondes (balloon-borne ozone sensors) [3].
• Monitoring also runs at Maitri and Bharati, India's stations in Antarctica [3].
• IMD contributes to WMO's Global Atmosphere Watch programme [3].

7. Troposphere and stratosphere compared

Feature	Troposphere	Stratosphere
Height	0–13 km average (8 km poles, 18 km equator)	Tropopause to about 50 km
Temperature with height	Falls (about 6.5°C/km)	Rises
Cause of temperature trend	Heated from below by the ground	Heated by ozone absorbing UV
Water vapour and dust	Nearly all	Very little; dry
Weather and clouds	All weather	Nearly cloud-free and calm
Ozone	About 10% [4]	About 90%; ozone layer at 15–35 km [4]
Air movement	Strong convection and turbulence	Stable, little vertical mixing

Prelims Hooks

• Layers from the ground up: troposphere → stratosphere → mesosphere → thermosphere → exosphere.
• Temperature falls with height only in the troposphere and mesosphere. It rises in the stratosphere (because of ozone) and in the thermosphere.
• The troposphere is thickest at the equator (about 18 km) and thinnest at the poles (about 8 km), because of convection. NCERT average: 13 km (Class 11) and 12 km (Class 9).
• Normal lapse rate: 1°C per 165 m, about 6.5°C per km.
• Trap: the tropopause is colder over the equator (about −80°C) than over the poles (about −45°C), because it is higher there.
• Jet streams blow near the tropopause.
• About 90% of ozone is in the stratosphere. The "ozone layer" lies at about 15–35 km [4].
• Mount Pinatubo (Philippines, June 1991): stratospheric sulphate aerosols cooled the globe by about 0.6°C for about 15 months [5][6].
• IMD's National Ozone Centre is the WMO secondary regional ozone centre for RA II (Asia). Ozone is also monitored at Maitri and Bharati (Antarctica) [3].
• IMD upper-air observation began at Shimla (1905). Today there are 39 radiosonde stations [2].

Mains Points

• Why the vertical structure matters for life and weather (GS-I):
• The troposphere holds nearly all water vapour, so the whole water cycle (monsoon rain, cyclones, western disturbances) happens in it.

• The stratosphere's ozone layer blocks harmful UV rays, so protecting it, as the Montreal Protocol does, protects health and crops.

• How the equator and poles differ (GS-I):

• Strong convection makes the tropical troposphere deep (about 18 km). This leads to tall cumulonimbus clouds, thunderstorms and very cold cloud tops over India.

• The jet streams near the tropopause help control when the Indian monsoon starts and ends.

• Volcanoes and climate (GS-I/GS-III):

• Stratospheric aerosols (Pinatubo 1991: about 0.6°C global cooling for about 15 months [5][6]) show that small particles can change the Earth's heat budget.

• This is also the idea behind proposals for stratospheric aerosol injection (deliberately spraying particles into the stratosphere to cool the Earth). Such proposals raise questions about risk and global governance.

• Watching the atmosphere is a disaster-preparedness tool (GS-III):

• IMD's radiosonde network (39 stations) and ozone network (National Ozone Centre, Antarctic stations) give the vertical data needed for forecasts and climate monitoring [2][3].
• This data feeds cyclone and heavy-rain warnings, and India's commitments to WMO.