Troposphere

Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 7 "Composition and Structure of Atmosphere"

Meaning

The troposphere is the lowest layer of the atmosphere. It starts at the Earth's surface and goes up to about 13 km on average: about 8 km over the poles and about 18 km over the equator. It is the layer where temperature falls as you go higher. It matters because it holds nearly all the water vapour and dust in the air, so all weather happens here. It is also the layer all living things on land live in.

Explanation

Position and height

• The atmosphere has five layers. They are told apart by how temperature changes with height, not by the gases in them.
• Order from the ground up: troposphere → stratosphere → mesosphere → thermosphere → exosphere. The troposphere is the first.
• Height figures in different books:
• Class 11 NCERT gives an average of 13 km.
• Class 9 NCERT gives about 12 km.
• Class 7 NCERT gives a range of 6–18 km.

• NOAA puts the top of the troposphere at about 10–15 km [3].

• Air density (how much air is packed into a given space) is highest at the surface.

• Gravity pulls most of the air close to the ground.
• So the troposphere has the thickest air. Air gets thinner, and air pressure falls, as you go up.

Why it is thicker at the equator than at the poles

• Equator (about 18 km):
• The Sun heats the equator strongly. This causes strong convection (warm air rising and carrying heat upward).

• The heat is carried high, so the layer reaches higher.

• Poles (about 8 km):

• Heating is weak, so convection is weak.

• The layer stays shallow.

• Result: the troposphere is thickest at the equator and thinnest at the poles.

Temperature and the lapse rate

• In the troposphere, temperature falls with height. This happens in only one other layer, the mesosphere.
• Normal lapse rate (the usual rate at which air cools as you go up): about 1°C for every 165 m, or about 6.5°C per km.
• Why it gets colder as you go up:
• The ground absorbs sunlight and warms the air from below.
• Air far from the ground gets less of this heat.

• So the higher air is colder.

• On a temperature-height graph (Class 11 Fig. 7.1; Class 9 Fig. 3.3), the line slopes left (colder) through the troposphere. It then runs straight up at the tropopause.

What it holds and why weather happens here

• Contents:
• Nearly all the water vapour in the atmosphere.
• Nearly all the dust particles.

• Only about 10% of Earth's ozone [3].

• Result:

• All weather forms here: clouds, rain, storms and fog. Climate change also takes place here.

• Air keeps moving up and down because of convection and turbulence (rough, mixed-up air movement).

• Dust here does not stay up for long. Rain washes it out within days or weeks. Particles in the dry stratosphere stay up for a year or two.

The tropopause: the upper boundary

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

Location	Height of tropopause	Temperature at tropopause
Over the equator	About 18 km	About −80°C
Over the poles	About 8 km	About −45°C

• Why the equator's tropopause is colder:
• Over the equator, the tropopause is much higher.
• Air keeps cooling at the lapse rate all the way up.

• More height means more cooling. So it reaches about −80°C, even though the ground below is the hottest on Earth.

• Jet streams (fast, narrow bands of strong wind high in the atmosphere, blowing mostly from west to east) blow near the tropopause.

• Long-haul jets cruise at about 10–12 km, near the tropopause.

Where it occurs

• All over the world: the troposphere covers the whole Earth from the surface up. Its height changes with latitude:
• Equatorial belt: deepest, about 18 km, because of strong convection.

• Polar regions: shallowest, about 8 km, because of weak heating.

• Tropical India: strong convection makes the troposphere deep. This builds tall cumulonimbus clouds (huge, tower-like thunderclouds), thunderstorms and very cold cloud tops.

• Weather systems in India: the whole water cycle happens in the troposphere. This includes monsoon rain, cyclones and western disturbances.
• How India measures the troposphere:
• The India Meteorological Department (IMD), set up in 1875, measures the troposphere and lower stratosphere with balloons.
• A radiosonde is 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.
• India has 39 radiosonde and 62 pilot balloon observatories [2].
• Systematic upper-air observation began at Shimla in 1905. An upper-air observatory followed at Agra in 1912 [2].
• IMD makes its own radiosondes at its Delhi workshop. Hydrogen for the balloons is produced at Agra [2].

Don't confuse with

• Stratosphere: temperature rises with height there, because ozone absorbs UV rays. In the troposphere it falls, because the air is heated from below by the ground. The stratosphere is also dry, nearly cloud-free and calm.
• Tropopause: this is the boundary zone above the troposphere, not a part of the weather layer. Temperature stays nearly constant through it, and it is colder over the equator (about −80°C) than over the poles (about −45°C).
• Mesosphere: temperature also falls with height there. But it lies above the stratosphere and holds no weather. Do not pick "temperature falls only in the troposphere". It falls in both.
• Ozone layer: this is the densest ozone band at about 15–35 km, in the stratosphere [3]. The troposphere holds only about 10% of Earth's ozone [3].

Prelims Hooks

• The troposphere is thickest at the equator (about 18 km) and thinnest at the poles (about 8 km), because of convection. NCERT gives an average of 13 km (Class 11) and 12 km (Class 9).
• Normal lapse rate: 1°C per 165 m, about 6.5°C per km.
• Temperature falls with height only in the troposphere and mesosphere. It rises in the stratosphere and thermosphere.
• Trap: the tropopause over the equator (about −80°C) is colder than over the poles (about −45°C), because it is higher there.
• Jet streams blow near the tropopause.
• Only about 10% of ozone is in the troposphere, and about 90% is in the stratosphere [3]. IMD upper-air observation began at Shimla (1905) [2].

Mains Points

• Why the troposphere matters for life and weather (GS-I):
• It holds nearly all water vapour and dust, so the whole water cycle happens here: monsoon rain, cyclones and western disturbances.

• All living things on land live in it, so changes in it directly affect farming, health and settlements.

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

• Strong convection makes the tropical troposphere deep (about 18 km). This drives tall cumulonimbus clouds and thunderstorms over India.

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

• Measuring the troposphere helps disaster preparedness (GS-III):

• IMD's network of 39 radiosonde stations gives the vertical temperature, pressure and humidity data that forecasts need [2].
• This data feeds cyclone and heavy-rain warnings.

Related concepts

• Structure of the atmosphere
• Tropopause
• Stratosphere

Read more

• Structure of the atmosphere: the troposphere and stratosphere