Stratosphere

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

Meaning

The stratosphere is the second layer of the atmosphere. It starts at the tropopause and goes up to about 50 km [4]. In this layer, temperature rises as you go higher, because ozone absorbs the Sun's ultraviolet (UV) rays.

It matters because it holds about 90% of Earth's ozone [4]. This ozone blocks harmful UV rays and protects life. Its dry, stable air can also keep volcanic particles high up for a year or two, and these particles can cool the whole planet.

Explanation

Position and height

• The stratosphere is the second of five layers: troposphere → stratosphere → mesosphere → thermosphere → exosphere.
• Scientists separate these layers by how temperature changes with height, not by the gases in them.
• Lower boundary: the tropopause (the boundary zone where temperature stays nearly constant with height).
• Over the equator, the tropopause is high (about 18 km).
• Over the poles, it is low (about 8 km).

• So the stratosphere starts lower over the poles and higher over the equator.

• Upper boundary: about 50 km [4]. Above it lies the mesosphere, where temperature starts falling again.

• Air density keeps falling with height, so the air here is much thinner than at the ground.

Why temperature rises with height

• Ozone absorbs UV:
• Ozone (O₃, a form of oxygen with three atoms) takes in the Sun's ultraviolet rays.
• The absorbed UV energy turns into heat.

• The upper stratosphere gets more UV, so it is warmer than the lower part.

• Result: temperature rises with height. This is the opposite of the troposphere, which is heated from below by the ground.

• On a temperature-height graph (Class 11 Fig. 7.1; Class 9 Fig. 3.3):
• The line runs straight up at the tropopause (temperature stays the same).
• It then slopes right (warmer) through the stratosphere up to about 50 km.
• Above that, it slopes left (colder) again in the mesosphere.

The ozone layer

• About 90% of Earth's ozone is in the stratosphere. Only about 10% is in the troposphere [4].
• The densest band of ozone, at about 15–35 km, is called the "ozone layer" [4].
• It protects living things by blocking harmful UV rays.

Calm, dry and stable air

• Dry: it has almost no water vapour, so it is nearly cloud-free.
• Stable:
• Warm air sits on top of colder air.
• Warm air does not sink and cold air does not rise, so there is very little up-and-down air movement.

• So there are no storms or turbulence like those in the troposphere.

• Flying: Class 9 calls it ideal for flying aeroplanes. In practice, long-haul jets cruise at about 10–12 km, near the tropopause. That is the boundary between the two layers, not deep inside the stratosphere.

• No rain to clean it: particles that reach the stratosphere are not washed out. They can stay up for a year or two. Dust in the troposphere is washed out by rain within days or weeks.

Where it occurs

• All over the Earth: the stratosphere surrounds the whole planet above the tropopause. It starts lower over the poles (tropopause about 8 km) and higher over the equator (tropopause about 18 km).
• Mount Pinatubo, Luzon island, Philippines (15 June 1991) [5]:
• 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 haze went around the globe in about three weeks. Stratospheric winds then spread it over the whole world over the next two years [5][6].

• Result: the haze blocked some sunlight, and average global temperature fell by about 0.6°C (1°F) over the next 15 months [5][6].

• India: watching the stratosphere (IMD, set up in 1875):

• Radiosondes (small instruments carried up by hydrogen balloons that radio back temperature, pressure and humidity) measure the troposphere and lower stratosphere. India has 39 radiosonde and 62 pilot balloon observatories [2].
• Upper-air observation began at Shimla in 1905. An upper-air observatory followed at Agra in 1912 [2]. IMD makes radiosondes at its Delhi workshop, and hydrogen for the balloons is produced at Agra [2].
• 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 the ground to space) is measured with Dobson and Brewer spectrophotometers at five places. Ozone at different heights is measured with ozonesondes (balloon-borne ozone sensors) [3].
• Ozone is also monitored at Maitri and Bharati, India's stations in Antarctica. IMD contributes to WMO's Global Atmosphere Watch [3].

Don't confuse with

• Troposphere: temperature falls with height (about 6.5°C per km), because it is heated from below. It holds nearly all water vapour and all weather. The stratosphere warms with height, because ozone absorbs UV, and it is dry and calm.
• Tropopause: this is the boundary zone where temperature stays nearly constant. It is not part of the stratosphere. Jet streams blow and long-haul jets cruise near the tropopause, not deep in the stratosphere.
• Mesosphere: this layer lies just above the stratosphere, beyond about 50 km. Temperature falls with height there, the opposite of the stratosphere.
• Thermosphere: temperature also rises with height here, but the thermosphere is above the mesosphere. In exam questions about ozone heating, the answer is the stratosphere.

Prelims Hooks

• Order of layers from the ground: troposphere → stratosphere → mesosphere → thermosphere → exosphere. The stratosphere is the second layer.
• The stratosphere runs from the tropopause to about 50 km [4].
• Trap: temperature rises with height in the stratosphere and the thermosphere. It falls only in the troposphere and mesosphere.
• 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 in Antarctica [3].

Mains Points

• The ozone shield and human welfare (GS-I/GS-III):
• The ozone layer in the stratosphere blocks harmful UV rays.
• So protecting it, as the Montreal Protocol does, protects human health and crops.

• India helps through IMD's National Ozone Centre and its Antarctic stations at Maitri and Bharati [3].

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

• The stratosphere has no rain and very little vertical mixing, so aerosols stay there for a year or two.
• Pinatubo in 1991 cooled the globe by about 0.6°C for about 15 months [5][6]. This shows that tiny particles can change the Earth's heat budget.

• The same idea lies behind stratospheric aerosol injection (deliberately spraying particles into the stratosphere to cool the Earth). Such proposals raise questions about risk and about who should control them globally.

• Upper-air monitoring as a disaster-preparedness tool (GS-III):

• IMD's 39 radiosonde stations [2] and its ozone network [3] give data on how conditions change with height.
• This data supports cyclone and heavy-rain warnings, climate monitoring and India's commitments to WMO.

Related concepts

• Structure of the atmosphere
• Troposphere
• Tropopause

Read more

• Structure of the atmosphere: the troposphere and stratosphere