NDA Current Affairs · Science & Technology · 10 Aug 2026

Unusual Ozone Layer over the North Bay of Bengal: An NDA Atmospheric Science Explainer

By Aditya Tiwari · 10 Aug 2026 · 14 min read

On 10 August 2026, the Ministry of Science & Technology announced that scientists from a national network of institutions had detected an unusually thick and ozone-rich layer at an altitude of about 21 to 23 km over the North Bay of Bengal — well below the height at which ozone normally peaks — and had traced the cause not to chemistry but to air movement. The Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under the Department of Science and Technology (DST), was a key participant, and the results were published in Earth and Space Science, a journal of the American Geophysical Union (AGU).

For an NDA aspirant this is an unusually generous topic: it is a fresh, datable Indian science achievement, and it sits on top of one of the most reliably examined chapters in the General Ability paper — the structure and composition of the atmosphere.

What the instruments actually saw

The observation, stripped to its essentials:

An enhanced ozone layer at roughly 21–23 km altitude over the North Bay of Bengal.
Ozone amounts in that layer were much higher than is normally measured over eastern India.
The layer persisted for more than a day — it was not a momentary blip.
It was present both during the day and at night.
The work formed part of the Phase-I NetRAD-ASMA campaign, a nationwide programme that pooled several Indian research institutions.
Scientists named in the release include Dr Manish Naja and Dr Samaresh Bhattacharjee of ARIES.

That last detail — daytime and night-time — is the hinge of the whole investigation, and we will come back to it.

Why 21–23 km is the "wrong" altitude

The ozone layer occupies roughly 15 to 35 km of altitude, and its concentration normally peaks at about 25–30 km. Finding the thickest, richest ozone several kilometres below that peak, and finding it sitting there stubbornly for over a day, is the kind of anomaly that demands an explanation.

To see why, you need the vertical structure of the atmosphere clearly in mind — the foundation the NDA notes on atmospheric structure build on:

Layer	Approximate extent	Temperature behaviour
Troposphere	Surface to ~8 km (poles) / ~18 km (equator)	Falls with height (~6.5 °C/km)
Stratosphere	Tropopause to ~50 km	Rises with height — inversion
Mesosphere	~50 to ~85 km	Falls with height; coldest atmospheric layer
Thermosphere	~85 to ~600 km	Rises steeply
Exosphere	Above ~600 km	Merges into space

Two points that examiners love:

The tropopause is highest over the equator (about 16–18 km) and lowest over the poles (about 8 km), because vigorous tropical convection pushes it up. So at the latitude of the Bay of Bengal, an altitude of 21–23 km is comfortably in the lower stratosphere.
The stratosphere warms with height because of ozone itself — ozone absorbs ultraviolet radiation and converts it to heat. The temperature inversion is an effect of the ozone layer, not a coincidence. That inversion also makes the stratosphere very stable, which is why aircraft prefer the lower stratosphere and why volcanic aerosols injected there linger for years.

How ozone is made and unmade: the Chapman cycle

Ozone (O₃) is continuously created and destroyed in the stratosphere in a four-step scheme first set out by Sydney Chapman:

Photodissociation: high-energy ultraviolet radiation splits molecular oxygen — O₂ + UV → O + O.
Formation: an oxygen atom joins an oxygen molecule — O + O₂ → O₃ (a third body carries away the excess energy).
Photolysis: ozone absorbs ultraviolet and breaks apart — O₃ + UV → O₂ + O. This is the step that shields the surface, and it consumes the UV rather than the ozone permanently.
Recombination: O + O₃ → 2O₂, the genuine loss step.

The essential insight for the exam: steps 2 and 3 form a cycle in which ozone is destroyed and immediately remade, absorbing UV each time. The ozone layer is therefore a dynamic steady state, not a static shell of gas.

Notice also where step 1 needs the most sunlight — the tropics, where the Sun is most nearly overhead. Ozone is manufactured mainly over the equator. Yet, as every atlas of total ozone shows, the column of ozone is thinnest over the tropics and thickest over the mid and high latitudes. Resolving that paradox is exactly what explains the Bay of Bengal finding.

The Brewer–Dobson circulation — the concept that cracks the case

The resolution is a slow, planet-wide overturning of the stratosphere known as the Brewer–Dobson circulation:

Air rises into the stratosphere over the tropics, where deep convection and the high tropopause feed it upward.
It then drifts poleward through the stratosphere.
Finally it sinks (subsides) in the middle and high latitudes, carrying accumulated ozone downward.

Two consequences follow, and both are examinable:

Ozone is produced in the tropics but stored elsewhere. That is why the total ozone column is lowest over the equator despite maximum production there.
Where stratospheric air descends, ozone-rich air is pushed to lower altitudes and squeezed. Sinking air undergoes adiabatic compression: it is compressed by the higher pressure below, it warms, and the ozone within it becomes more concentrated per unit volume.

That is the mechanism the Indian team identified. Ozone-rich air was transported horizontally into the region, then pushed downward by persistent subsidence in the lower stratosphere, and compressed — producing an abnormally rich layer at an abnormally low altitude.

How the team ruled out chemistry

Good science is as much about eliminating explanations as proposing them, and the reasoning here is clean enough to quote in an interview:

The night-time test. Ozone production by the Chapman cycle requires sunlight. The enhanced layer was observed at night as well as by day, and it survived more than 24 hours. Photochemistry therefore could not be the primary cause.
The independent-instrument test. Balloon-borne measurements were confirmed by satellite observations — the Aura MLS (Microwave Limb Sounder) and INSAT-3DR, ISRO's meteorological satellite. Agreement between an in-situ platform and two remote-sensing platforms rules out an instrument artefact.
The dynamics test. The team then examined the candidate physical processes in turn — convection, wind shear, horizontal transport and vertical air motion — and found persistent downward air motion in the lower stratosphere. Radars measure vertical velocity directly, which is precisely why they were decisive.

The conclusion: horizontal transport of ozone-rich air, followed by downward motion and compression of the air mass.

The instruments — an Indian capability story

The measurement chain is worth learning as a set, because "which instrument does what" is a classic matching question:

The ARIES Stratosphere–Troposphere (ST) Radar, Nainital — an indigenously developed 206.5 MHz radar, the workhorse of this study. An ST radar is a wind profiler: it senses tiny refractive-index irregularities in clear air and returns profiles of horizontal and vertical wind through the troposphere and lower stratosphere. Crucially, it measures vertical velocity, which balloons and satellites cannot supply directly.
A national radar network — companion radars at Haringhata (West Bengal), Gadanki (Andhra Pradesh) and Kochi (Kerala). Simultaneous observation from widely separated sites let the team compare wind patterns across the country rather than guess from a single point.
Ozonesondes and radiosondes — balloon-borne packages measuring ozone partial pressure, temperature, pressure and humidity as they ascend. These give the vertical profile in situ.
Satellites — Aura MLS for limb-sounding of stratospheric composition, and INSAT-3DR for meteorological context.
Atmospheric models — used to trace where the air mass had travelled from.

ARIES itself is worth a line of general knowledge: located near Nainital in Uttarakhand, an autonomous institute of DST, and the host of the 3.6-metre Devasthal Optical Telescope, the largest optical telescope in India. Its radar work and the wider study of winds and air masses belong to the same syllabus family.

Measuring ozone: Dobson Units and the "hole"

Since ozone is spread thinly through a deep column, it is measured by imagining it compressed into a single layer at the surface:

One Dobson Unit (DU) = a layer 0.01 mm thick at standard temperature and pressure.
The global average total column is roughly 300 DU — about 3 mm of pure ozone if brought to sea level. The shield that protects the biosphere is, quite literally, three millimetres thick.
An "ozone hole" is conventionally defined as a total column below 220 DU.

The Antarctic ozone hole, reported in 1985 by British Antarctic Survey scientists Farman, Gardiner and Shanklin, forms because of a specific chain: the polar vortex isolates Antarctic air in winter; temperatures fall below about −78 °C, allowing polar stratospheric clouds to form; reactions on those cloud particles convert inactive chlorine reservoirs into reactive forms; and when spring sunlight returns in September–October, chlorine destroys ozone catalytically — a single chlorine atom can destroy thousands of ozone molecules. The Arctic hole is far weaker because its vortex is less stable and its stratosphere warmer.

The treaty architecture follows directly, and is high-yield:

Vienna Convention, 1985 — the framework convention for protection of the ozone layer.
Montreal Protocol, 1987 — the operative treaty controlling ozone-depleting substances (CFCs, halons, carbon tetrachloride, methyl chloroform, HCFCs); universally ratified, and the reason the layer is now projected to recover around the middle of this century.
Kigali Amendment, 2016 — extends the Protocol to hydrofluorocarbons (HFCs), which do not deplete ozone but are potent greenhouse gases. India ratified it in 2021.
World Ozone Day: 16 September.

Good ozone, bad ozone

One distinction reliably separates a prepared candidate from an unprepared one — "good up high, bad nearby":

Stratospheric ozone is protective. It absorbs UV-B, the band implicated in skin cancer, cataracts and damage to phytoplankton, the base of the marine food chain.
Tropospheric ozone is a secondary pollutant — never emitted directly, but formed when oxides of nitrogen (NOx) and volatile organic compounds (VOCs) react in sunlight. It is the defining ingredient of photochemical smog, it aggravates asthma and it measurably reduces yields of wheat and rice. It is also a short-lived climate pollutant, i.e. a greenhouse gas in its own right, and it is one of the pollutants monitored under India's National Ambient Air Quality Standards.

The revision hook: an unusually thick ozone layer was found at 21–23 km over the North Bay of Bengal, below the normal 25–30 km peak, persisting more than a day and present at night; the cause was horizontal transport of ozone-rich air plus downward motion and adiabatic compression, not photochemistry; instruments were the indigenous 206.5 MHz ARIES ST Radar at Nainital with radars at Haringhata, Gadanki and Kochi, ozonesondes, Aura MLS and INSAT-3DR; part of Phase-I of the NetRAD-ASMA campaign; published in Earth and Space Science (AGU); ozone layer 15–35 km; 1 DU = 0.01 mm at STP, global average ~300 DU, hole below 220 DU; Vienna Convention 1985, Montreal Protocol 1987, Kigali Amendment 2016, World Ozone Day 16 September.

Why it matters beyond the headline

Sovereign observing capability. The decisive instrument was indigenously developed. A country that can only download other people's satellite data cannot investigate its own atmosphere; one that operates its own radar network can. That argument transfers directly to a lecturette on self-reliance in science.
Networks beat single stations. No single site could have distinguished local chemistry from long-range transport. The finding is an advertisement for coordinated, multi-institution campaigns.
Monsoon relevance. The upper atmosphere over the Bay of Bengal in the summer season is one of the most dynamically active regions on Earth, and understanding how air is exchanged between troposphere and stratosphere there feeds into better modelling of the Indian monsoon — a subject of direct national consequence.
Ozone recovery is being monitored, not assumed. The Montreal Protocol is the most successful environmental treaty ever negotiated, but verifying recovery requires exactly this kind of patient measurement.

Exam relevance in one paragraph

For NDA General Ability, retain: on 10 August 2026 the Department of Science and Technology reported that Indian scientists had observed an unusually thick and ozone-rich layer at about 21 to 23 kilometres over the North Bay of Bengal, several kilometres below the altitude of 25 to 30 kilometres at which ozone concentration normally peaks within the 15-to-35-kilometre ozone layer; the enhancement lasted more than a day and appeared at night as well as by day, which ruled out sunlight-driven photochemical production and pointed instead to horizontal transport of ozone-rich air followed by persistent downward motion and adiabatic compression in the lower stratosphere, consistent with the Brewer-Dobson circulation in which air rises over the tropics, moves poleward and subsides at higher latitudes; the observations came from the indigenously developed 206.5 megahertz ARIES Stratosphere-Troposphere Radar at Nainital together with radars at Haringhata, Gadanki and Kochi, balloon-borne ozonesondes, the Aura Microwave Limb Sounder and INSAT-3DR, under Phase-I of the NetRAD-ASMA campaign, and were published in Earth and Space Science of the American Geophysical Union; ozone is measured in Dobson Units where one unit equals a layer 0.01 millimetre thick at standard temperature and pressure, the global average column being about 300 units and an ozone hole being defined below 220 units; the governing treaties are the Vienna Convention of 1985, the Montreal Protocol of 1987 and the Kigali Amendment of 2016 on hydrofluorocarbons, with World Ozone Day on 16 September.

🎯 Practice MCQs

Q1. The unusually thick ozone layer reported in August 2026 was observed over the: (a) North Bay of Bengal (b) Arabian Sea (c) Gulf of Mannar (d) Rann of Kutch → (a) — the North Bay of Bengal.

Q2. The enhanced ozone layer was found at an altitude of about: (a) 21–23 km (b) 5–7 km (c) 50–55 km (d) 80–85 km → (a) — in the lower stratosphere.

Q3. Ozone concentration in the atmosphere normally peaks at about: (a) 25–30 km (b) 10–12 km (c) 45–50 km (d) 2–3 km → (a) — which is why the observation was anomalous.

Q4. The ARIES ST Radar used in the study is located at: (a) Nainital (b) Gadanki (c) Kochi (d) Haringhata → (a) — Nainital; the other three hosted companion radars.

Q5. ARIES is an autonomous institute under the: (a) Department of Science and Technology (b) ISRO (c) Ministry of Earth Sciences (d) DRDO → (a) — DST.

Q6. The scientists concluded that the ozone enhancement was mainly caused by: (a) transport of ozone-rich air and downward motion (b) photochemical production (c) volcanic eruption (d) industrial emissions → (a) — dynamics, not chemistry.

Q7. The main evidence against a photochemical cause was that the layer: (a) was present at night as well (b) was very thin (c) lasted only minutes (d) appeared only in winter → (a) — sunlight-driven chemistry cannot operate at night.

Q8. In the stratosphere, temperature increases with altitude mainly because: (a) ozone absorbs ultraviolet radiation (b) of proximity to the Sun (c) of water vapour (d) of carbon dioxide → (a) — absorption by ozone heats the layer.

Q9. One Dobson Unit corresponds to a layer of ozone: (a) 0.01 mm thick at STP (b) 1 cm thick (c) 1 m thick (d) 1 km thick → (a) — the global average column is about 300 DU.

Q10. An "ozone hole" is conventionally defined as a total column below: (a) 220 DU (b) 300 DU (c) 500 DU (d) 100 DU → (a) — 220 Dobson Units.

Q11. Tropospheric ozone is best described as a: (a) secondary pollutant (b) primary pollutant (c) protective shield (d) noble gas → (a) — formed from NOx and VOCs in sunlight.

Q13. The Kigali Amendment (2016) deals with: (a) hydrofluorocarbons (b) chlorofluorocarbons (c) methane (d) sulphur dioxide → (a) — HFCs, potent greenhouse gases.

Q14. The circulation in which air rises in the tropics, moves poleward and sinks at higher latitudes in the stratosphere is the: (a) Brewer–Dobson circulation (b) Hadley cell (c) Walker circulation (d) Ferrel cell → (a) — Brewer–Dobson.

Q15. The tropopause is highest over the: (a) equator (b) poles (c) Tropic of Cancer only (d) mid-latitudes only → (a) — about 16–18 km over the equator against roughly 8 km at the poles.

📋 How this gets asked (PYQ pattern)

Ozone is a perennial NDA topic, and it is asked in three recognisable ways. First, structure: which layer contains ozone, at what altitude it peaks, and why the stratosphere shows a temperature inversion — nearly always a straight one-liner. Second, the good-versus-bad distinction: statement pairs that invert stratospheric and tropospheric ozone, which is the single most common trap in this chapter. Third, treaty matching: Vienna 1985 with the framework, Montreal 1987 with ozone-depleting substances, Kigali 2016 with HFCs, and 16 September with World Ozone Day. The fresh 2026 hook adds a fourth angle — an institution-and-instrument item (ARIES / Nainital / DST / ST Radar / INSAT-3DR), which is how "science in the news" is normally converted into a question. Note the pattern, not any specific past question number.

Preparing for NDA? Atmospheric science pays twice — once in the General Ability paper and again in the SSB, where "explain the ozone hole" is a standard lecturette and GD topic. Follow our daily NDA current affairs and train with serving and retired officer faculty in the upcoming Cavalier courses in Delhi.

✍️ Written by Aditya Tiwari — Science, geography & current-affairs faculty at The Cavalier. Reviewed by the Cavalier Faculty Desk. The Cavalier, founded by ex-Army officers, has trained NDA/CDS/SSB aspirants since 2001 (Facebook · YouTube).

Source: PIB / Ministry of Science & Technology (DST), 10 August 2026. Facts cross-verified with independent sources.

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