BharatNotes Why this chapter matters for UPSC: Weather and disaster management are recurring themes across GS1 (physical geography, climate), GS3 (disaster management), and even GS2 (governance of IMD, NDMA). Prelims ask on cyclone behaviour, Coriolis effect, weather instruments, and alert systems. Mains links extreme weather to climate change, India's disaster preparedness evolution, and urban resilience.
PART 1 — Quick Reference Tables
| Weather Element | Definition | Instrument Used |
|---|---|---|
| Temperature | Degree of hotness/coldness of air | Thermometer |
| Atmospheric Pressure | Weight of air column above a point | Barometer (mercury or aneroid) |
| Humidity | Amount of water vapour in air | Hygrometer (wet and dry bulb) |
| Wind Speed | Rate of air movement | Anemometer |
| Wind Direction | Direction from which wind blows | Wind vane |
| Precipitation | Water falling from atmosphere | Rain gauge |
| Cloud Cover | Fraction of sky covered by clouds | Okta scale (visual observation) |
| IMD Alert Colour | Meaning | Action |
|---|---|---|
| Green | No severe weather | Watch / Normal operations |
| Yellow | Moderate risk; watch required | Be updated; prepare if in vulnerable area |
| Orange | High risk; be prepared to act | Alert; precautionary measures required |
| Red | Extreme risk; take action now | Emergency action; evacuation if ordered |
| Weather Disaster | India's Vulnerability | Key Statistic |
|---|---|---|
| Cyclones | Bay of Bengal = most cyclone-prone basin globally | ~80% of world's tropical cyclone deaths historically in Bay of Bengal |
| Floods | NE India, Bihar, Odisha most affected | ~5,000 deaths/year average |
| Droughts | Vidarbha, Bundelkhand, Saurashtra chronically prone | El Niño years → 20–50% deficit monsoon possible |
| Heatwaves | North-central and peninsula interior | 1,500–2,000 deaths/year; increasing trend |
PART 2 — Detailed Notes
Weather vs Climate
Weather: The state of the atmosphere at a particular place and at a specific point in time. It includes temperature, humidity, rainfall, wind speed, and cloud cover — all of which can change within hours.
Climate: The average atmospheric conditions (weather) of a place over a long period — conventionally 30 years or more (as defined by the World Meteorological Organization, WMO).
The classic distinction: "Climate is what you expect; weather is what you get."
Weather is short-term and highly variable; climate is long-term and relatively stable. A sudden thunderstorm is a weather event; monsoon rainfall patterns averaged over 30 years are climate.
Elements of Weather
Seven major elements describe weather at any location:
- Temperature — measured in °C or °F; varies with latitude, altitude, season, and time of day; recorded as maximum, minimum, and mean daily temperature.
- Atmospheric Pressure — the weight of the overlying column of air pressing down on a surface; measured in millibars (mb) or hectopascals (hPa); standard sea-level pressure = 1013.25 mb.
- Humidity — the amount of water vapour present in the air; relative humidity is expressed as a percentage of the maximum moisture air can hold at that temperature; 100% = saturation (clouds, fog, dew form).
- Wind — horizontal movement of air from areas of high pressure to low pressure; speed measured in km/h or knots; direction given as the direction from which it blows (a "westerly" blows from west).
- Precipitation — any form of water falling from the atmosphere: rain, snow, sleet, hail, dew, frost.
- Cloud Cover — measured in oktas (eighths of sky covered); affects temperature (daytime cooling, nighttime warming).
- Visibility — distance at which objects can be clearly seen; reduced by fog, haze, dust storms.
Weather Instruments
Stevenson Screen: A ventilated, louvred white wooden box mounted 1.25 m above the ground, housing thermometers, hygrometer, and barometer. The design ensures instruments measure true air temperature, not direct sunlight or ground radiation. Used at all standard meteorological stations worldwide.
- Thermometer: Measures air temperature; max-min thermometers record the daily range; digital sensors used in modern automatic weather stations (AWS).
- Barometer: Measures atmospheric pressure. Mercury barometer (Torricelli, 1643) — mercury column height proportional to pressure. Aneroid barometer — portable, no liquid; uses metal capsule that expands/contracts with pressure changes. A rapidly falling barometer indicates an approaching storm.
- Hygrometer (Wet and Dry Bulb): Two thermometers side by side — one with a wet wick around its bulb. Evaporation cools the wet bulb; the difference between readings indicates relative humidity (greater difference = drier air).
- Rain Gauge: A cylindrical container that collects and measures precipitation; standard IMD gauge has a funnel opening of 20 cm diameter; readings taken daily at 8:30 AM IST.
- Anemometer: Rotating cups measure wind speed; a wind vane (arrow) indicates direction.
Atmospheric Pressure and Wind
Atmospheric pressure decreases with altitude — at 5,500 m altitude, pressure is roughly half of sea-level pressure. This is why high-altitude areas have thinner air and why aircraft cabins are pressurised.
Pressure and wind relationship:
- Hot air is less dense → rises → creates low pressure (cyclone) at surface
- Cold air is denser → sinks → creates high pressure (anticyclone) at surface
- Wind blows from high pressure to low pressure to equalise the difference
Coriolis Effect: Due to Earth's rotation, moving air (and water) is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection causes cyclones (low pressure systems) to rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Anticyclones (high pressure) rotate in the opposite direction.
The Coriolis effect is why Indian Ocean cyclones approaching India spiral in from the Bay of Bengal in a characteristic anticlockwise pattern.
Indian Meteorological Department (IMD)
UPSC GS1/GS3 — IMD: India's Oldest Scientific Department: The Indian Meteorological Department (IMD) was established in 1875, making it one of the oldest scientific departments of the Government of India. It predates many ministries. IMD functions under the Ministry of Earth Sciences (MoES). Headquarters: New Delhi.
Key functions: weather forecasting (up to 15 days now, previously 5 days), monsoon prediction (seasonal forecast released April, updated June), cyclone warnings, aviation meteorology, agro-meteorology.
IMD's technology infrastructure:
- INSAT satellite network: Geostationary satellites provide real-time cloud imagery and weather data every 30 minutes.
- Doppler Weather Radars (DWR): Network of radars across India for cyclone tracking and rainfall estimation; critical for 12–24 hour cyclone warnings.
- Automatic Weather Stations (AWS): ~5,000+ stations across India providing real-time data.
- Mausam App: IMD's official mobile application for real-time weather and forecasts.
- Damini App: Lightning alert app developed by IITM Pune and ESSO; provides advance warning of lightning strikes — crucial for outdoor workers and farmers.
IMD's colour-coded alert system (Green/Yellow/Orange/Red) is now standard for all weather events — cyclones, heatwaves, heavy rainfall, cold waves, fog. State governments trigger evacuation and emergency responses based on these alerts.
Weather and Disasters
Cyclones:
UPSC GS3 — India's Cyclone Preparedness: A Success Story: The Bay of Bengal accounts for approximately 5–6% of the world's tropical cyclones but historically caused ~80% of global cyclone deaths due to storm surges in low-lying Bangladesh and coastal India. India's preparedness transformation is dramatic:
- 1999 Odisha Super Cyclone: ~10,000+ deaths despite some warning
- Cyclone Fani (2019, Category 5): 2.6 million evacuated in 48 hours; fewer than 100 deaths
This improvement resulted from: better IMD forecasting (from 3-day to 5-day track forecasts), the National Cyclone Risk Mitigation Project (NCRMP), improved communication to last mile, a strong NDRF deployment culture, and community preparedness training.
The National Disaster Management Authority (NDMA), established under the Disaster Management Act 2005, coordinates this response.
Floods: India loses approximately 5,000 lives per year to floods on average (NDMA data). Most vulnerable states: Assam, Bihar, Uttar Pradesh, West Bengal, Odisha. The Brahmaputra carries one of the world's highest sediment loads, making Assam's flood problem structurally intractable. The National Flood Control Programme (NFCP) funds embankments, drainage, and flood forecasting.
Droughts: India classifies drought as meteorological (rainfall deficit), hydrological (water body depletion), or agricultural (soil moisture deficit for crops). Vidarbha, Bundelkhand, and Saurashtra are chronically drought-prone. El Niño events (warming of central-eastern Pacific Ocean) typically correlate with below-normal monsoon in India. The National Drought Management Policy (2016) emphasises demand management over supply augmentation.
Heatwaves: A heatwave is declared when maximum temperature is ≥40°C (plains) or ≥30°C (hills) and is 4.5°C or more above normal for at least 2 days. India records 1,500–2,000 heat-related deaths annually (likely an undercount). Heat Action Plans (HAPs) — pioneered by Ahmedabad after the 2010 heatwave — now exist in most states.
Wet-Bulb Temperature: A measure combining temperature and humidity that indicates how effectively the human body can cool itself through sweating. A wet-bulb temperature of 35°C is considered the survivability threshold for a healthy adult — beyond this, the body cannot cool itself even in shade. As climate change intensifies, parts of South Asia risk crossing this threshold by 2100.
Climate Change and Weather
UPSC GS3 — Climate Change and Extreme Weather: The IPCC Sixth Assessment Report (AR6, 2021) confirmed with "high confidence" that climate change is increasing the frequency and intensity of extreme weather events in South Asia:
- More intense rainfall events (even as total monsoon days may decrease)
- Longer and more severe heatwaves
- Increased intensity of cyclones (though not necessarily frequency)
- Accelerated glacier melt → increased GLOF risk
Urban Heat Island (UHI) effect: Cities are 2–5°C warmer than surrounding rural areas due to concrete surfaces, reduced vegetation, waste heat from vehicles and air conditioners. Delhi, Mumbai, Bengaluru all show measurable UHI effects. Green roofs, urban forests (miyawaki method), and cool pavements are being explored as mitigation.
[Additional] 2a. Mission Mausam — India's Doppler Radar Network Expansion (2024–26)
The chapter covers weather instruments and forecasting but does not mention India's flagship meteorological modernisation initiative: Mission Mausam, approved by the Cabinet on September 11, 2024, with a Rs.2,000 crore outlay. The mission is scaling India's Doppler Weather Radar (DWR) network from 14 radars (2014) to 50 radars (2025) and targeting ~100 radars — directly enabling the chapter's concept of "weather prediction" to be covered across much of India's terrain.
Key Terms — Mission Mausam:
| Term | Meaning |
|---|---|
| Doppler Weather Radar (DWR) | A radar system that measures both the intensity and the radial velocity of precipitation — allowing forecasters to detect storm structure, rainfall intensity, and wind shear inside clouds; far more powerful than conventional radar |
| IMD | India Meteorological Department — nodal agency under Ministry of Earth Sciences; established 1875; headquarters New Delhi; issues weather forecasts, cyclone warnings, monsoon outlooks |
| Mission Mausam | Cabinet-approved (September 11, 2024) national weather modernisation mission; Rs.2,000 crore over 2 years; aims to improve forecast accuracy and disaster preparedness across India |
| NWP (Numerical Weather Prediction) | Mathematical models that simulate the atmosphere using physics equations — the core of modern weather forecasting; requires data from dense observation networks (radars, AWS, radiosondes) |
| Mausamgram | IMD's localised hyperlocal weather forecast service launched January 2024 — provides village-level forecasts via mobile app to farmers, fishermen, and disaster managers |
| Lead time | How far in advance a forecast is made — e.g., "7-day lead time for cyclogenesis" means IMD can predict a cyclone forming 7 days before it develops |
[Additional] Mission Mausam — IMD Modernisation and Forecast Accuracy (GS3 — Disaster Management / GS2 — Science Policy):
Cabinet approval:
- Approved: September 11, 2024 by the Union Cabinet
- Ministry: Ministry of Earth Sciences (MoES)
- Outlay: Rs.2,000 crore over 2 years (2024–26)
- Nodal agency: India Meteorological Department (IMD)
Doppler Weather Radar expansion — the core deliverable:
| Year | DWRs Operational | Coverage |
|---|---|---|
| 2014 | 14 DWRs | Limited to metro areas and coasts |
| 2025 | 50 DWRs | Expanded coast, hill states, interior plains |
| Target (Mission Mausam) | ~100 DWRs | Near-complete national coverage including Northeast India |
Why DWRs matter:
- Conventional radar tells you where rain is; Doppler radar tells you how fast raindrops are moving — allowing detection of tornadoes, microbursts, and the internal wind structure of cyclones
- More DWRs = fewer "blind spots" — currently Northeast India and interior Madhya Pradesh have large areas with no radar coverage
- Data from DWRs feeds directly into NWP models, improving their output
Forecast accuracy improvements (2014–2025):
- Cyclone track forecast accuracy: Improved by 20–25% since 2014
- Cyclone landfall accuracy: Improved by 35–45%; 24-hour landfall position error now under 20 km (was ~100 km in 2000)
- Cyclogenesis lead time: IMD can now give 7-day advance warning when a cyclone is likely to form in the Bay of Bengal or Arabian Sea
- Rainfall prediction accuracy: ~85% for seasonal monsoon forecasts
Mausamgram (January 2024):
- Village-level hyperlocal forecasts for farmers and fishermen
- Delivered via IMD mobile app and SMS
- Covers temperature, rainfall, wind speed, and humidity at block/panchayat level
UPSC synthesis: Mission Mausam = GS3 disaster management + GS2 science policy. Key exam facts: Cabinet approved September 11 2024; Rs.2,000 crore; Ministry of Earth Sciences; IMD nodal agency; DWR network 14 (2014) → 50 (2025) → target ~100; cyclone track accuracy up 20-25%; landfall accuracy up 35-45%; 24-hr position error <20 km; 7-day cyclogenesis lead time; rainfall accuracy 85%; Mausamgram launched January 2024 (hyperlocal village-level forecasts). The DWR expansion is the single most impactful infrastructure change in Indian meteorology since independence.
[Additional] 2b. Cyclone Dana (October 2024) — Odisha's Zero-Casualty Model vs 1999 Super Cyclone
The chapter describes cyclones and coastal flooding but provides no modern Indian case study. Cyclone Dana — a Severe Cyclonic Storm that made landfall on the Odisha coast on October 25, 2024 — is the most recent major Indian cyclone and an ideal UPSC case study contrasting disaster preparedness across 25 years: the 1999 Odisha Super Cyclone killed ~10,000 people; Dana resulted in approximately 4–5 deaths despite hitting the same coastal zone with advance notice, mass evacuation (3.62 lakh people), and pre-deployed NDRF teams.
Key Terms — Cyclone Dana:
| Term | Meaning |
|---|---|
| Severe Cyclonic Storm | IMD classification: 3-minute sustained winds 89–117 km/h (Category 1 on Saffir-Simpson scale); the category under which Dana made landfall |
| Landfall | The moment and location where a cyclone's eye crosses the coastline from sea to land |
| NDRF | National Disaster Response Force — India's dedicated disaster response body; 13 battalions; deployed pre-landfall across Odisha and West Bengal for Dana |
| Zero-casualty model | Odisha's stated approach to cyclone management: prioritising mass pre-emptive evacuation so that no preventable deaths occur; adopted after the 1999 disaster |
| Storm surge | Abnormal rise in sea level driven by a cyclone's low pressure and winds — can temporarily raise coastal sea level by 2–5 metres; the primary cause of death in most cyclones |
| Bhitarkanika | Mangrove delta area in Kendrapada district, Odisha — one of India's largest mangrove ecosystems and the landfall zone for Cyclone Dana |
[Additional] Cyclone Dana (October 2024) — Odisha's Disaster Management Success (GS3 — Disaster Management):
Cyclone Dana — key facts:
- Name: Cyclone Dana (named by Qatar; each cyclone in the North Indian Ocean is named by member nations of the WMO panel; the 2024 list was prepared by the 13-member WMO/ESCAP panel)
- Classification: Severe Cyclonic Storm (SCS)
- Landfall: Early hours of October 25, 2024, between Bhitarkanika (Kendrapada district) and Dhamara (Bhadrak district), Odisha
- Wind speed at landfall: 100–110 km/h sustained, gusting up to 120 km/h
- Deaths: Approximately 4–5 total (across Odisha and West Bengal)
- Evacuation: 3.62 lakh (362,000) people evacuated from coastal districts of Odisha; over 3 lakh in West Bengal
Response timeline:
- IMD issued landfall warning 72 hours in advance (accuracy enabled by expanded DWR network and NWP models)
- 25 NDRF teams pre-positioned across Odisha before landfall
- Schools and colleges in 14 Odisha districts declared holiday on October 24 (day before landfall)
- Cyclone shelters activated; vulnerable populations shifted to shelters
- Odisha's Special Relief Commissioner (SRC) coordinated real-time with district collectors
Comparison with 1999 Odisha Super Cyclone:
| Parameter | 1999 Super Cyclone | Cyclone Dana (2024) |
|---|---|---|
| Classification | Super Cyclonic Storm (300 km/h) | Severe Cyclonic Storm (100-110 km/h) |
| Landfall | October 29, 1999; Paradip, Odisha | October 25, 2024; Bhitarkanika-Dhamara |
| Deaths | ~10,000 | ~4–5 |
| Evacuation | Negligible pre-landfall | 3.62 lakh in Odisha + 3 lakh+ in WB |
| Early warning | 24 hours, limited reach | 72 hours, SMS to every coastal household |
| NDRF | Did not exist (formed 2005) | 25 teams pre-positioned |
Why Odisha's model works:
- Cyclone shelters: Odisha has built over 879 permanent multi-purpose cyclone shelters along the coast (funded by NDMA and State Disaster Management Fund)
- Community-level preparation: Village Disaster Management Committees; trained community volunteers; last-mile warning via local governance
- Early warning reach: IMD SMS alerts + All India Radio + NDRF/SDRF coordination
- Zero-casualty political commitment: Odisha CM and SRC treat any preventable cyclone death as a governance failure — creates accountability at all levels
- Mangrove protection: Bhitarkanika's mangroves attenuate storm surge — a natural coastal buffer
UPSC synthesis: Cyclone Dana = GS3 disaster management case study. Key exam facts: landfall October 25 2024; Bhitarkanika-Dhamara Odisha; SCS 100-110 km/h; ~4-5 deaths; 3.62 lakh evacuated Odisha; 25 NDRF teams; 72-hour advance warning; Dana named by Qatar (WMO 13-member panel); 1999 Super Cyclone comparison (10,000 deaths → 4-5 deaths = 99.9% reduction); Odisha zero-casualty model = shelters (879+) + community committees + early warning + political accountability. Paired with: Mission Mausam (IMD radar expansion enabling 72-hour accuracy) and NDMA (formed 2005 post-2004 tsunami and 1999 Odisha cyclone lessons).
Exam Strategy
Prelims traps:
- IMD established 1875 (not 1947, not 1950) — one of India's oldest scientific institutions.
- IMD falls under Ministry of Earth Sciences, not Environment or Home Ministry.
- Cyclones in Northern Hemisphere rotate anticlockwise (Coriolis effect deflects right → inward spin is anticlockwise around a low-pressure centre).
- A falling barometer = storm approaching; rising barometer = fair weather returning.
- Mawsynram (not Cherrapunji) is currently recognised as the world's wettest place by annual average — Cherrapunji holds the record for highest rainfall in a calendar month and a year (historic records).
- Bay of Bengal cyclones are more frequent and more deadly than Arabian Sea cyclones because the Bay is shallower, more enclosed, and SSTs are higher for longer.
Mains angles:
- India's cyclone preparedness transformation (Odisha model, Fani 2019) — governance success story
- Heat Action Plans as climate adaptation — Ahmedabad's leadership
- Urban Heat Island — urban planning, green infrastructure
- IMD modernisation and disaster early warning systems — role of science in governance
Practice Questions
Prelims:
With reference to the Indian Meteorological Department (IMD), which of the following statements is correct?
(a) IMD was established in 1947
(b) IMD functions under the Ministry of Environment
(c) IMD was established in 1875 and functions under the Ministry of Earth Sciences
(d) IMD issues only monsoon-related forecastsCyclones in the Northern Hemisphere rotate in which direction?
(a) Clockwise
(b) Anticlockwise
(c) Direction depends on the season
(d) Direction depends on proximity to the equator
Mains:
India's disaster management response to cyclones has improved dramatically over the past two decades. Analyse the institutional, technological, and community-level factors behind this transformation. (CSE Mains 2020, GS Paper 3, 15 marks)
Examine how Urban Heat Islands are formed and suggest measures to mitigate their impact in the context of India's rapidly urbanising cities. (CSE Mains 2023, GS Paper 1, 15 marks)
