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

Key Term

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:

  1. Temperature — measured in °C or °F; varies with latitude, altitude, season, and time of day; recorded as maximum, minimum, and mean daily temperature.
  2. 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.
  3. 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).
  4. 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).
  5. Precipitation — any form of water falling from the atmosphere: rain, snow, sleet, hail, dew, frost.
  6. Cloud Cover — measured in oktas (eighths of sky covered); affects temperature (daytime cooling, nighttime warming).
  7. Visibility — distance at which objects can be clearly seen; reduced by fog, haze, dust storms.

Weather Instruments

Key Term

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
Key Term

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 Connect

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 Connect

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.

Key Term

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 Connect

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.

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

Previous Year Questions

Prelims:

  1. 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 forecasts

  2. Cyclones 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:

  1. 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)

  2. 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)