Weather Glossary
Key meteorological terms explained in simple language.
Absolute Humidity
The actual mass of water vapour in a cubic metre of air, in g/m³. It is what makes monsoon air feel heavy, and it is the number relative humidity hides.
MeasurementAnticyclone
A circulation of sinking air around a high-pressure centre, turning clockwise north of the equator. It suppresses cloud and rain, and drives South Asian winter and heatwave weather.
PhenomenaAQI (Air Quality Index)
A single number that tells you how polluted the air is and what that means for your health. In South Asia it usually refers to the CPCB National AQI, which runs from 0 to 500.
MeasurementAtmosphere
The envelope of gases surrounding Earth, extending ~100 km up. Contains all weather, life-supporting oxygen, the protective ozone layer, and the climate system that shapes life on Earth.
BasicsAtmospheric Pressure
The weight of the atmosphere above a location, measured in hPa (hectopascals) or millibars. Sea-level standard is 1013.25 hPa. Falling pressure warns of storms; rising pressure brings clear skies.
BasicsBarometer
An instrument that measures the weight of the air above it. The direction and speed of the change matters more than the reading itself for forecasting.
ForecastingBarometric Pressure
Atmospheric pressure as an instrument reports it, corrected to sea level and given in hectopascals. Standard sea-level pressure is 1013.25 hPa, and the direction of change matters more than the value.
MeasurementCarbon Dioxide (CO2)
The long-lived greenhouse gas released when coal, oil and gas are burned. Concentrations have risen from about 280 ppm before industrialisation to past 420 ppm today.
ClimateCelsius
The temperature scale used by every weather service in South Asia. Water freezes at 0°C and boils at 100°C, and the thresholds that trigger heat warnings are all set in it.
MeasurementClear Sky
A sky with no significant cloud, reported as zero to one okta. It brings the strongest sunshine by day, the fastest cooling at night, and the widest temperature swing.
BasicsClimate Change
Long-term shifts in global temperatures and weather patterns driven primarily by greenhouse gas emissions from burning fossil fuels. South Asia is among the most vulnerable regions.
ClimateClimate Zone
A region whose temperature, rainfall and seasonal rhythm are broadly alike. South Asia packs hot desert, tropical monsoon, savanna and permanent ice into one subcontinent.
ClimateCloud Cover
The fraction of the sky hidden by cloud, counted in oktas or shown as a percentage — the number behind every sun, sun-and-cloud or grey-cloud icon in a forecast.
MeasurementCloud Types
Clouds are grouped into ten WMO genera by height and shape, from wispy cirrus to towering cumulonimbus — the storm cloud that delivers most of South Asia rainfall.
BasicsCold Front
The leading edge of an advancing cold air mass. Produces sudden temperature drops, gusty winds and showery weather. Common in north India winter via Western Disturbances.
PhenomenaCumulonimbus
A towering thunderstorm cloud that can reach 15 km altitude, producing heavy rain, lightning, hail and downbursts. The factory of South Asian monsoon weather.
BasicsCyclogenesis
The formation and intensification of a cyclone — the process by which a tropical disturbance becomes a depression, then a cyclonic storm.
PhenomenaDew
Water that condenses directly onto cool surfaces overnight when they chill to the dew point — heaviest across the northern plains on the clear, calm nights of winter.
BasicsDew Formation
Water vapour condensing onto surfaces that have cooled below the dew point overnight, heaviest across the northern plains of India and Pakistan in winter.
BasicsDew Point
The temperature at which air becomes saturated and moisture starts to condense. Higher dew points mean more humid, oppressive conditions.
MeasurementDoppler Radar
A weather radar that measures both precipitation location and the speed of raindrops, revealing storm structure, rotation and downbursts. Backbone of modern monsoon forecasting.
ForecastingDownpour
A short, intense burst of rain from a convective cloud, usually lasting minutes rather than hours. Downpours drive much of South Asia's urban flooding and waterlogging.
BasicsDrizzle
Very light rain of droplets under 0.5 mm falling slowly from low stratus cloud, common in South Asian hill stations and behind winter western disturbances.
BasicsDrought
An extended period of below-normal rainfall causing water shortages, crop failure and ecological stress. The slowest-moving but most economically damaging weather hazard.
PhenomenaDust Storm
A wall of dust and sand swept up by strong winds. Pre-monsoon "andhi" storms over the Thar Desert and Indo-Gangetic plain are routine April-June events that kill dozens each year.
PhenomenaEl Niño
A climate pattern where Pacific Ocean surface temperatures warm above normal, often weakening the Indian monsoon and bringing drought to parts of Asia.
PhenomenaElevation
Height above mean sea level. Air cools by roughly 6.5 °C per 1,000 metres climbed, which is why Shimla and Darjeeling are cool while the plains below them bake.
BasicsENSO (El Niño–Southern Oscillation)
The coupled ocean-atmosphere climate cycle in the tropical Pacific that drives global year-to-year weather variability. Modulates the Indian monsoon.
PhenomenaEquator
The line of zero latitude circling the Earth. No part of India, Pakistan or Bangladesh touches it, but distance from it sets sun angle, day length and cyclone behaviour.
ClimateEvaporation
The conversion of liquid water to water vapor. Cools surfaces and powers the atmosphere with moisture. The reason humid heat feels so much worse than dry heat.
BasicsEye of the Storm
The calm, often clear centre of a mature tropical cyclone, ringed by the eyewall where the strongest winds and heaviest rain are found. The sudden calm when it passes overhead is the most deceptive moment of a cyclone, not the end of it.
PhenomenaFeels-Like Temperature
A combined "apparent" temperature that accounts for humidity (in heat) or wind (in cold), reflecting how hot or cold the air really feels on your skin.
MeasurementFlash Flood
A sudden, rapid flood caused by intense rainfall over a short period, often in steep terrain or urban areas with poor drainage. Warning time is typically only 1–3 hours.
PhenomenaFlooding
Water overflowing onto normally dry land, caused by heavy rain, storm surge, dam failure or river overflow. The most common natural disaster in South Asia.
PhenomenaFog
A cloud at ground level that reduces visibility below 1 km. Dense fog across the Indo-Gangetic plain in December–January is one of South Asia's most disruptive weather phenomena.
PhenomenaFrost
Ice crystals forming on surfaces when air cools below freezing. Common across north India and Pakistan in December-January; can devastate winter crops (mustard, gram, potato).
BasicsGale
A sustained wind of roughly 62 to 88 km/h — Beaufort Force 8 to 9 — strong enough to strip roofs, bring down trees and keep fishing fleets in harbour.
BasicsGreenhouse Effect
The warming produced when gases in the atmosphere absorb heat radiating from the surface and send part of it back down. It is natural and necessary; human emissions have strengthened it.
ClimateHail
Solid ice balls produced by severe thunderstorms. Can range from peas (5 mm) to cricket balls (61–75 mm), causing crop, vehicle and roof damage.
BasicsHaze
Reduced visibility from dry suspended particles — dust, smoke, pollutants — not water droplets. North India and Pakistan suffer severe winter haze.
PhenomenaHeat Index
A "feels-like" temperature that combines air temperature and humidity. At 36°C with 70% humidity, the heat index reaches 52°C — dangerous.
MeasurementHeat Stroke
A life-threatening medical emergency where body temperature exceeds 40°C and the body can no longer cool itself. Untreated heat stroke kills within hours.
PhenomenaHeatwave
A prolonged period of abnormally hot weather, usually lasting several days or weeks. Definitions vary by region.
PhenomenaHemisphere
Half of the Earth — north or south of the equator, east or west of the Prime Meridian. All of India, Pakistan and Bangladesh sit in the northern and eastern halves.
ClimateHigh Pressure System
A region of above-average pressure where air sinks and warms, producing clear skies, calm winds and dry weather. Drives the South Asian winter and the northeast monsoon.
PhenomenaHumidity
The amount of water vapour in the air, expressed as relative humidity (%) — higher means more moisture and a stickier feel.
BasicsIndian Ocean Dipole (IOD)
A climate mode in the Indian Ocean where east-vs-west sea-surface temperatures swing. Can either rescue or wreck the Indian monsoon.
PhenomenaInundation
Normally dry land left standing under water — by a river spilling its banks, by rain a city cannot drain, or by the sea pushed inland during a Bay of Bengal cyclone.
PhenomenaITCZ (Intertropical Convergence Zone)
A belt of rising air near the equator where the northern and southern trade winds collide, producing persistent cloud and rainfall. Drives the South Asian monsoon.
PhenomenaJet Stream
Narrow ribbons of fast wind (150–400 km/h) at 9–12 km altitude that steer weather systems and affect monsoon timing.
PhenomenaLa Niña
The cool phase of ENSO — Pacific surface temperatures fall below normal. Generally strengthens the Indian monsoon and intensifies Bay of Bengal cyclone activity.
PhenomenaLandslide
The downslope movement of rock, soil or debris triggered by heavy rain, earthquakes or human activity. A major monsoon hazard in the Himalayas and Western Ghats.
PhenomenaLightning
An electrical discharge between clouds or between a cloud and the ground, caused by charge separation in thunderstorms. Lightning kills more people in India than any other weather hazard.
PhenomenaLow Pressure System
A region of below-average atmospheric pressure where air rises, cools and condenses — bringing clouds, rain and stormy weather. Drives monsoon depressions and tropical cyclones in South Asia.
PhenomenaMausam
The Hindi and Urdu word for weather (मौसम / موسم), from the Arabic mawsim meaning "season" — the same root that gave English the word monsoon.
VocabularyMist
Thin fog reducing visibility to 1–5 km. Lighter than fog but still meaningful for transport, especially in Himalayan foothills and hill stations.
PhenomenaMonsoon
A seasonal wind reversal that brings heavy rainfall to South and Southeast Asia, driven by temperature differences between land and ocean.
PhenomenaMonsoon Depression
A large, slow-moving low-pressure system that forms within the summer monsoon, usually over the north of the Bay of Bengal, and drifts inland across India bringing three to five days of heavy, widespread rain. Together with weaker monsoon lows, it brings much of the monsoon rain over central India.
PhenomenaMonsoon Onset
The date when the monsoon arrives at a location, marking the transition from dry hot summer to the wet season. India's most-watched annual weather event.
PhenomenaMonsoon Retreat (Withdrawal)
The seasonal end of the monsoon when rains taper off and dry weather returns. Begins in north India in mid-September and reaches the southern tip by mid-October.
PhenomenaNowcasting
Forecasting the next few hours from live radar, satellite and lightning data rather than from a model run. It is how a Kalbaisakhi squall or a Mumbai cloudburst gets a warning at all.
ForecastingOvercast
A sky completely covered by cloud — eight oktas out of eight. The default sky of a monsoon spell on the Konkan coast and of a winter western disturbance over Punjab.
BasicsOzone (O₃)
A molecule of three oxygen atoms. Protective in the stratosphere (10-50 km up — blocks UV); toxic at ground level (key component of photochemical smog). South Asian summer ozone often exceeds WHO limits.
MeasurementPartly Cloudy
A sky of scattered cloud with plenty of blue between — the default South Asian afternoon outside the monsoon, when heating builds cumulus that fade again after sunset.
BasicsPM10
Airborne particles up to 10 micrometres across — road dust, desert dust, smoke and pollen. Coarser than PM2.5, and the dominant pollutant across much of South Asia.
MeasurementPM2.5
Fine particulate matter smaller than 2.5 micrometres — small enough to penetrate deep into the lungs and enter the bloodstream. The most harmful common air pollutant.
MeasurementPost-Monsoon Season
October–December: the transition between the southwest monsoon and winter. The most dangerous cyclone season in the Bay of Bengal.
PhenomenaPrecipitation
Any form of water — rain, drizzle, snow, sleet, hail — falling from clouds to the ground. Measured in millimetres of liquid-equivalent depth.
BasicsRainbow
An arc of colour produced when sunlight is refracted and reflected inside falling raindrops. Seen only with the sun low and behind you and rain ahead of you.
PhenomenaRainfall
Liquid precipitation measured in millimetres (mm). 1 mm equals 1 litre of water spread evenly over 1 square metre.
BasicsRelative Humidity
The percentage of moisture in the air relative to the maximum it can hold at that temperature. 100% means saturated — fog, rain or dew are likely.
MeasurementSleet
Frozen raindrops or refrozen melted snow — small ice pellets that bounce on impact. Occurs at the transition between rain and snow in Himalayan winter storms.
BasicsSnow
Frozen precipitation in the form of ice crystals. Covers the Himalayas, Karakoram and Hindu Kush from December to March — vital for South Asian river systems.
BasicsSolar Radiation
The sun energy reaching the ground, measured in watts per square metre. It sets the pace of heating, evaporation and the monsoon circulation across South Asia.
BasicsSquall
A sudden, sharp burst of wind — typically 60–120 km/h — lasting minutes to an hour, usually from a thunderstorm downburst. Common in pre-monsoon Nor'westers.
PhenomenaStorm Surge
The rise in sea level that a tropical cyclone drives onto a coast, pushed by onshore winds and drawn up by low pressure. Historically the deadliest part of a cyclone, and nowhere on Earth more so than the northern Bay of Bengal.
PhenomenaStratosphere
The stable atmospheric layer above the troposphere, holding the ozone layer and warming with height. Over South Asia it begins unusually high, around 16 to 18 km.
BasicsSunshine
The hours per day when the sun shines brightly enough to cast a sharp shadow — abundant over the Thar and the Deccan, scarce during the monsoon and the winter fog season.
BasicsTemperature
A measure of how hot or cold the air is, reported in Celsius across South Asia — the single number that decides whether a pre-monsoon afternoon is merely hot or genuinely dangerous.
BasicsTemperature Inversion
A reversal of the normal temperature profile — warm air sitting above cooler ground-level air, trapping pollution, fog and smoke below. Root cause of Delhi-Lahore winter smog.
PhenomenaThunder
The sound produced when lightning superheats air to ~30,000°C, causing explosive expansion. Heard up to 25 km from the strike.
BasicsThunderstorm
A storm with lightning, thunder, heavy rain and sometimes hail or strong wind. South Asia experiences thousands of thunderstorms every monsoon season.
PhenomenaTropical Cyclone
A rotating low-pressure weather system with organised thunderstorms that forms over warm tropical waters. Called a typhoon, hurricane or cyclone depending on the ocean basin.
PhenomenaTropical Depression
The first organized stage of a tropical cyclone with sustained winds below 62 km/h. Many monsoon depressions never get stronger but still deliver torrential rain.
PhenomenaTropical Monsoon Climate
A hot climate with no cold season and a short dry season carried by heavy monsoon rain. In South Asia it covers the Malabar and Konkan coasts, much of Bangladesh and the northeast.
ClimateTropical Storm
A tropical cyclone with sustained winds of 62–117 km/h — the stage between tropical depression and a hurricane/typhoon/very severe cyclonic storm.
PhenomenaTropics
The band of Earth between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S) where the sun can be directly overhead. South Asia spans the northern half of the tropics.
ClimateTyphoon
A powerful tropical cyclone in the northwest Pacific with sustained winds above 119 km/h. The same phenomenon is called a hurricane in the Atlantic and a cyclone in South Asia.
PhenomenaUrban Heat Island
Cities are 2–7°C warmer than surrounding rural areas because concrete, asphalt and reduced vegetation absorb and retain heat. A major risk for South Asian megacities.
PhenomenaUV Index
A 0–11+ scale measuring the strength of ultraviolet radiation from the sun. In Delhi, Mumbai and Chennai the index can reach the Extreme category (11+).
MeasurementVisibility
The greatest horizontal distance at which a prominent object can be clearly identified. Reduced by fog, haze, smog, rain, dust storms and air pollution.
MeasurementWater Cycle (Hydrological Cycle)
The continuous movement of water between oceans, atmosphere, land and rivers via evaporation, condensation, precipitation and runoff. Powers all weather and life.
BasicsWeather Forecast
A prediction of future weather using supercomputer models, satellite observations and radar. Accurate about 90% of the time for a five-day forecast and about 80% for a seven-day forecast.
ForecastingWeather Model (NWP)
A supercomputer simulation that predicts future weather by solving fluid-dynamics equations on a global 3D grid. ECMWF and GFS are the leaders; Mausam Online uses ECMWF via Open-Meteo.
ForecastingWeather Radar
An instrument that sends out microwave pulses and listens for the echo from raindrops, showing where rain is falling right now, how hard, and which way it is moving.
ForecastingWestern Disturbance
A mid-latitude low-pressure system, forming over the Mediterranean or further east, that delivers winter rain and snow to Pakistan and northern India. Critical for rabi crops and Himalayan snowpack.
PhenomenaWet-Bulb Temperature
The lowest temperature air can reach via evaporative cooling. A wet-bulb above 35°C is fatal — and South Asian coasts are approaching it.
MeasurementWind Direction
The compass direction wind blows FROM, not towards. In South Asia it is the single clearest signal of the season: southwesterly means monsoon, northwesterly means dry.
MeasurementWind Gust
A brief peak in wind speed, measured by convention as the strongest three-second burst — and the figure that does most of the damage in a South Asian thunderstorm.
BasicsWind Chill
The temperature it feels like when wind speeds up heat loss from exposed skin. Critical for Himalayan winter trekking and high-altitude cold safety.
MeasurementWind Speed
How fast air moves past a fixed point, measured by anemometer ten metres above open ground and reported in km/h, m/s or knots alongside a separate gust figure.
Basics