🌦️ Weather Glossary
Key meteorological terms explained in simple language.
Absolute Humidity
The actual mass of water vapour in a volume of air, measured in grams per cubic metre (g/m³).
measurementAcclimatisation
The body's gradual adjustment to a new climate, typically taking 7-14 days for heat acclimatisation.
generalAir Quality Index (AQI)
A standardized scale that rates air pollution levels from "Good" to "Hazardous", based on concentrations of pollutants like PM2.5, PM10, ozone and NO2.
measurementAlbedo
The fraction of sunlight reflected by a surface. Snow has high albedo (~0.8); ocean and forests have low albedo (~0.1).
climateAnemometer
An instrument that measures wind speed, usually using spinning cups or ultrasonic sensors.
forecastingAnticyclone
A large area of high atmospheric pressure with clockwise wind circulation in the Northern Hemisphere. Brings settled, dry weather.
phenomenaAQI (Air Quality Index)
A standardized index measuring air pollution levels, indicating how clean or polluted the air is and associated health risks.
air-qualityAtmosphere
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.
basicAtmospheric 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.
basicBarometer
An instrument that measures atmospheric pressure. Falling pressure indicates approaching storms; rising pressure signals clearing weather.
forecastingBarometric Pressure
Atmospheric pressure measured by a barometer. Standard sea-level pressure is 1013.25 hPa.
measurementBeaufort Scale
A scale from 0 to 12 that classifies wind speed by its observed effects — from calm (0) to hurricane force (12).
measurementBlack Ice
A thin, transparent layer of ice on road or ground surfaces, nearly invisible and extremely hazardous.
phenomenaBlizzard
A severe snowstorm with strong winds (>56 km/h), heavy snow and very low visibility (<400 m) for at least 3 hours.
phenomenaCarbon Dioxide (CO2)
A greenhouse gas released by burning fossil fuels. Atmospheric levels have risen from 280 ppm pre-industrial to over 420 ppm today.
climateCelsius
The temperature scale used in most of the world (except the US). Water freezes at 0°C and boils at 100°C.
measurementCirrus Clouds
Thin, wispy high-altitude clouds made of ice crystals. Often indicate fair weather but can signal an approaching warm front.
basicClear Sky
A sky with little or no cloud cover (0–1 okta). Maximum solar radiation and temperature reach.
basicClimate 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 of the world with similar temperature, rainfall and seasonal patterns, classified by systems like Köppen.
climateCloud Cover
The fraction of the sky covered by clouds, expressed in oktas (eighths) or percent.
basicCloud Types
Clouds are classified by altitude and shape: cirrus (high, wispy), cumulus (puffy), stratus (layered), nimbus (rain-bearing).
basicCold 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.
phenomenaCondensation
Water vapour turning into liquid droplets, forming clouds, fog and dew. Releases latent heat that powers storms.
basicConvection
The vertical movement of warm air rising and cool air sinking, the primary mechanism for tropical thunderstorm formation.
basicCumulonimbus
A towering thunderstorm cloud that can reach 15 km altitude, producing heavy rain, lightning, hail and downbursts. The factory of South Asian monsoon weather.
basicCyclogenesis
The formation and intensification of a cyclone — the process by which a tropical disturbance becomes a depression, then a storm, then a cyclone.
phenomenaDew
Water droplets that form on cool surfaces overnight when temperature drops to the dew point.
basicDew Formation
The condensation of moisture onto cool surfaces at night, forming water droplets visible at dawn.
basicDew Point
The temperature at which air becomes saturated and moisture starts to condense. Higher dew points mean more humid, oppressive conditions.
measurementDiurnal Temperature Range
The difference between the day maximum and night minimum temperature. Smaller near the equator, larger in deserts.
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 sudden, heavy fall of rain, typically from a cumulonimbus cloud. Common in tropical afternoons during monsoon season.
basicDrizzle
Very light rain consisting of tiny droplets less than 0.5 mm in diameter, falling slowly and steadily.
basicDrought
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.
phenomenaEl Niño Modoki
A variant of El Niño where warming occurs in the central Pacific rather than the eastern Pacific. Has different effects on Asian monsoons.
phenomenaElevation
Height above sea level. Temperature drops about 6.5°C for every 1,000 metres gained in elevation.
basicENSO (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 imaginary line at 0° latitude dividing Earth into Northern and Southern Hemispheres. Receives the most direct sunlight.
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.
basicEye of the Storm
The calm, clear centre of a tropical cyclone, surrounded by the most intense winds and rain of the eyewall.
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).
basicGale
Strong sustained winds of 63-87 km/h (Beaufort Force 8-9). Can cause structural damage and dangerous seas.
basicGreenhouse Effect
The trapping of heat by gases in the atmosphere (CO2, methane, water vapour), warming the planet. Essential for life but enhanced by human emissions.
climateHail
Solid ice balls produced by severe thunderstorms. Can range from peas (5 mm) to cricket balls (8 cm+), causing crop, vehicle and roof damage.
basicHaze
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 35°C with 70% humidity, the heat index reaches 46°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, divided either north-south by the equator or east-west by the Prime Meridian.
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.
basicHygrometer
An instrument that measures atmospheric humidity. Modern versions use capacitive or resistive sensors.
forecastingIndian 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
The covering of normally dry land by water, whether from rivers, storm surge, rainfall or rising sea levels.
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.
phenomenaKnot
A unit of speed equal to one nautical mile per hour (1.852 km/h). Standard for maritime and aviation weather.
measurementLa Niña
The cool phase of ENSO — Pacific surface temperatures fall below normal. Generally strengthens the Indian monsoon and intensifies Bay of Bengal cyclone activity.
phenomenaLand Breeze
A cool wind blowing from land to sea at night, caused by the land cooling faster than the ocean.
basicLandslide
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.
phenomenaLapse Rate
The rate at which air temperature decreases with altitude. The environmental lapse rate averages about 6.5°C per 1,000 metres.
measurementLeeward (Lee Side)
The sheltered side of a mountain or island, facing away from the prevailing wind. Typically drier (rain shadow).
basicLightning
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.
phenomenaMadden-Julian Oscillation (MJO)
A 30-60 day cycle of enhanced and suppressed rainfall that moves eastward across the tropical Indian and Pacific Oceans.
phenomenaMaritime Climate
A climate strongly influenced by the ocean, with small temperature ranges, high humidity and frequent rainfall.
climateMicroclimate
A local climate that differs from the surrounding area due to elevation, vegetation, water bodies or urban features.
climateMillibar (mbar/hPa)
A unit of atmospheric pressure. 1 millibar = 1 hectopascal (hPa). Standard sea-level pressure is 1013.25 mbar.
measurementMist
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 low-pressure system that forms during the monsoon season, bringing sustained heavy rain over wide areas for several days.
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.
phenomenaMudslide (Lahar/Debris Flow)
A fast-moving flow of mud, water and debris down a slope, triggered by heavy rain on steep terrain.
phenomenaNimbostratus
A thick, dark grey cloud layer that produces continuous moderate rain or snow over a wide area.
basicNowcasting
Very short-range weather forecasting (0-6 hours ahead), using radar and satellite data rather than numerical models.
forecastingOrographic Lift
The forcing of air upward over mountains, causing cooling, condensation and rainfall on the windward slope.
phenomenaOvercast
A sky completely covered by clouds (8 oktas). Reduces solar radiation and creates diffused lighting.
basicOzone (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 with scattered clouds covering roughly 3–5 oktas (about 40–60% of the sky).
basicPermafrost
Ground that remains frozen year-round for at least two consecutive years. Not present in Southeast Asia.
climatePM10
Particulate matter smaller than 10 micrometres. Includes dust, pollen and sea salt. Less harmful than PM2.5 but still a respiratory irritant.
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.
basicPsychrometer
A device using two thermometers (wet-bulb and dry-bulb) to measure humidity by comparing their readings.
forecastingRain Shadow
A dry area on the leeward side of a mountain range, where moisture is blocked by the mountains on the windward side.
climateRainbow
An arc of colours visible when sunlight is refracted and reflected by water droplets in the air.
phenomenaRainfall
Liquid precipitation measured in millimetres (mm). 1 mm equals 1 litre of water spread evenly over 1 square metre.
basicRelative 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.
measurementRidge (High Pressure)
An elongated area of high pressure on a weather map, associated with fair weather and suppressed rainfall.
phenomenaSaffir-Simpson Scale
A 1-5 scale rating tropical cyclone intensity by maximum sustained winds: Category 1 (119-153 km/h) to Category 5 (>252 km/h).
measurementSea Breeze
A cool wind blowing from the sea toward land during daytime, caused by the land heating faster than the water.
basicSea Level Rise
The gradual increase in global ocean levels due to thermal expansion and melting ice sheets. A major threat to low-lying Southeast Asian coasts and islands.
climateSleet
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.
basicSnow
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.
basicSolar Radiation
Energy from the sun reaching Earth, measured in watts per square metre (W/m²). Drives weather and climate.
basicSquall
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
An abnormal rise of sea level during a storm, caused by low pressure and strong onshore winds. The most deadly aspect of tropical cyclones.
phenomenaStratosphere
The atmospheric layer above the troposphere (12-50 km altitude). Contains the ozone layer. Temperature increases with altitude.
basicSunshine
Direct solar radiation reaching the ground. Measured in hours per day.
basicTemperature
A measure of how hot or cold the air is, expressed in Celsius (°C) or Fahrenheit (°F).
basicTemperature 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.
basicThunderstorm
A storm with lightning, thunder, heavy rain and sometimes hail or strong wind. South Asia experiences thousands of thunderstorms every monsoon season.
phenomenaTornado
A violently rotating column of air in contact with both a cumulonimbus cloud and the ground. The most destructive small-scale weather phenomenon.
phenomenaTrade Winds
Persistent easterly winds in the tropics, blowing from subtropical high pressure toward the equatorial low. They drive oceanic circulation.
basicTropical 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 63 km/h. Many monsoon depressions never get stronger but still deliver torrential rain.
phenomenaTropical Monsoon Climate
A climate with high temperatures year-round and a pronounced wet monsoon season, typically with annual rainfall over 1500 mm.
climateTropical Rainforest Climate
A climate with high temperatures (25-35°C), high rainfall (>2000 mm/year) and no real dry season. Found near the equator.
climateTropical Savanna Climate
A tropical climate with a longer dry season than monsoon climates. Drier grassland or parkland vegetation.
climateTropical Storm
A tropical cyclone with sustained winds of 63–117 km/h — the stage between tropical depression and a hurricane/typhoon/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.
climateTrough (Low Pressure)
An elongated area of low pressure on a weather map, associated with increased cloudiness, rain and unsettled conditions.
phenomenaTurbulence
Irregular, chaotic air movement that causes sudden jolts for aircraft. Common near thunderstorms, mountains and jet streams.
basicTyphoon
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. Tropical South Asia routinely reaches UV Index 10–13 — extreme.
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.
measurementVolcanic Ash
Fine rock and glass particles ejected during volcanic eruptions. Hazardous to aviation and human health.
phenomenaWarm Front
The leading edge of advancing warm air, producing gradual cloud thickening and steady rain over a wide area.
phenomenaWater 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.
basicWaterspout
A rotating column of air and water spray extending from a cumulonimbus cloud to the sea surface.
phenomenaWeather Forecast
A prediction of future weather using supercomputer models, satellite observations and radar. Accurate to 90%+ for next 24 hours, 60-70% for next week.
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
A system that uses radio waves to detect precipitation, its intensity and movement. Essential for tracking rain, thunderstorms and typhoons in real time.
forecastingWeather Satellite
A spacecraft that orbits Earth and captures images of clouds, storms and atmospheric conditions for weather monitoring.
forecastingWestern Disturbance
A mid-latitude low-pressure system from the Mediterranean 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 FROM which wind blows. A north wind blows from north to south.
basicWind Gust
A brief, sudden increase in wind speed, typically lasting less than 20 seconds.
basicWind 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 Shear
A sudden change in wind speed or direction over a short distance. Dangerous for aircraft and critical for tropical cyclone development.
basicWind Speed
How fast air is moving, measured in km/h, m/s or knots. Stronger winds increase cooling and wave height.
basicWindward
The side of a mountain or island facing the prevailing wind. Typically wetter because moist air is forced to rise.
basicZenith
The point directly overhead in the sky. The sun reaches near-zenith at solar noon in the tropics.
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