NCERT
Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Fundamentals of Physical Geography — all chaptersWater in the Atmosphere
Start here: why this chapter matters
In simple wordsThink of the air as a sponge. Warm air is a big sponge that can soak up lots of water vapour; cold air is a small sponge. Squeeze the sponge — cool the air — and water drips out as dew, fog, cloud and finally rain. This chapter is about how much water the air holds, how it lets it go, and where on earth it falls.
Water vapour is only a small part of the air — from zero to four per cent by volume — but it drives the weather. Water exists in the air in three forms: gas (vapour), liquid (droplets) and solid (ice crystals). It enters the air from water bodies by evaporation and from plants by transpiration, and returns by condensation and precipitation. This continuous exchange links the atmosphere, the oceans and the continents.
- Builds on Chapter 7 (water vapour as a variable gas) and Chapter 8 (cooling of air; latent heat in the heat budget).
- Uses Chapter 9: rising air in lows, fronts and cyclones is what makes clouds and rain.
- Leads into Chapter 11 (climate types are defined by rainfall as well as temperature) and the water chapters.
- UPSC: GS1 Geography (precipitation, rain-shadow, world rainfall) and GS3 Disaster management (cloudbursts, floods, smog and fog).
| Word | What it means here |
|---|---|
| Humidity | The water vapour present in the air |
| Saturated air | Air holding all the vapour it can at its temperature |
| Dew point | The temperature at which a sample of air becomes saturated |
| Evaporation / condensation | Liquid → vapour / vapour → liquid |
| Hygroscopic nuclei | Tiny particles (dust, smoke, sea salt) that attract water, on which droplets form |
| Precipitation | Water falling from the air in liquid or solid form — rain, snow, sleet, hail |
| Windward / leeward | The side of a mountain facing the wind / the sheltered side away from it |
At a glance — the six ideas to carry away
- Water vapour: 0–4% of the air by volume; it comes from evaporation and transpiration.
- Relative humidity = vapour present ÷ what the air can hold at that temperature (as %).
- Dew point: the temperature at which air becomes saturated; cool it more and vapour condenses.
- Dew needs a dew point above 0°C; frost forms when it is at or below 0°C.
- Four basic clouds: cirrus (highest), cumulus, stratus, nimbus.
- Three kinds of rain: convectional, orographic (relief), cyclonic (frontal).
1. Humidity, saturation and dew point
In simple wordsA glass that holds 300 ml has 150 ml of water in it. The absolute amount is 150 ml; the glass is 50% full — that percentage is like relative humidity. Now swap the glass for a smaller one (cool the air): the same 150 ml may fill it completely — the air is saturated. The temperature at which that happens is the dew point.
| Term | Meaning | Unit / note |
|---|---|---|
| Absolute humidity | The actual weight of water vapour in a unit volume of air | grams per cubic metre; differs from place to place |
| Relative humidity | The moisture present as a percentage of the air's full capacity at that temperature | %; changes when temperature changes; greatest over oceans, least over continents |
| Saturated air | Air holding moisture to its full capacity — it cannot take any more at that temperature | RH = 100% |
| Dew point | The temperature at which saturation occurs in a given sample of air | Cool below it → condensation |
The key idea: the ability of air to hold water vapour depends entirely on its temperature. Warm air can hold much more than cold air. So if air is warmed while its vapour stays the same, its relative humidity falls; if it is cooled, its relative humidity rises — until it reaches 100% at the dew point.
Beyond NCERT — the numbers behind the curveA cubic metre of air can hold roughly 5 g of water vapour at 0°C, 17 g at 20°C and 30 g at 30°C — the capacity roughly doubles with every 10–11°C rise. Relative humidity is measured with a hygrometer. UPSC 2003 tested exactly this chain: moisture in the air varies with latitude because the capacity to hold moisture depends on temperature (both true, the second explains the first). UPSC 1981 accepted that humidity increases as air temperature rises — warmer air can hold, and over warm seas usually does hold, more vapour.
Exam trap
- Air ‘that contains moisture to its full capacity’ is saturated air — not relative humidity (NCERT Q1-iii).
- Cooling air does not change its absolute humidity (the grams of vapour stay the same) — it raises its relative humidity.
- Relative humidity is higher over oceans; absolute humidity is highest in warm, wet regions.
2. Evaporation and condensation
In simple wordsWet clothes dry fastest on a hot, dry, windy day: heat gives the water energy to escape, dry air has room to take it, and wind keeps carrying the damp air away. The opposite — vapour turning back into water — happens when air cools: think of the droplets on the outside of a glass of cold water.
Evaporation turns liquid water into vapour; heat is its main cause. The latent heat of vaporisation is the heat needed to turn a unit mass of liquid into vapour without a change in temperature. Evaporation increases when: (1) the temperature is higher (air can absorb and hold more); (2) the air's moisture content is low; (3) the air moves — wind replaces the saturated layer next to the water with unsaturated air.
Condensation turns vapour back into water; it is caused by the loss of heat. When moist air cools to the point where it can no longer hold its vapour, the excess condenses. In free air, droplets form around tiny hygroscopic condensation nuclei — particles of dust, smoke and sea salt, which are good nuclei because they absorb water. Condensation also happens when moist air touches a colder object, or when the temperature is close to the dew point. It depends on the amount of cooling and the relative humidity, and is influenced by the air's volume, temperature, pressure and humidity.
- Condensation takes place when the temperature falls to the dew point with the volume constant.
- …when both volume and temperature are reduced.
- …when moisture is added to the air through evaporation.
- The most favourable condition of all: a fall in air temperature.
NCERT slipThe book numbers these conditions (i), (ii), (iv) — there is no (iii). Nothing is missing from the meaning; learn the three conditions plus ‘cooling is the most favourable’.
⤵ From vapour to a droplet
Moist airfrom evaporation and transpiration
Air coolsrising, touching a cold surface, or losing heat at night
Reaches the dew pointrelative humidity 100% — saturated
On a cold surfacedew or frost
In free air, on nucleifog, mist or cloud
Droplets grow until air cannot hold them upprecipitation
Beyond NCERTThe latent heat absorbed during evaporation is released during condensation — the same heat that warms the atmosphere in the heat budget (19 units, Chapter 8) and fuels cyclones (Chapter 9). This is also why evaporation cools: a fan cools you by speeding up evaporation from your skin.
3. Forms of condensation: dew, frost, fog and mist
In simple wordsWhere the water appears tells you its name. On a cold surface on a clear night: dew (if it's above freezing) or frost (if it's below). In the air near the ground: fog or mist. High in the sky: clouds.
| Form | Where | Dew point | Ideal conditions |
|---|---|---|---|
| Dew | Water droplets on cool surfaces — stones, grass blades, leaves (not on nuclei in the air) | Above freezing | Clear sky, calm air, high relative humidity, cold and long nights |
| Frost | Minute ice crystals on cold surfaces | At or below 0°C | Same as dew, but the air temperature must be at or below freezing |
| Fog | Condensation within an air mass on fine dust particles — a cloud with its base at or very near the ground | — | Moist air cools suddenly; common where warm air meets cold currents |
| Mist | Like fog but each nucleus has a thicker layer of moisture | — | Frequent over mountains, where warm air rising up the slopes meets a cold surface |
| Smog | Fog mixed with smoke | — | Urban and industrial centres, where smoke gives plenty of nuclei |
| Fog | Mist | |
|---|---|---|
| Horizontal visibility (NCERT) | Less than 1 km | 1 km to 2 km |
| Moisture (NCERT) | Drier than mist | Contains more moisture |
| Typical place (NCERT) | Where warm currents of air meet cold currents | Over mountains |
NCERT puzzle — fog vs mistNCERT says fog cuts visibility more than mist, yet calls mist the wetter of the two. Learn both lines as NCERT states them for Prelims. (Beyond NCERT, in meteorology the difference is defined by visibility: below 1 km is fog, above 1 km is mist.)
⤵ Dew or frost? (NCERT exercise Q3-ii)
Long, clear, calm nightthe ground radiates its heat away
Ground and grass cool below the air's dew pointhigh relative humidity helps
Dew point above 0°C→ DEW: water droplets
Dew point at or below 0°C→ FROST: ice crystals
Beyond NCERT (asked by UPSC)
- Less dew forms on cloudy nights: clouds absorb the earth's outgoing long-wave radiation and send part of it back, so the ground does not cool enough (UPSC 1979: ‘radiation takes place very slowly’; UPSC 2019's key: ‘clouds reflect back the earth's radiation’).
- Fog is common off Newfoundland because the warm Gulf Stream meets the cold Labrador current (UPSC 1988) — NCERT's ‘warm currents of air meet cold currents’ in action.
- NCERT's smog is fog + smoke. Photochemical smog (from vehicle exhaust and sunlight) is a different, environment-syllabus topic.
4. Clouds
In simple wordsA cloud is just fog that formed high up instead of at the ground. Its shape tells you how it formed: feathery wisps very high (cirrus), cotton-wool heaps from rising bubbles of air (cumulus), flat grey sheets from a whole layer cooling (stratus), and dark, heavy masses ready to rain (nimbus).
A cloud is a mass of minute water droplets or tiny ice crystals formed by condensation in free air at considerable heights. By height, expanse, density and transparency, clouds are grouped into four types.
| Type | Height (NCERT text) | Look and features |
|---|---|---|
| Cirrus | 8,000–12,000 m (highest) | Thin, detached, feathery; always white |
| Cumulus | 4,000–7,000 m | Like cotton wool; in scattered patches; flat base |
| Stratus | — | Layered, covering large parts of the sky; from loss of heat or mixing of air masses of different temperatures |
| Nimbus | Middle levels or very near the ground | Black or dark grey; extremely dense, opaque to the sun; shapeless masses of thick vapour; can seem to touch the ground |
| Combination family | Clouds |
|---|---|
| High clouds | Cirrus, cirrostratus, cirrocumulus |
| Middle clouds | Altostratus, altocumulus |
| Low clouds | Stratocumulus, nimbostratus |
| Clouds with extensive vertical development | Cumulus, cumulonimbus |
Reading NCERT Figures 10.1 and 10.2 — cloud photographs (page 88)
- The book asks you to identify the clouds; it gives no answer.
- Figure 10.1: separate white heaps with flat bases and rounded, cauliflower tops, scattered across a blue sky above a lake and bare mountains — cumulus (fair-weather cumulus).
- Figure 10.2: a sky almost completely covered by a dark grey, shapeless, dense layer, with sunlight breaking through a gap, above a reservoir — nimbus / nimbostratus, a rain cloud.
- Use the clues the text gives: shape (heaps vs sheets), colour (white vs dark grey) and density (light passing through or not).
NCERT inconsistency — cumulus heightThe chapter puts cumulus at 4,000–7,000 m, but the book's own glossary gives cumulus an altitude range of 300–2,000 m (and cumulonimbus from a few hundred m to over 12,000 m; cirrostratus and cirrocumulus 5,000–18,000 m). Cumulus bases are usually low and their tops can tower high, which is why the figures differ. For an NCERT-based MCQ, the safe facts are: cirrus is the highest, nimbus the lowest and darkest, cumulus has a flat base.
Beyond NCERT (asked by UPSC)Cirrus clouds are made of ice crystals (UPSC 2004), and a halo round the sun or moon is produced by light refracting through the ice crystals of cirrus-type clouds (UPSC 2002). Clouds float because their tiny droplets and the air around them have low density, and rising air keeps them up (UPSC 1995).
5. Precipitation: rain, snow, sleet and hail
In simple wordsCloud droplets are so tiny that even gentle rising air keeps them afloat, like dust in a sunbeam. They bump, join and grow. Once a drop is too heavy for the air to hold up, it falls. What it becomes on the way down — rain, snow, sleet or hail — depends on the temperatures of the layers it falls through.
Continuous condensation in free air makes the particles grow. When air resistance can no longer hold them against gravity, they fall to the earth — precipitation. It may be liquid or solid. Rainfall is precipitation as water. When the temperature is below 0°C, it falls as fine flakes of snow — snowfall; the moisture is released as hexagonal crystals that form snowflakes. Sleet and hail are less common and sporadic in time and space.
| Form | How it forms | What reaches the ground |
|---|---|---|
| Rain | Drops fall through air above freezing | Water drops |
| Snow | Temperature below 0°C; vapour forms hexagonal ice crystals | Snowflakes |
| Sleet | A layer of air above freezing lies over a sub-freezing layer near the ground; raindrops from the warm layer freeze in the cold layer below | Frozen raindrops and refrozen melted snow-water — small ice pellets no bigger than raindrops |
| Hail | Raindrops released by clouds pass through colder layers and solidify | Rounded pieces of ice with several concentric layers |
Beyond NCERTHail grows its onion-like layers because strong updrafts in a cumulonimbus cloud carry the ice up and down through super-cooled water several times (Chapter 9, thunderstorms). The NCERT glossary gives hailstone sizes from 5 to 190 mm and lists cloudbursts as a form of precipitation — very intense rain over a small area in a short time, common on Himalayan slopes (UPSC Mains 2024).
6. Types of rainfall
In simple wordsRain needs air to rise and cool. There are three ways to lift air: heat it from below so it bubbles up (convectional), push it over a mountain (orographic), or slide it up over a wedge of colder air at a front (cyclonic or frontal).
Convectional rain. Heated air becomes light and rises in convection currents; it expands, loses heat, and condenses into cumulus clouds. Heavy rain falls with thunder and lightning, but it does not last long. It is common in summer or the hotter part of the day, and very common in the equatorial regions and the interiors of continents, especially in the northern hemisphere.
Orographic (relief) rain. Saturated air meeting a mountain is forced to rise; it expands, cools and its moisture condenses. The windward slopes get the most rain. Crossing to the other side, the air descends and warms; its capacity to hold moisture increases, so the leeward slopes stay rainless and dry — the rain-shadow area.
Cyclonic (frontal) rain. NCERT refers you back to Chapter 9: in extra-tropical cyclones, warm air glides over cold air at the warm front (wide clouds, steady rain) and is pushed up sharply at the cold front (cumulus, heavy showers).
| Convectional | Orographic | Cyclonic / frontal | |
|---|---|---|---|
| What lifts the air | Heating from below | A mountain barrier | Colder, denser air at a front |
| Clouds | Cumulus, cumulonimbus | Clouds on the windward slope | Layered clouds ahead of the warm front; cumulus at the cold front |
| Rain | Heavy, short, with thunder | Heavy on windward side; rain-shadow on leeward side | Long and steady (warm front) or heavy showers (cold front) |
| Where / when | Equatorial belt; continental interiors in summer afternoons | Mountains across moist winds | Middle latitudes (temperate cyclones) |
⤵ How a rain-shadow forms
Moist wind meets a mountaine.g. winds from the sea
Forced up the windward slopeexpands and cools
Condensation and heavy rainwindward side is wet
Air descends the leeward slopewarms; can hold more moisture
Little or no rainthe rain-shadow area
Beyond NCERT (asked by UPSC)
- Areas near the equator get rain throughout the year because high temperature and humidity bring convectional rain most afternoons (UPSC 1996 and 2003 — both times the reason correctly explained the assertion).
- Rainfall is scanty east of the Western Ghats because it lies on the leeward side — a rain-shadow (UPSC 1989).
7. World distribution of rainfall
In simple wordsRain follows a few simple rules: warm places get more than cold ones, coasts more than interiors, oceans more than land, and the side facing the rain-bearing wind more than the sheltered side.
- Rainfall decreases steadily from the equator towards the poles.
- Coastal areas get more rain than continental interiors; oceans get more than land (they are great sources of water).
- Between 35° and 40° N and S, rain is heavier on the eastern coasts and decreases westward.
- Between 45° and 65° N and S, the westerlies bring rain first to the western margins; it decreases eastward.
- Where mountains run parallel to the coast, rain is greater on the coastal plain on the windward side and decreases towards the leeward side.
| Annual rainfall | Regions (NCERT) |
|---|---|
| Over 200 cm | The equatorial belt; windward slopes of mountains along western coasts in the cool temperate zone; coastal areas of the monsoon lands |
| 100–200 cm | Interior continental areas (moderate); coastal areas of the continents also receive moderate rain |
| 50–100 cm | Central parts of tropical lands; eastern and interior parts of temperate lands |
| Less than 50 cm | Rain-shadow areas in the continental interiors; high latitudes |
Seasonal distribution matters too: rain that is spread evenly through the year is more useful than the same amount in one burst. Rainfall is distributed evenly in the equatorial belt and the western parts of cool temperate regions.
Exam trapThe east-coast rule (35°–40°) and the west-coast rule (45°–65°) are opposites — don't swap them. The west-coast rule comes from the westerlies; the NCERT text gives no reason for the east-coast rule (Beyond NCERT: these coasts face onshore summer winds such as the monsoons of China and the south-eastern USA).
NCERT exercise — answer outlines
Try each question yourself first, then open the outline to compare. MCQs (ii)–(iv) are in the self-check below.
MCQ (i): Which one of the following is the most important constituent of the atmosphere for human beings? — water vapour, nitrogen, dust particle, oxygen
NCERT publishes no answer key. For survival, oxygen is the obvious answer (we breathe it). Read in this chapter's context, water vapour is the constituent that makes weather, rain and fresh water possible — some guides choose it. In an exam, read the wording: ‘for human beings’ in a general sense → oxygen.
Name the three types of precipitation. (30 words)
By form: rain (liquid), snow (hexagonal ice crystals below 0°C), and sleet and hail (ice pellets). By origin, rainfall is convectional, orographic and cyclonic/frontal — mention both readings.
Explain relative humidity. (30 words)
The moisture present in the air as a percentage of its full capacity at that temperature. Warming the air lowers it, cooling raises it; it reaches 100% at the dew point. Higher over oceans than continents.
Why does the amount of water vapour decrease rapidly with altitude? (30 words)
Vapour comes from evaporation at the surface, and air cools with height (6.5°C per km), so its capacity to hold vapour falls; rising air condenses its moisture into clouds and rain at lower levels.
How are clouds formed? Classify them. (30 words)
Moist air rises and cools to its dew point; vapour condenses on hygroscopic nuclei in free air at height. Types: cirrus (high, feathery), cumulus (cotton-wool, flat base), stratus (layered), nimbus (dark rain clouds).
Discuss the salient features of the world distribution of precipitation. (150 words)
Intro: precipitation varies in amount and season across the globe.
General rules: decreases from the equator to the poles; coasts wetter than interiors; oceans wetter than land; windward sides wetter than leeward (rain-shadow).
Coast rules: 35°–40° N/S — east coasts wetter, decreasing westward; 45°–65° N/S — westerlies make west coasts wetter, decreasing eastward.
Regimes: over 200 cm — equatorial belt, windward western slopes in cool temperate zones, monsoon coasts; 100–200 cm — continental interiors, many coasts; 50–100 cm — central tropical lands, eastern and interior temperate lands; under 50 cm — rain-shadow interiors and high latitudes.
Seasonality: even all year in the equatorial belt and western cool temperate regions; seasonal in monsoon and Mediterranean lands.
Diagram: sketch a hypothetical continent with these zones.
General rules: decreases from the equator to the poles; coasts wetter than interiors; oceans wetter than land; windward sides wetter than leeward (rain-shadow).
Coast rules: 35°–40° N/S — east coasts wetter, decreasing westward; 45°–65° N/S — westerlies make west coasts wetter, decreasing eastward.
Regimes: over 200 cm — equatorial belt, windward western slopes in cool temperate zones, monsoon coasts; 100–200 cm — continental interiors, many coasts; 50–100 cm — central tropical lands, eastern and interior temperate lands; under 50 cm — rain-shadow interiors and high latitudes.
Seasonality: even all year in the equatorial belt and western cool temperate regions; seasonal in monsoon and Mediterranean lands.
Diagram: sketch a hypothetical continent with these zones.
What are forms of condensation? Describe the process of dew and frost formation. (150 words)
Forms: dew, frost, fog (and mist, smog) and clouds — classified by temperature (dew point above or below 0°C) and location (on surfaces or in free air).
Dew: on clear, calm, long cold nights with high relative humidity, the ground radiates heat and cools; air touching cool surfaces (stones, grass, leaves) is chilled below its dew point; vapour is deposited as water droplets — on surfaces, not on nuclei in the air. The dew point must be above freezing.
Frost: same conditions, but the dew point is at or below 0°C, so the excess moisture is deposited as minute ice crystals (white frost).
Why cloudy or windy nights give little dew: clouds keep the ground warmer; wind mixes the air (Beyond NCERT).
Diagram: a 2×2 grid — dew, frost, fog, cloud.
Dew: on clear, calm, long cold nights with high relative humidity, the ground radiates heat and cools; air touching cool surfaces (stones, grass, leaves) is chilled below its dew point; vapour is deposited as water droplets — on surfaces, not on nuclei in the air. The dew point must be above freezing.
Frost: same conditions, but the dew point is at or below 0°C, so the excess moisture is deposited as minute ice crystals (white frost).
Why cloudy or windy nights give little dew: clouds keep the ground warmer; wind mixes the air (Beyond NCERT).
Diagram: a 2×2 grid — dew, frost, fog, cloud.
What to remember
Numbers
Water vapour in the air0–4% by volume
Absolute humidity unitgrams per cubic metre
Relative humidity% of capacity at that temperature
Dew forms when the dew point isabove 0°C
Frost forms when the dew point isat or below 0°C
Fog visibilityless than 1 km
Mist visibility1–2 km
Cirrus height8,000–12,000 m
Cumulus height (text)4,000–7,000 m (glossary: 300–2,000 m)
Basic cloud types4 — cirrus, cumulus, stratus, nimbus
Types of rainfall by origin3 — convectional, orographic, cyclonic
East coasts wetter35°–40° N and S
West coasts wetter (westerlies)45°–65° N and S
Heavy rainfallover 200 cm
Very low rainfallunder 50 cm
Hailstone size (glossary)5–190 mm
One-liners
- Warm air holds more vapour than cold air.
- Cooling raises relative humidity; at 100% the air is saturated (dew point).
- Dust, smoke and sea salt are hygroscopic nuclei.
- Fog is a cloud with its base at or near the ground; smog = fog + smoke.
- Cirrus is the highest cloud; nimbus is dark, dense and low.
- Sleet = warm layer above a freezing layer; hail has concentric ice layers.
- Windward slope wet, leeward slope dry (rain-shadow).
- Convectional rain: heavy, short, afternoon, with thunder.
Traps to avoid
- Air holding moisture to full capacity is saturated air, not ‘relative humidity’.
- Dew forms on surfaces, not on nuclei in the air.
- Frost needs a dew point at or below freezing — not just a cold night.
- NCERT: fog has the lower visibility, yet mist is called the wetter one.
- Cumulus height: 4,000–7,000 m in the text, 300–2,000 m in the glossary.
- 35°–40°: east coasts wetter; 45°–65°: west coasts wetter — not the other way round.
- Cloudy nights give less dew — the ground stays warmer.
- The condensation list in NCERT skips (iii) — nothing is missing.
How to strategise
Prelims — what UPSC asks
- Reason-based questions dominate: dew and cloudy nights (1979, 2019), convectional rain near the equator (1996, 2003), moisture vs latitude and temperature (2003), rain-shadow east of the Western Ghats (1989).
- Cloud facts: cirrus = ice crystals (2004), halos (2002), why clouds float (1995). Learn the four types with height and look.
- Basic definitions: water cycle (1992), humidity and temperature (1981), fog and ocean currents (1988).
- Expect statement pairs that swap words — saturated vs relative humidity, dew vs frost, windward vs leeward, east vs west coast.
Mains — where it appears and how to answer
- GS1: cloudbursts — what they are and why they hit mountain slopes (2024). Link to orographic lifting, saturated air and intense convection.
- GS3 links: urban floods, cloudburst disasters, winter fog and smog in north India — explain the physical process first (this chapter), then impacts and mitigation.
- Answer frame: define → process (draw the orographic or convectional sketch) → world and India examples → impacts.
- A neat orographic-rain sketch with ‘windward / leeward / rain-shadow’ labels earns marks in any rainfall answer.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Which one of the following process is responsible for transforming liquid into vapour?
2. The air that contains moisture to its full capacity:
3. Which one of the following is the highest cloud in the sky?
4. Frost forms instead of dew when:
5. A layer of air above freezing lies over a sub-freezing layer near the ground. Precipitation will most likely reach the ground as:
6. Air at 30°C holds 17 g of water vapour per m³ and is cooled without adding or removing any. Its relative humidity will:
7. Between 45° and 65° N and S, rainfall is heaviest on:
UPSC questions from this chapter
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