Humidity, Clouds & Rainfall – NDA Geography Notes

Water is constantly moving between the Earth’s surface and the atmosphere, evaporating from oceans and lakes, rising into the atmosphere, condensing into clouds, and falling back as precipitation. This movement is the water cycle or hydrological cycle: one of the most important processes shaping Earth’s climate and supporting all life.

NDA tests this chapter through cloud types and their altitude, types of rainfall and the conditions that produce them, and humidity-related terms. These are direct, factual questions with no ambiguity. A student who knows the cloud classifications and rainfall mechanisms will score full marks from this chapter.

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Evaporation and the Water Cycle

Evaporation is the process by which liquid water at the surface converts to water vapour and enters the atmosphere, driven by solar energy.

Factors that increase evaporation: high temperature | low humidity | strong wind | large surface area.

Transpiration is the loss of water vapour from plants through stomata in their leaves.

Evapotranspiration = evaporation + transpiration combined: the total water loss from land surfaces.

The hydrological cycle: Evaporation → Water vapour rises → Condensation into clouds → Precipitation → Runoff → Evaporation again. The hydrological cycle is driven entirely by solar energy: the Sun powers evaporation, which powers the entire cycle.

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Humidity

Humidity is the amount of water vapour present in the air.

Absolute Humidity: the actual amount of water vapour in a given volume of air, expressed in grams per cubic metre (g/m³). Absolute humidity decreases with altitude.

Specific Humidity: the mass of water vapour per unit mass of air (including water vapour), expressed in grams per kilogram (g/kg). It does not change when pressure or temperature changes alone, only when water vapour is added or removed. More useful in meteorology than absolute humidity.

Relative Humidity: the ratio of the actual amount of water vapour in the air to the maximum it could hold at that temperature: expressed as a percentage. [NDA 2014-II]

Relative Humidity (%) = (Actual water vapour ÷ Maximum possible at that temperature) × 100

When relative humidity reaches 100%, the air is fully saturated. Any further moisture addition or temperature drop will cause condensation.

Relative humidity changes with temperature. As temperature rises, air can hold more vapour, so relative humidity falls even if no moisture is added. As temperature falls, relative humidity rises.

Dew Point: the temperature to which air must be cooled (at constant pressure) for it to become saturated, when relative humidity reaches 100%. Below the dew point, condensation begins.

Humidity Term	Definition	Unit	NDA Tag
Absolute Humidity	Actual water vapour per volume	g/m³	—
Specific Humidity	Water vapour mass per air mass	g/kg	—
Relative Humidity	Actual / Maximum × 100%	%	[NDA 2014-II]
Dew Point	Temperature at which RH = 100%	°C

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Condensation and Its Forms

When air cools below the dew point, water vapour condenses into tiny liquid droplets. For condensation to occur, air must cool below the dew point AND there must be tiny particles (condensation nuclei: dust, pollen, salt) for droplets to form around.

Dew: water droplets forming on cold surfaces at night when the surface temperature falls below the dew point. Forms best on clear, calm nights. Radiation escapes freely, cooling the surface rapidly.

Frost: when the dew point is below 0°C, water vapour deposits directly as ice crystals on cold surfaces. Same mechanism as dew but at sub-zero temperatures.

Fog: a cloud at ground level. Forms when a large mass of air near the surface cools below its dew point, creating tiny suspended droplets that reduce visibility below 1 km. [NDA 2016-I]

Mist: similar to fog but thinner: visibility reduced below 2 km.

Smog: a mixture of smoke, dust, and fog. Common in industrial cities when temperature inversion traps pollutants near the surface. (The word combines smoke and fog.)

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Types of Fog

Radiation Fog: forms on clear, calm nights when the ground radiates heat and cools rapidly. The air near the ground cools to the dew point. Most common in valleys where cold air drains and accumulates.

Advection Fog: forms when warm, moist air moves horizontally over a cold surface. The air near the surface cools below its dew point. Classic example: fog along the California coast when warm Pacific air moves over the cold California Current.

Steam Fog (Arctic Sea Smoke): forms when very cold air moves over relatively warm water. Water evaporates and immediately condenses in the cold air above, creating wisps of fog.

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Clouds: Classification

A cloud is a visible mass of tiny water droplets or ice crystals suspended in the atmosphere. Clouds form when air rises, expands, cools below its dew point, and condensation occurs on condensation nuclei.

Air is forced upward by: convection (surface heating) | orographic lifting (mountain) | frontal lifting (warm air over cold) | convergence.

The cloud classification system was devised by Luke Howard in 1803.

Clouds are classified by two characteristics:

Altitude: high (above 6,000 m), middle (2,000–6,000 m), or low (below 2,000 m)

Form: layered/flat (stratus type) | heaped/puffy (cumulus type) | wispy (cirrus type)

The prefix “nimbo-” or “nimbus” in a cloud name indicates it is rain-producing.

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High, Middle, Low, and Vertical Clouds

High Clouds (above 6,000 m)

Made entirely of ice crystals because temperatures at these altitudes are well below freezing.

Cirrus: thin, wispy, feathery clouds made of ice crystals. White wisps against a blue sky. Do not produce rain. Often signal approaching weather (particularly a warm front).

Cirrostratus: a thin sheet-like layer of ice crystals covering much of the sky. Creates a halo effect around the Sun or Moon. Indicates approaching precipitation.

Cirrocumulus: small, white puffs of cloud in rows at high altitude. Sometimes called a “mackerel sky.”

Middle Clouds (2,000–6,000 m)

Altostratus: a grey or blue-grey sheet of cloud covering the whole sky. Often thick enough to block the Sun. Produces widespread, steady rain or snow.

Altocumulus: white or grey patches, sheets, or layers in rows. May produce light rain.

Low Clouds (below 2,000 m)

Stratus: a flat, grey, featureless sheet of cloud close to the ground. Like fog that does not reach the surface. Produces only drizzle. Common on overcast days.

Stratocumulus: low, lumpy, grey clouds in patches or rolls. The most common cloud type globally. May produce light rain.

Nimbostratus: a dark grey, thick layer of cloud producing continuous, moderate to heavy rain or snow. No distinct base. The rain merges with the cloud. The “nimbo” prefix means rain-producing.

Vertically Developed Clouds (span all altitudes)

Cumulus: puffy, heaped, white clouds with flat bases and rounded tops. The classic “fair weather” cloud. Formed by convection. Small cumulus do not usually produce rain but can grow into cumulonimbus.

Cumulonimbus: the largest and most powerful cloud type. Extends from low altitude to the top of the troposphere (tropopause), sometimes 15 km tall. Dark, anvil-shaped top. Produces heavy rain, hail, lightning, thunder, and tornadoes. The cloud associated with thunderstorms. [NDA 2022-I]

★ IMPORTANT Three features of cumulonimbus tested together in NDA 2022-I: Thunderstorm + Violent upward air movement + Hail: all three correct. [NDA 2022-I]

Altitude Zone	Cloud Type	Key Feature	Precipitation
High (>6,000 m)	Cirrus	Wispy; ice crystals; halo with cirrostratus	None (cirrus); indicates weather approaching
High (>6,000 m)	Cirrostratus	Thin sheet; halo effect	None
High (>6,000 m)	Cirrocumulus	Mackerel sky	None
Middle (2,000–6,000 m)	Altostratus	Grey sheet; blocks Sun	Widespread steady rain/snow
Middle (2,000–6,000 m)	Altocumulus	Patches/rows	Light rain
Low (<2,000 m)	Stratus	Flat grey; ground fog-like	Drizzle only
Low (<2,000 m)	Stratocumulus	Most common cloud globally	Light rain
Low (<2,000 m)	Nimbostratus	Dark, thick; no distinct base	Continuous moderate-heavy rain
Vertical (all)	Cumulus	Puffy; fair weather	Usually none
Vertical (all)	Cumulonimbus	Tallest; anvil top; THUNDERSTORM	Heavy rain, hail, thunder [NDA 2022-I]

Height sequence lowest to highest: Nimbostratus → Altostratus → Cirrus

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Precipitation and Types of Rainfall

Precipitation is any form of water, liquid or solid, that falls from clouds to the Earth’s surface.

Types: rain | drizzle | snow | sleet | hail | freezing rain.

There are three main types of rainfall based on the mechanism that forces air upward.

Convectional Rainfall

Mechanism: The surface heats up intensely. Air near the surface warms rapidly, becomes less dense, and rises vigorously in convection currents. As it rises, it cools and condensation occurs, forming large cumulonimbus clouds.

Characteristics: Intense and heavy | short duration (a few hours) | accompanied by thunder and lightning | very localised.

Where it occurs: Equatorial regions (almost every afternoon) | continental interiors in summer.

Orographic Rainfall (Relief Rainfall)

Mechanism: Moisture-laden winds blow against a mountain range and are forced upward. As the air rises, it cools, condenses, and produces rainfall on the windward side. After crossing the mountains, air descends on the leeward side: warming and drying: creating the rain shadow.

Characteristics: Heavy on windward side | dry rain shadow on leeward side | persistent.

Examples: Western Ghats (windward) vs Deccan Plateau (rain shadow) | Southern slopes of Himalayas vs Tibetan Plateau.

Cyclonic (Frontal) Rainfall

Mechanism: When a warm air mass meets a cold air mass at a front, the warm air is forced upward: steeply at a cold front, gradually at a warm front.

Characteristics: At cold front: heavy intense rain over narrow belt | at warm front: steady widespread moderate rain | most common in mid-latitudes.

Where it occurs: Temperate regions: particularly Western Europe and North America.

Rainfall Type	Mechanism	Region	Character	NDA Tag
Convectional	Surface heating → air rises	Equatorial; continental interior summers	Heavy, short, thundery
Orographic	Moist air forced up by mountain	Windward slopes of ranges	Heavy windward; dry rain shadow leeward
Cyclonic/Frontal	Warm air forced up over cold at front	Temperate mid-latitudes	Widespread, steady

Snow forms when temperatures in the cloud are below freezing. Precipitation at the poles is mostly snow. Snowflakes have a hexagonal (six-sided) crystal structure.

Hail forms in cumulonimbus clouds where strong updrafts carry raindrops up into freezing temperatures, which grow in concentric layers until heavy enough to fall. Hail is always associated with cumulonimbus and thunderstorms.

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Global Rainfall Distribution

High rainfall areas: Equatorial zone (convectional daily) | windward sides of mountains | tropical monsoon coasts | mid-latitude west coasts (frontal rainfall).

Low rainfall areas: Subtropical high pressure belts (~25°–35° latitude): the world’s hot deserts (Sahara, Arabian, Thar, Australian) | rain shadows | continental interiors | polar regions.

Why subtropical deserts are dry: The subtropical high pressure belt is characterised by sinking, diverging air. Sinking air warms, increasing its capacity to hold moisture rather than releasing it. Precipitation is suppressed.Measurement:Rain gauge: a cylindrical container measuring rainfall depth in millimetres. Isohyets: lines on a map connecting places of equal annual rainfall (equivalent of isotherms for temperature, isobars for pressure).

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